Puller vs Pusher Aircraft - Which is More Efficient?
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- čas přidán 13. 06. 2024
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The DarkAero 1 is engineered to fly fast while maintaining high efficiency, and we located the propeller at the front of the airplane even though it has long been theorized that placing the prop at the back so it pushes would be better for speed and efficiency. So why is our propeller at the front? Would the DarkAero 1 actually be better with the prop in back?
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Great discussion. A few items you didn't mention: for prop clearance, pushers often just add blades and reduce diameter in order to gain ground clearance, and to prevent prop strikes during over-rotation.
You alluded to frontal profile, but let me expand a bit: With a tractor design, you have a relatively tall engine (particularly if it's a traditional Lycoming/Continental with the induction on the bottom of the engine) which means the cowl must be tall in order to fit it. The pilot then needs to sit high and upright to provide visibility over the cowl. On a pusher with the engine mid-mounted or rear-mounted, the front of the aircraft can be quite slim, both vertically and horizontally. This means the pilot can be seated lower and reclined (like in a Long EZ, or as in my Cozy), which means less overall frontal profile, and less drag.
Lastly, one of the big downsides to pushers that you didn't mention is FOD. If you are landing on an unimproved strip or on a runway with loose gravel, stones or any other foreign matter, your nosewheel can (and usually does) kick this up directly into the prop arc. This can cause serious damage to the prop. On a composite prop, smaller nicks can usually be repaired, aluminum props can be dressed if it's not too bad. Otherwise, you're looking at a prop replacement. On a constant speed prop like you are using, this could get extremely expensive in a hurry.
The tradeoff with decreasing diameter is decreased prop efficiency, but your other points are excellent reasons to consider both options.
Another difficulty with conventional layout pusher is the lost weight needed to provide the drive shaft and increased structure to support it in torsion and vibration. I think when you are trying for light weight as hard as the DarkAero project that loss in weight efficiency is not trivial.
but you can also completely eliminate the need to reduce prop diameter, by placing the prop as close to the main gear as possible. Airco DH.2, SAAB 21, J7W, XP-55, Cessna O-2, and Long EZ are examples of this. And of course we cannot forget the Wright Flyer with its very large diameter props.
But that doesn't alleviate the FOD issue in many pushers. Though not all pushers have the FOD issue either. Seabird Seeker, Aircam, Cessna O-2, and "The Prototype" being but a few examples.
And of course, a lot of this changes when you look at twin-engine canards.
For a student project for a long range aircraft we looked at pusher props because they ingest the boundary layer, reducing the pressure rise and thereby delaying the transition of the flow over the fuselage from laminar to turbulent. I dont remember the exact numbers but they seemed quite favorable. We were a bit worried about the added vibration loads on the blades. Reinforcing them would have reduced the benefits off course.
I guess the range of the DarkAero is not enough to make a real difference.
I would add another factor. It seems like FOD prop damage is much more common with pushers. Rocks and debris are often kicked up by wheels, especially on the more civilian airstrips that small planes are commonly used. A pusher has a blade that is much more at risk of FOD. I agree...puller is an appreciably better solution for a civilian small plane.
Prop erosion, and nicks are challenging enough with 12” of clearance in a tractor configuration….
Having a prop with less clearance, will have you rushing the run-up procedure…
Having the prop inline with a nose wheel… will lead to lots of dents and scratches on the prop’s leading edge…
Props are expensive…. 😃
Not all pushers have the FOD issue though either. Seabird Seeker, Aircam, Cessna O-2, and "The Prototype" being but a few examples.
Just add some mud flaps with metal silhouettes of naked ladies! Style AND functionality!
@@damnsong8675309 mudflaps for the takeoff and landing procedures, basically an added excuse to have an inline flap that doubles as guard for the propeller.
You can build a extendable shield to protect the prop during take off and landing
I think the pusher configuration offers some interesting opportunities in terms of cabin configuration-- for example, you could install a ground-view window that could provide incredible scenic and filming opportunities (which definitely falls well outside the scope of DA1's stated design goals, but could be worth considering for a separate project). I really enjoy seeing just how meticulous everyone at DA has been when tackling these design challenges and walking us through the process, and as always, I'm looking forward to seeing more from your team! Great job, guys!
I think there are other advantages considering how the Learfan was based on this configuration and it was designed by an industry veteran, but it was a different application than this small aircraft. Maybe the propeller efficiency can be increased with some changes, the potential reduction in drag is much higher, the higher crusing speeds justified this design choice, ...
This is fantastic idea.
Tractor props are typically considerably quieter than pushers. Pusher blades chopping through the shear created by the wing lift create noise initially, and that also can cause vibration in the blades that tractor props would not be subject to.
Just like how wind turbines have the blade before the tower in a horizontal axis style turbine. If the blades were behind the tower, you wouldn't need a yaw gear, but your blade would flex every time it passes into the wake of the tower. This loading cycle is much worse for a high aspect ratio wing, like a wind turbine blade than for an airplane propeller. The propeller does spin considerably faster, though, so more cycles. Does anyone know whether props fail sooner for pusher configurations?
The twin beam Bayraktar TB2 drone is a good example of a pusher with a non interfering tail and Rotax 912.
