Thanks for all the responses! So, I get that the pusher configuration allows for greater lift because the airflow over the wings is not disrupted… and that the B-36 was able to use it because it had a lot of fuselage forward of the wings… I’m still not clear on why it (the pusher configuration) didn’t become the preferred design for multi-engined aircraft though.
The reason why it doesn’t work so well for single-engined applications was pretty obvious once I thought about how many times I’ve seen R/C aircraft scraping their propellers on takeoff (and crashing immediately afterward).
Well, I can think of a couple reasons right off the top of my head:
One, maintaining those engines, buried in the B-36’s wing as they are, had to be a real pain.
Two, the B-36 was about the last major propeller-driven multi-engine airplane. It was replaced by the B-47 and B-52 (and on the commercial airplane side, the 707 came along) which were jet-propelled; and on a jet airplane, it makes a lot of sense to hang the engines way out in front of the wing, where they can be used to counteract wing flexing.
It’s not like the engine would need to be buried any more deeply if you flipped it around, though; on the contrary, since the trailing edge of the wing is thinner, there would be less wing blocking access.
I think that suggests another reason, though - if the engine cowling is in the rear of the wing, you can’t put control surfaces there.
As mentioned upthread, the pusher configuration cuts drag significantly in a large plane. This was crucial for the B-36, which pushed the available technology to the absolute limit for maximum range with a heavy load above all other considerations. In this it succeeded. The price to be paid was heavy wear and maintainance on the engines and props. The air intakes on the B-36 were much more prone to icing up than comparable tractor configuration planes, and the props took a heavy beating because they continually cycled through the vortex between the upper and lower surfaces of the wings. It’s worth noting that there is not a single B-36 left in flying condition today, despite the interest of restorers. And as a footnote, several modifcations were tried with the B-36, including the YB-36C, which moved the props to tractor position.
No, the clean airflow over the wings reduces drag, but it doesn’t increase lift; a tractor prop downwash over the wing generates more lift, since the downwash is faster than the undisturbed airflow, is as if the plane would be flying faster than it really is.
THE B-36 (to the tune of Battle Hymn of the Republic)
The B-36 it flies at 40,000 feet
The B-36 it flies at 40,000 feet
The B-36 it flies at 40,000 feet
But it only drops a teensey, weensey bomb
Tons and tons of ammunition
Tons and tons of ammunition
Tons and tons of ammunition
But it only drops a teensey, weensey bomb
Ask this: When an airplane is in flight, wouldn’t the air passing under the wing negate that effect?
The propwash is more or less equal on top of the wing and on the bottom. (Lots of variables such as engine mounting position, angle, etc.) So it’s more or less like a wing in flight, except that the propwash is over just part of the wing instead of the whole wing. Also, wings have a positive angle of incidence; so you get some lift from plain old Newtonian deflection.
One obvious advantage is that in the event of a “mid-air collision with a planet”, the engine - possibly the single most expensive component of the model - isn’t as prone to damage. (My Electrafun had another problem, though; if a bad landing grounded the wingtip hard enough to push the wing out of place, the trailing edge would get chewed by the prop.)
Not a “vortex” as such but different pressure field, and not just the props but the drive shafts too. Otherwise, that’s the answer - the wings were more efficient, but the props less so, and the system was unreliable due to heavy vibratory loading inducing fatigue cracking that the technology of the time was not up to controlling. I understand it was constantly prone to losing prop blades and even entire props from the problem, and that at least one plane was almost lost due to losing all 6 engines at different times on a flight - only the 4 jets used for takeoff thrust (on the later “six turning and four burning” models) kept the plane aloft.
Plus, the prop noise created by the 2/rev pressure pulsing meant you could hear the plane from 20 miles away. You can get some feel of that just from the flying scenes in"Strategic Air Command" with Jimmy Stewart (outrageous propaganda, sure, but where else can you see B-36 and B-47 footage like that?).
This is what I though in my first post. And then I read Santo Rugger’s post and it seemed to say that mounting the engine behind is more efficient. So I went back and re-read it:
So it’s the wings that are more efficient, and not the props; right? As I said, I thought the props were less efficient. (And noisy, as you point out.)
Right - the wings are more efficient, the props are less efficient. The wings get generally less turbulent airflow. But the prop blades can’t be at their most efficient angle throughout a revolution because of the different pressure and velocity fields they encounter.
My memory of larger pushers is that one of the biggest drawbacks is overheating. When you stick the engine out in front of the wing and build a fan to keep air going across it in all conditions, it is much easier to cool the engine than when you bury it inside a wing and hope that the air sucked in by any scoops is always sufficient. Yanks used air-cooled engines in bombers and transports almost exclusively. The Brits and Germans used a variety of engines including a lot of water-cooled ones, but even water cooled engines did better when they were exposed to air (given the technology of the times.)
As noted, earlier, maintenance on a buried engine was a lot more work, as well.
Which makes sense, as it was an attempt to buld a cargo plane out of the original B-36 design.
Some airplanes have “contra-rotating” engines & other do not. Generally, larger aircraft do not. A tally by type would probably be 80+% all-same & 20-% contra.
There isn’t exactly a gyroscope effect, but there is an effect called “P-factor”, which causes a prop to develop a torque which tends to pull the nose left or right.
On small airplanes which are relatively under-powered, an engine failure on the side where the assymetrical thrust is augmented by P-factor can be much more difficult than a failure on the other side where the two forces partially cancel out. This is the so called “critical engine” effect
Building the plane so both sides cancel out & neither is “critical” is the main goal of contra-rotating engines.
Piston-powered airplanes generally do not have a gearbox between engine & prop, so the only way to make the prop turn teh opposite way is to build the engine to turn the opposite way. This leads to lots of extra costs, left- or right- handed parts to be stocked, etc. So there are good non- aero engineering reasons that contra has always been a bit of a niche.
Turboprops usually have a gearbox between engine & prop, so in theory a reversing gear could be added to one side to make the prop rotate opposite the other without needing to make the engine turn opposite. But even then, this is still pretty rarely done.
Another approach sometimes used is axial contra-rotating propellors, where you have two props on the same engine, one ahead of the other, and each rotating in opposite directions. The engine still produces some net torque & p-factor, but much less. The mechanical complexity of this is huge, as are the vibration & efficiency problems.
Contrarotating props are uncommon in twins for the reasons LSLGuy states - the difference in yaw with one out just isn’t all that much in most of them. In fact, the only exception I can think of was the P-38, which had so much power that it really did matter. It even had to have the engines switched to rotate tops-outboard, creating even more yaw moment with one out, to get rid of a tail flutter problem. It didn’t have a reversing gear, though - one engine had a mirror-image crankshaft and other parts so that it ran counterclockwise.
The EADS A400 (looks like a blown-up C-130), if it ever flies, will have the outboard engines turn top-inboard and the inboard engines top-outboard, for reasons they must consider significant but are not obvious to outside observers.