I have a strong feeling you could design unconventional Propellers to reduce the issue, and arrange the wings in a way it works, perhaps a lifting body aircraft. I may be able to draw something out of this
It was beyond scope here but I'm sure some sort of flow conditioner could be added to make the airflow more laminar and gain both efficiency and quieter props. But that would also add complexity and weight and I'm not sure how well it would work through the speed range.
I have a velocity, one other downfall of the pusher style is anything that comes off the airframe ends up going through the prop. Cowling screws, stones or rocks from a soft field or even a wrench resting on the wing when doing a dry run up……. Ask me how I know. 😂
just like helicopter tail rotors
In my experience with R/C aircraft, which tend to have higher drag, a rear prop gives noticeably better efficiency but a lot more noise due to the disturbed air entering the prop disc.
For beginner RC craft, the pusher prop is much more protected in bad landings and when you hit obstructions like trees and such. Also, some small unmanned aircraft are catapault launched and retrieved by flying into a net. So it is better to keep the prop away from the front of the plane if you are going to "land" by flying into a net.
@@mckenziekeith7434 I like the single engine tractor arrangements found on the Polaris and Seawind, mounted high on the tail away from the fuselage. It isn't as noisy as a pusher as well as still being efficient, especially if there is adequate clearance between prop tips and fuse, however It can cause a downward pitching moment with any rapid throttle increase.
Something not discussed is the ability of the traditional "tail dragger" tractor arrangement to reduce takeoff distance. Most high performance STOL aircraft use this arrangement in competitions.
Noise is synonymous of power loss, as noise is a result of energy loss.
Because smaller rudders required to vercome the p effect of the propeller so one is NOT wasting horse power turning the aircraft in the correct direction. Big advantage, in fact this is the LARGEST advantage compared to all the BS the host brought up, but then I would expect that as he does not know aerodynamics.
@@w8stral Why would a pusher configuration not suffer the same p effect? Because the air that has been moving over the aircraft is moving more in line with the pitch angle of the aircraft instead of the horizon? I am not sure how big that effect is, and I also wonder what happens when you enter a (partial) stall, which then also seriously disrupts the airflow into the propeller.
I like pusher configurations, but I would probably mount it on the top of the fin, for minimum air disturbance and biggest ground clearance. But this position pushes the nose down, so you probably need to mount the propeller at an angle to counteract this (like the DC-10)
Prop at the front also gives you more stick authority and lower stall speed. The air goes over the wings and control sufaces.
a huge factor
Seabird Seeker
Chinook PLUS 2
Challanger II
Sky Arrow 600
Cessna O-2
.....
All have airflow over the tail.
This channel is everything that the Raptor development channel wasn’t.
Probably this plana will fly
@@bernhardjordan9200 The Raptor flew...
@@flexairz "The Raptor flew..."
Into that cornfield.
I used to like the guy’s enthusiasm until he lost it…
These guys research their stuff and listen to criticism. The Raptor guy thought he knew it all and got cranky if someone questioned his decisions. So he ended up in the cornfield due to his ego. His latest iteration of the Raptor was seen as a CAD render as an effin biplane several months ago but doesn't seem to have advanced any further!
While I've long thought that the prop wash from the forward mounted prop would create more drag over the airframe, I'd never considered that the airframe would reduce the efficiency of a rear mounted prop. This was a really interesting and well produced examination of both arrangements.
yup, propellers of all type really want clean air. this is a challenge with helicopters. But not all pushers are equal. some have little to no real interference to the prop disk (e.g. Aircam multiengine)
A video about canards and the reasons you chose not to use one would be very interesting.
Have a look at my channel, I have a video about canards and why I DID choose one (both pros and cons).
This was going to be my question since this project started. Canard is much more efficient and safer and I'm sure these guys could out do the DarkAero One.
The "safety" of a canard aircraft comes from the fact that you are so afraid of a main wing stall, which is unrecoverable, that you engineer the canards to stall well before the main wing. This means your aircraft has a higher stall speed than it would have if you put the same wing on a traditional layout.
The other issue is that because you've engineered the canard to stall before the main wing, adding flaps to the main wing doesn't reduce your stall speed because your stall speed is based on the canards rather than the main wing.
A higher overall stall speed, which you then can't reduce with flaps, means a very fast approach speed (or you need to oversize your wing to give yourself enough lift for a slower stall speed). A fast approach speed means you need a very long runway to land, and you need to engineer your landing gear+brakes to handle higher speeds and more energy dissipation. Oh, and if you ever have to ditch you'll be ditching at higher speeds too. So much for safety.
I too was on the "rutan is a genius with his weird looking planes" bandwagon until I read more about the downsides of the canard layout.
@@StephanAhonen You're correct on most of the points you make, I talk about all of those in the video I mentioned in my channel above, the "Why I bought this airplane" video - I go over the pros and cons of canard aircraft, and do go a bit into the weeds talking about aerodynamics.
The stall speed is not high because of a "fear" of stalling the main wing, it's because it is impractical to put flaps on a canard (although it can be done - the Beech Starship had flaps, and a complex mechanism to make adjustments to the canard - again, I talk about that in my video).
That said, the stall speed is not THAT high. The stall speed of my Cozy is 63 kts, which is pretty close to the 61 kt stall speed of a Lancair (with flaps down), an airplane with a comparable performance envelope.
Runway length for canards for landing is not the limitation - it's takeoff. You need enough speed to get the canard to fly before you can rotate the main wing into lifting. That takes some runway. You will land a bit longer than a Cessna, but again - comparable to an equivalently performing aircraft like a Lancair.
@@StephanAhonen the safety of the standard aircraft comes from the fact that you are so afraid of a tail stall, which is unrecoverable, that you engineer the main wing to stall well before the tail.
My grandfather was one of the few people who actually built and finished his BD-5. I used to sit in it and pretend I was a fighter pilot.
You’re lucky you didn’t die just being around that POS!
Wow that is so not even a little interesting, this isn't about you
@@slowery43 Just glad you survived…😂😂😂😂
when the bd came out the wife said no way no chance
now she's long gone and in my shop under construction a carbon fiber BD5
with a zero radar return(the American inventor also invented me )
power ....a m8 Harley Davidson motor twin plug no shake programable in flight ejection, turbo ,and its being rebuilt to about 400 hp..
hey at my age..nursing home suck any way !
@@NeroontheGoon
@@user-qx6wb2kf3o Carbon fiber fiber BD5, the horror continues.
Great lesson on the trade offs in aircraft design and iterative process. Since constraints are universal, that’s why air planes end up looking quite similar and innovative designs are few and far in between.
Interesting discussion. You mentioned balance - with a tractor, you tend to keep the fuel tank and passenger payload close to the CG. With a pusher, you typically need to offset that weight in the back by moving other things forward. That puts limits on how changes in payload and potentially fuel weight affects your CG envelope. The big problem with canards is that flaps cause a large pitching moment when deployed due to the NP being further forward than a rear tail configuration. Most canards land without flaps - and fast. Although in a different class of aircraft, the Piaggio Avanti did a nice job of overcoming both of those compromises with a three-surface design and twin pusher engines behind the wings instead of behind the tail. It is one of the most efficient business class twin turbo prop planes ever built.
Most pushers also simply keep variable payload and fuel near the CG.
Seabird Seeker, Aircam, Cessna O-2, SAAB 21, Wright flyer, and many more.
Also, look how helicopters handle CG, as they have the same issue with the engines being in the back.
I like both.
An electric and gas.
One of the front the other in the back.
Best of both worlds.
@@jtjames79 battery weight and drag penalty is going to be huge.
The 500l does efficiency to the nth degree. If you're going for full laminar flow pusher is the only option. The way it is implemented on the 500l though, there's no improved visibility lol!
Anyone else impressed with the white board game! My sketches would not be this neat! JS!
There have been several forays into pusher props over the years. The B-36 Peacemaker, the Lear Fan, the Beech Starship, the Piaggio P-166 and the P-180. The B-36 initially had problems with engine cooling. The Lear Fan was only test flown but never went into production. The P-166 was the piston-powered ancestor of the Turboprop P-180. The Starship was considered a loser business deal and Raytheon stopped making parts for it. Pushers sound like a logical idea. They look sleek and sexy. But some of them come with very real issues. On takeoff, if you rotate too quickly, there’s a chance you’re going to get a prop strike. And on landings, you can’t have the nose too high for the same reason. Landings are a real consideration because we typically have a higher nose-up attitude to stall just before touchdown. The Piaggio P-180 is a really sleek looking aircraft and it absolutely is. It’s a head-turner on any ramp even from the jet folks. It’s the fastest production turboprop in the world. But it’s a turboprop. Yes, it uses a variant of the PT-6 engine but it needs all its own parts. If you own a King Air, access to parts is a breeze because King Airs are operated all over the world. The P-180? Not so much. The home-built market has really outpaced the "Big" companies in terms of cost and that has allowed developers to play around more with the pusher concept and there are a couple of companies that make pushers. So overall, pushers are a fascinating idea but they come at a price.
Keep going! Great content as always. Proud reservation holder!
Love these explanations videos, thanks, loving following your project
Just thought of a cool idea. What about keeping the engine up front and run a shaft to the rear for a pusher? That would maintain the cooling challenge and ease of access to the engine. A F86/mig15 look would be awesome!
That would require a significant increase in weight and reduced interior space. Depends on your requirements I guess.
There's an additional problem aside from added weight and complexity. The P-39 was a midengined tractor aircraft, similar to what you're proposing. The driveshaft had to be lengthened, and the additional forces that came with that length meant that a quick enough power increase would actually break it.
@@HikariKobayashiuse a high RPM, low torque engine ( V8 automotive). Torque tube from Corvette 6 . Planetary gear at the rear. Fast 5 blade prop.
I would guess that for a tractor, the shaft needs to be as short as possible, especially with low blade loading. The 6 cylinders should be close together ( water cooled ) and almost stick into the propeller hub.
Then again a modern aluminum engine block isn’t that heavy.
Inline six in the front. Two pilots . No fat clutch or gears. Just a “divider” between both pilots. Upright engine, not slant. Might be interesting. Not so great for visibility and collides with the nose gear. No flat firewall.
I just love your design process walk-throughs. Great work!
Brilliantly straightforward video...Thank you for making these.
Subbed!
Based in this analysis it might be interesting to evaluate wingtip prop configuration, especially on the tail wings
Love your tutorial videos! Please keep them coming. I'm hoping you guys will expand your planes to include a 4-6-seat plane. Maybe a dual-engine canard-style pusher airplane that is powered by the new Deltahawk? I know some of you will say there is such a plane in the Velocity Twin, but that plane is a dated design made of fiberglass. I'm sure these Darkaero guys would come up with something amazing!
I fly a quicksilver ultralight with a pusher prop that is mid-body. It's really nice not having to look through a prop, plus no possibility of a prop strike even on over-rotate, plus it gives the elevators and rudder huge authority with the prop blasting right onto them.
Interesting, as a layman I've always wondered why we don't see pushers with a heat exhaust ducted similar to a center-mounted cannon in WW2 fighters to keep the turbulences on the blades to a minimum.
If you wanted max efficiency, you could do like the Sunseeker Duo which puts the engine (actually, motor) at the front of the top of the T-tail empennage where you ingest clean air but don't dump disturbed air over the fuselage, just a portion of the tail. (It's also a sailplane, so the propeller folds in gliding/soaring flight so doesn't even disturb the tail during much of the flight). Sunseeker Duo: czcams.com/video/uBA4XeMddMY/video.html
That can have issues in high angle of attack flight in possibly deep stalling with challenging recovery. But, has been done, and possibly airplane parachutes are a good fix for deep stall crew survivability.
Well it helps with sailplanes that don't need a motor to maintain stable flight.@@Georgewilliamherbert
You also have higher trim drag with this arrangement
Excellent presentation, thank you.
Great explanations. Thanks!
A pusher configuration was used for the Edgley Optica - which provided a good observation platform. But the rear engine configuration and ducted fan was complicated.
Great job and thank you sooo much for the education. You guys are amazing and I wish you the very best!
Incredible sharing of knowledge and design :) Just outstanding
Amazing video. Great explanation, just earned a subscriber.
Fantastic stuff thanks for another great video!
Good and understandable explanation. Thank you.
Great video. I haven't been around aircraft in over 30 years since school. I never received my A&P, I changed fields. I guess I had a great instructor, because everything came back while you were talking. I even anticipated several potential problems. :)
Great video and lots of other good considerations mentioned in the comments, particularly FOD and noise being big problems for pushers. Another important factor that I did not see mentioned was short field performance. Pushers take a lot longer to get off the runway due to the lack of prop wash making the tail surfaces effective at slower speeds. Also, you talked about forward engine placement with long prop shafts to the tail, like the BD-5 or VK-30. An issue here tends to be torsional vibrations in the prop shafts, usually requiring some kind of flex coupling, which adds weight and is another maintenance item.
Another consideration is safety in a crash scenario. With the engine and prop in front, the mass of the engine absorbs some of the impact forces, whereas with it at the rear, the tendency is that you will need to insure that the engine cannot come loose and move forward in the event of a crash. In event of a crash with the engine at the rear, there is a greater likelihood that fuel lines will be severed, and increased risk of fire inside the cabin, in my opinion.
Yes, and that's why mid-engined Porsche Boxsters and Caymans have a cable, or wire rope, to restrain the engine in case of frontal collision.
Is there anyone named Lindberg in your family?
"In event of a crash with the engine at the rear, there is a greater likelihood that fuel lines will be severed, and increased risk of fire inside the cabin,"
this is baseless.
it is wholly dependent upon where the fuel is located, even in a tractor, and where the fuel lines are run. If the engine never impacts the ground, how exactly are the fuel lines magically severed?
Also, even helicopters like the OH-6/MD-500 are famously crash resistant with the engine in the rear.
Airlines and corporate jets also do well as pushers.
how does the Aircam result in fuel in the cockpit, or engine hitting the pilot?
A great many ultralights have been pushers, and rarely are pilots killed by the engine/prop specifically upon crashing.
Formula 1 race cars crash into brick walls at over 200mph with the engine in the rear, and the engine has never come through the cockpit. engines are TERRIBLE crumple zones.
@@SoloRenegade F1 cars also don't have to worry about not getting off the runway because they burned too much weight budget making the driver tub. And of course they don't crash into brick walls, but energy dissipation walls and the relative speed between the car and the wall is more like 40 mph because of the angles. Maybe we should compare hydrofoil boats to airplanes next.
6:28 Something that some people don't know: The propeller also cools down the pilot. The pilot will often start sweating when the propeller stops spinning.
Fr, great vid
That graphic you drew explains so much, but I’m intrigued anyway. Instant like)
Nice drawings! Somebody there is pretty handy with a whiteboard marker
I would love it if these guys did a thorough analysis of the celera 500l or the long forgotten synergy aircraft. The way they explain complex equations with ball park figures is amazing.
Excellent analysis and consideration of the multiple tradeoffs. I do like pushers partly because of the better visibility, though as you mentioned, efficiency is about equal overall. People often point to the Cessna 337 as proof of the pusher configuration's higher efficiency, because the climb performance and maybe cruise speed is a little higher with only the rear engine running, versus only the front engine running. But the airframe shape may be directing more air to the rear prop than some other pusher configurations might experience.
I was going to make the same observation. The reason for this behaviour is that air accelerates as it is drawn towards the prop, and accelerating flow is more stable. That might be particularly important for the 337 layout as the rear end is pretty blunt.
Easygoing, well researched, and it’s evident the presenter really likes the topic. Plus a nod to the cool factor, which surely is what’s really on everyone’s mind. One question-is there a difference in noise between the arrangements?
Great video!
Thanks very informative video
Great video, it was very well explained.
Amesome video guys, Thank you so much for explaining it to slower guys like myself
I am grateful for this excellent explanation and comparison of plane designs. I love the clean drawings and the filtered explanations of many topics. I think both design are relatively close in performance (oddly in model airplanes the pusher seem to be slightly more efficient/ faster) but this plane is well conceived. Also you can design the rear as a T tail pusher to avoid the turbulent air ( I know its not ideal but at least a way around the standard configuration.)
We have advanced very far since first looking at birds and desire to fly.
Appreciate the summary table.
That's why I like the Cessna 337, you don't have to decide, it has both a pusher engine , and a puller prop in a centerline thrust configuration, and the weight of the engines at both ends makes the CG equation easier. The added bonus is you have a second engine if one fails. Only downside is two engine maintenance and fuel burn rates.
It would be very easy (and understandable) for a set of people so focused on performance and spend so much effort on meticulous engineering to disregard something like "cool factor" out of hand. Very nice that you considered that in your final comparison!
- just like the tailfin rake of "later model" Cessnas - if it looks fast on the ground it sells better (Pilatus never cared).
A very interesting video, and comments section. Worth a watch and a read. Cheers all!
I would argue that one of the biggest advantages to a tractor configuration is that you always have high energy airflow moving over the tail control surfaces, even when airspeeds get low or when higher angles of attack cause flow separation in the wake of the wing.
You can quickly lose control authority of the tail when it falls into the wake of the wing at high alpha, and that phenomenon also kills your pusher prop efficiency too. This can cause inconsistent thrust through the maneuvering window, at the point at which the airframe is also experiencing the highest aerodynamic induced drag.
Conversely, the tractor configuration effectively "forces" airflow over the tail even in these awkward flight regimes. It doesn't always solve the problem (particularly at high speeds), but it certainly improves it by a significant margin. It can mean the difference between preventing a spin, and falling into a spin.
The Dornier Do 335 was one of the fastest propeller driven fighters of the war, although it came too late to see any service and was a developmental dead end at the start of the jet age. Still I can't help but think it looks pretty sleek and wonder why that design hasn't really ever been used since?
Cessna 336/337 (get the numbering ?) Moynet Jupiter, Rutan Defiant, Adam 500 et al -- the C 337 is noticeably noisier than other aircraft and had some quirks such as engine failure accidents still under full control but the pilot being unaware, for the critical instant, of losing the rear engine and failing to clear trees etc on take off . I knew a pilot that had experienced exactly this on night ops. They did well in Vietnam as FAC spotters and for fish spotting given the better visibility over low wing conventional twins.
I would have a view from building a fairly light weight single engine plane. My thinking is to put a pusher prop up on a "pod" above the fuselage, or at the top of the (reinforced) rudder. An idea also just came to me to make this engine electric. However, it would be powered a typical engine running a (small/compact) generator. This arrangement gives flexibility to where the weight of the propulsion system is located. Particularly the prop, which could be placed in clear air above the (rear of the) fuselage. The motor/generator combination could be placed in the nose to make maintenance easier. The electrical wiring to the electric propeller motor would be easy to route, unlike a pusher prop driven by some long drive shaft.
I like the approach of your team into the development of this project, I have a feeling that behind your project there is someone with a very very mature work ethic and state of mind, no hurries at all but a firm and steady pase. Congratulations you will go far!!
Hello. A bit off topic to this latest video, however: You May wish to consider a fore and aft jig system, where you can rotate the aircraft for most efficient mechanical work / painting / installation, etc. There are automotive engine rotating stands… you guys can figure it out. Thanks for your videos! You give many of us enthusiasm for your continued successes 👍.
This was a really cool video. I know its a fictional plane, but the Sanka mk b has made me obsessed with pusher props for the last 10 or so years. The canard and wing design along with the contra rotating propellers just look awesome to me
This was an excellent overview of two common propulsion designs! Loved it.
I used to live near the final approach of a regional airport. There was a rear-engine style similar to the Gyroflug SC 01 Speed Canard that would sometimes overfly. That thing was incredibly loud and annoying pitch of sound too. I hated the sound, but it did look very cool. The noise was incredibly bad, and it was on final approach at probably 1/2-throttle at most. I hate to be on the other end of this airport where such a craft was taking off at 100% throttle!
I would love a follow-up video that describes *twin-engine* modes of puller and pusher propellers.
Because my personal fav propeller style planes the twin engine pullers like the classic Cessna 310 series. Given what was stated in the video at 3:55, the twin pullers will have reduced airframe drag since the engine are situated outboard and only cover the thin profiles of the wings. So, a twin-engine pusher would also have outboard motors so may benefit from decreased dirty air and maybe get better thrust and reduced noise too?
Another great video! Cheers
Actually the MOST efficient is to have both pusher and tractor pulling propellers together. Better known as centerline thrust. A great example of this concept was Nazi Germany's Dornier Do 335 Pfiel (Arrow) aircraft, which was Germany's fastest piston powered fighter plane of WWII. The concept is also utilized in the Cessna 337 Skymaster and the militarized Cessna 02 Skymaster. The idea actually cancels out propeller torque, thus increasing efficency. Its been proven to work quite well. Its possible to have one engine power both propellers utilizing driveshafts and gearboxes.
Yes the dornier was an awesome designs super fast but they had problems with the engine over heating on the rear engine and also needed super tall landing gear to have clearance for that huge rear prop. I also found the skymaster very interesting but apparently it wasn't quite as fast as a standard twin setup probably due to the wing configuration . The maintenance was more expensive, and had a noisier cockpit. But the cool Factor outweighs it's deficiencies.
I thought of this too. One engine driving both props. I guess if that's too much load you could reduce prop pitch or surface area so the engine isn't overloaded. The main advantage would be the balanced torque. Another might be to reverse the pitch of the rear prop on landing so the thing would stop on a dime.
Aesthetics are underrated. Tractor config looking familiar and understandable means a lot.
Keep it up Guys.
thank you for the presentation.. for me the best option is to have two small engines on the two sides left and right between the wings and the tail in a pusher or puller configuration .. two engines will make your flight safe if one failed the other one will keep you going on till you land in a safe area
A nice thorough analysis, but I'd add another factor against the pusjher; the prop shaft.
It adds weight, although I sippose a carbon-fibre tube might not weigh much. In addition, it has to be very carefully balanced and not have any flexibility or resonance frequencies with anything in the airframe. Generally solvable problems, but potential unnecessary development effort.
I had to give a chuffing laugh when I saw the Mini-IMP listed on your white board under pushers. After I got my A&P I purchased a set of plans for a Mini-Imp (many years ago when they were still available) and over a three year period got about +-80% done. It actually was a pretty cool design and the cockpit was quite comfortable. If you ever look at the plane, not how the landing gear wheels fit up into the wing (and was part of the air intake system) but folded down and gave pretty good ground clearance.
Talking about problems, one of which ended the project: accessing the engine was a nightmare, I had the cog belt reduction system manufactured twice and it wasn't made right either time but the killer was the torsional resonance of the long hollow drive shaft. The tail cone of the design just wasn't stiff enough to handle it. I could have reinforced the tail cone and added (considerable) weight in the nose to get the right CG but then the gross weight was becoming problematic because I'm a big guy and was already at the planes limits. Who wants to go flying with only half a tank of gas to not exceed the weight limit.
I tried a 3" diameter drive shaft, then a 3.75 and finally a 4" which aaaaallllmmost tamed the resonance but just not enough to be feeling safe.
I stored the plane for years then gave it to a old guy who wanted to try to see what he could do with it. I wasn't rich enough or smart enough to solve the problems. I suspect that's why there aren't more of them flying.
PS - I also bought into Jim Bede's fantasy and bought a set of BD-5 plans, even built the wings before the lack of a suitable engine killed that plane (certainly didn't have the money for a turbine like the Coors jet, haha)
Could you expand on the problem with the drive shaft/tailcone please? I would have expected all the propeller torque and any torsional vibration to be isolated from the tail cone via the thrust bearing --was it a case of tailshaft whipping or out of balance and how did it manifest (presumably during ground running --was there any 'ground resonance' via the landing gear springing involved possibly ? ( I just lost a longer post by hitting the wrong button ..#**&# -- I have had a lifelong interest in the tailpusher concept including designing building my own design "Opal" starting in 1975 - worked for Dick Schreder on sailplane design in 1974 and met Molt then (at Oshkosh) ,corresponded and went over to see him in 1990 etc etc -also met EdLesher and Jim Bede then and attended their forums . Maybe 'compare notes' on shaft drive issues etc ? I had thought that the flexidyne was a pretty good damping solution and it worked for Ed Lesher on the Teal and Nomad apparently but admittedly has not led to any significant uptake of the pusher configuration ... curious as to your experience to say the least.. (working on a current roadable using, notionally ,the flexidyne 'solution')
Apologies fot the 'crossing out' it is a quirk of my very old laptop and only happens intermittently and shows up after sending .
@@rossnolan7283 at the time of my build, flexidyne couplings were restricted because they freaked out that they were being used in aircraft and they thought it would expose them to massive liabilities. So they were only selling to established businesses. I tried going through / ordering from a couple of pump companies to acquire one but there were too many hoops to jump through. So I tried to use Gates cog (toothed) belts in a 1:1 drive to isolate the power pulses. Same 3" wide belt used to drive Roots blowers on race cars, I just used 2 on 6" toothed pulleys.
It was a long time ago, late 90's and honesty I've forgotten a lot details of what I tried.
Great vid - I'm tossing around a canard concept with a forward-swept wing with the trailing edge 2' forward of the first disc of a contra-rotating setup. NACA research says sufficient separation smooths out the airflow for the prop. It means the engine is ahead of the wing, but I'm using a liquid-cooled engine for it. I need to build a wind tunnel to test the design.
Just from a few model airplanes in my childhood - If the prop is at the back, it would be more agile, but energy would be wasted in level flight maintaining attitude. If the prop pulls the fuselage along, it's slower to respond, but with much better control.
My understanding, from before watching the video, is that pushing can cause a feedback loop of nose up/down, where as the front propeller is a negative feedback loop, dampening the pitch.
What about passenger comfort ? Pusher configuration should in theory have less noise, at least the part induced from the propeller
A spinning propeller in disturbed air creates more noise, than if were in clean air.
this thought came in my mind today morning😮
How about the efficiency of the pusher right behind the wing near the center of gravity?
A pusher like the Aircam or Seabird Seeker is a really good design. all payload, fuel, and engine is right near the wing.
Ground clearance not an issue. helicopter visibility. And can even have trigear or tailwheel. Even still get airflow over the tail for low speed tail authority, but no fuselage nor wing drag.
But a twin like the Aircam can have minimal drop in prop efficiency since not all of the airflow is disturbed, much remains laminar.
But a twin tractor also has the benefits of much reduced airframe drag due to propwash, while giving additional lift to the wings, while still getting improved visibility, ground clearance, ease of access to teh engines, etc.
lots of tradeoffs.
Thank you, @@SoloRenegade !
so well explained. im sure there must be more to it but it feels like ive really learned something lol. what do you think about planes that have both pusher and puller proppellers like the do 335? id love to hear your thoughts on that!
Inspired by the Dornier fighter, how about a plane with TWO props, one pusher and one puller. Front mounted engine drives both props. Rear prop spins opposite the front (due to an offset gear). Stable since torque is balanced out. Cooling is normal. Only thing is to have an engine with enough horsepower to power both props. Maybe if the rear prop is up high it would avoid some of the turbulence and at the same time some of the tail clearance issues.
An additional affect not mentioned is the wing lift generated by a puller, which may offset some percentage of the drag. The angle of attack of the outboard wing can be slightly lower. On the other hand, another possibility is to have a twin boom with T tail for the pusher.
Interesting comparison. I'm curious about crash survivability. CZcams is full of videos about pilots that walked away from low speed/altitude nose dives where a rear-mounted engine might have ruined their day.
Hi, DarkAero.
Thank you for an informative video. clear and concise and easy to understand. Much appreciated.
I'm no aeronautics engineer, 'justa pore, dumm bulldozer op'rator', but I do have some 'kwestyuns' if you wouldn't mind answering them.
1. The Pilatus Porter - when they switched over to a turbo-prop engine, they had to lengthen the nose by quite a bit to maintain trim due the lighter engine but it also resulted in a longer, sleeker nose - which leaves me wondering if placing the engine a little further back and running a drive shaft to a puller prop could also give a more streamlined shape, maybe with vents to trap cooling air for the engine? Again, it would likely need some modifications to cater for trim.
2. I am wondering if there is any real advantage - or disadvantage - to placing the engine and propeller on a pylon above and behind the cockpit in either a puller or pusher configuration? I suspect that this might mean some changes to other aspects of the aircraft, including the landing gear and I seem to remember seeing small aircraft with this configuration so I am really just wondering about the pros and cons of it.
3. There was some talk a couple of years ago of 'unstallable' wings that were made up of multiple aerofoil shaped 'mini-wings' placed in banks layered backwards from front to rear at varying pitches. You can find videos of them here on You Tube - I would post the links but You Tube doesn't seem to like that any more. Have you had anything to do with them and, if so, what is your take on them?
Thanks again.
Just my 0.02.
You have a wonderful day. Best wishes. Deas Plant.
I learned something today. That is a good day. Thank you! What about the engine above the cabin or midway the fuselage? The center of gravity, drag and prop efficiency would be optimized?
Although I generally agree with your arguments and your conclusion, I feel it is greatly oversimplified to think of a driven propeller like a magic layer which takes air from somewhere and throws it out the back while only influencing the air directly behind it. If you mount a pusher prop like shown you would of cause also disturb the air that flows over the wings, especially the elevators. The effect is not as drastic as with the puller prop, but every bit of air that is thrown out the back was accelerated before reaching the prop through a serious low pressure area and thus drawn in from the front. The pressure wave you mentioned for example also exist in front of the prop - just in reverse (to simplify it again).
There's also control characteristics. Eg you can keep the elevator effective at slower speeds with a bit of thrust, or even a pulse of thrust. But if it's a pusher, it will depend on where the prop is compared to the elevator.
very interesting! regarding airframe efficiency, what was the reason for choosing a conventional tail and not a T-tail used by very high performance gliders? this would also decrease prop wash on the elevator for the pull configuration.
Muito bom e muito instrutivo.
Excellent discussion. Could you do one on ducted props, and why there aren't any?
With prop efficiency decreasing due to airflow issues forward or aft of the aircraft, would you just need to consider propeller length and pitch to regain your efficiency ?
A lot of comments about noise, which is important. Not that its a huge general aviation problem, but a pusher would reduce the likelihood if flicker vertigo (not looking through the propeller). I haven't seen anyone discuss in the comments, so I thought I'd bring it up.
As an engineer I like to see how your thought process is, I like that you compare different options. I guess that the only way to know for sure whether the pusher or the puller configuration is the most efficient in your case is to try both, fluid dynamics is one of the most contra intuitive fields of physics and can yield some unexpected results.
I would also include crashworthiness. I’d rather have 200 lbs of concentrated mass leading me into the crash rather than coming in behind me.
I'm not sure about that. You could plausibly design a pusher aircraft to have a crumple zone in front of the pilot and a strong enough fuselage to keep the rear engine in place. you won't get any crumple zone in a tractor.
@@appa609 Crashworthiness is not just about crumple zone. The purpose of a crumple zone is to lessen the acceleration forces. If you hit a brick wall it doesn’t matter much which end of the airplane the engine is on. However, in a more typical crash where you hit something that has some give to it, you want the mass in the front as the heaviest mass will be hardest to decelerate. And if you hit say a tree or similar, the engine may well break if off before the cockpit passes through in a tractor. In a pusher, the cockpit will hit the tree first and likely lack the mass to break the tree. This means the cockpit will decelerate very quickly as the engine keeps pushing forward and will crush the cockpit between the tree in the engine. Ask any running back if he’d rather punch into the line behind the lineman or have the lineman behind him pushing him into the line. The physics are the same.
cite crash statistics showing pushers are less safe
@@SoloRenegade There are too few pushers to get good statistics, so I depend on physics. Show statistics or physics that show that they are more safe than tractors.
@@LTVoyager Ultralights are predominantly pushers, and have a significantly higher crash rate than larger licensed aircraft.
but the fact is, it doesn't matter how many pushers are out there. Just find me ONE NTSB report in which a pusher crashed and the engine or prop broke free and came through teh cockpit. find me ONE example of this, just one.
You depend upon physics? what physics, exactly, proves you correct? How does the engine separate from a fuselage in a crash exactly? what breaks? and how does a propeller, disconnected from the engine, pass THROUGH the engine and the airframe, to get to the cockpit while still spinning and intact? I'd like to see some physics on that.
On that note, ever notice how many aircraft crash nose first? and how the tractor engine gets shoved into the cockpit?
"Show statistics or physics that show that they are more safe than tractors."
you made the claim pushers are less safe, so it's one YOU to prove it. It is not my responsibility to prove you right for you. you made teh first claim, now defend it. it does not fall to others to prove a negative, or to prove your argument for you.
Thank u
Anyone seriously interested in this should be buying Raymer’s “Aircraft Design - A conceptual approach”, if you want a brief introduction his “Simplified airfield design for Homebuilders”.
But actually having this theory to application example worked out and explained by the Dark Aero team is immensely helpful in understanding it all.
This was very insightful. I'm curious how propeller position influences handling - does one have better responsiveness / manoeuvrability? The analogy with cars is that rear wheel driving cars have more traction during acceleration, sharper steering feel, and more balanced handling. While front wheel drive cars have a numb steering feel but are less prone to oversteer (safer).
The SAAB J-21 pusher prop fighter, the only one ever mass produced and in prolonged service, was less maneuverable than nose driven types, despitd a larger wing and lower wing loading.
Further factors: noise (big one), stone chips on the prop, exhaust gases on the prop
What was NOT mentioned is that on a "tractor" conf. the faster air of the prop wash, for the inner section of the wing that it passes over adds more lift, so the wings of a pusher may need to be a little bit longer for the same lift.
In the end it all comes down to aerodynamic quality of the propeller in relation to aerodynamic quality of the airframe:
First, the speed of the object moving thru the air has a huge impact: air resistance shoots up with speed squared. The propeller will move with a tip speed approaching sonic or trans-sonic speeds, so poor aerodynamic properties will have a profound effect on achievable thrust in relation to engine power applied. We also all should agree it's preferable to have a "clean" airframe in order to go properly fast.
But even the best propeller creates vortices which will travel along the fuselage if placed at the front, while it will be negatively affected by vortices created even by the cleanest of fuselage, wings and control surfaces if placed at the back.
You just can't win it all.
However, if you're in the situation where one of the two is "better" you'll chose to put the better one in the preferable location.
Thus, from the aerodynamic point of view it makes sense to place the propeller on a piper cub in the front while many modern military drones with extremely streamlined fuselage/wing setup have their propeller in the rear.
You could also evade all of the discussion and end up with a DO 335 just because there are some more engines lying around your workshop...😅
Also forgot that pusher config would have less sound and thus more cabin comfort
Twin boom tails can provide greater ergonomics for a pusher. The earliest planes included twin boom tails with push blades. This was useful during WW1 for mounting machine guns before synchronised guns. But the later conventional WW1 fighters became dominant for a number of reasons. Mainly better performance. With modern technologies like fly-by-wire controls, future aircraft can have more extreme designs. Now common on commercial and military Jets. And more flexible materials can produce new opportunities. The Bronco II is a good example of a twin boom pusher. The Cessna 02 Skymaster is also interesting. It can compare the efficiency of pullers and pushers on one airframe.
What about having the propeller elevated above the body like you see on some motor sail planes.
You could have also compared the different survivability of either configuration: the puller one is said to better absorb energy in a frontal impact
B-36 -- one of my favorite planes.
Wouldn't the disturbed air over the wings from the tractor prop harm lift thus requiring a higher AOA resulting in even more drag?