Aircraft propeller configuration 2

[kmpillai]

Hi friends,
What are all the merits and demerits of pusher over tractor configuration aircrafts?
Advance thanks.

KMP

 

[Felix107]

The benefits are less turbulence around the body caused by propeller disturbance at the front of the fuselage, and safety provided by a main wing that will not stall. The canard is designed to stall first and reduce the main wing angle of attack. Another benefit more lift. Both wing and canard produce lift whereas on a tractor the tailplane produces a downward force to balance the craft.
Though these benefits are real they do not always work as well as they should. Some canards have been subject to early stall when dirty, so pilots have resorted to vortex generators on the canard. The idea of reduced drag from laminar flow is great, 1) if you can maintain it with clean airfoils, and 2) if you don't end up increasing drag at the back portion of the fuselage.
Pushers, especially wide bodied, suffer from a top end speed limitation. They go just so fast and no amount of added power can speed them up. This is due to a large separated vortex around the fuselage's rear where the shape is a long, unfavorable pressure gradient. Oil traces on a Cozy showed the oil separating and flowing forward on the belly, more than two feet in front of the tail.
As a practical matter pilots don't like damaged propellers caused by gas caps, loose bolts, (anything coming loose in front) going through them.

 

[Sparweb]

Kmpillai,

Having the engine up front has a significant effect on the pitch and directional control of the aircraft. Any pilot of a typical single can attest to the change in the aircraft's behaviour when the engine is operating (sending an airstream backward that washes over the tail), and when they are flying without engine power (and the airflow over the tail doesn't contain this slipstream).

Ultimately, the key consideration in placing an engine is its WEIGHT. It makes up such a large portion of the mass of an aircraft, that its placement must not make the aircraft impossible to balance. In this regard it does affect the placement of wings, and a few designers stick with the pusher engine to solve some of the problems that they bring down on themselves when they try to design a canard aircraft.

Could we all avoid talking about canards from now on, please?

STF

 

[wilg]

The original question didn't
necessarily concern canard configuration.
One of the fastest propellor driven aircraft
of WWII was the DO 335 and it had both a pusher
and a tractor configuration. Some aircraft of
WWI were pusher types, designed that way
because no one had as of yet developed a
way of firing machine guns through the propellor arc.

 

[JohnFortier]

Wasn't the B36 a pusher design? With contra rotating props to boot! all six of them.

As I see it, the real problem with pushers in any single engine design is that, unless you want to mount the engine quite a way from the prop and connect them with a drive shaft, the engine must be at the rear of the aircraft as well.

This means that the rear of the aircraft, assuming the use of a reciprocating engine, is broader than it should be for efficient airflow.

This, however, is leading me to imagine all kinds of Rube Goldberg designs which I personally wouldn't want to test fly! -Engine in the fromt, prop in the rear and horizontal stabilizer on twin booms as in the P38 or DeHaviland Venom.

And of course, the old Aero-cobra had it the other way around; engine in the middle and prop in the front! Not the best ever fighter design.

The real question is, is it all worth it? Does any particular design provide such an advantage that a non standard layout is viable. Personally, I'm very much in favour of the 3LS design. But as Sparweb said, let's not open that canard worms again, and I guess that could extend to 3LS.

John

 

[wilg]

JohnFortier

The B-36 did not have contra-rotating props,
It was nick-named the aluminum cloud because of it's size
and slow speed. Our B-47's used to play games with
them. A favorite manuever was to pull along side,
drop the gear and then easily pull way ahead.
Sometimes it iritated them so much they would begin tracking with thier turrets, then it was time to get out of there.
Contra Rotating props were tried on planes like the
Douglas Skyshark without much success because of
gear box problems.The Contra Rotating prop was a great
idea, as it would be easy to fly, no torque to
counteract by using rudder and/or ailerons.

 

[GregLocock]

"Aluminium overcast" is the expression I remember. A good thing you didn't try to fly higher than them!



Cheers

Greg Locock

 

[JohnFortier]

You're quite right wilg, it's the XB35 that had the contra-rotating pusher props. A large, flying wing design and a competitor for the strategicv bomber contract eventually won by the B36.

Problems arose with the gear boxes, which were apparantly government issue (why did the government have a stock of contra-rotating propellor gear boxes!?), leading to the jet version of the same design, the YB49. This was basically unstable, even with the addition of four vertical stabilizers.

Given modern fly by wire systems, the B49 would have been an excellent aircraft, with a far, far smaller radar cross section than the B52, or even your B47.

Interestingly enough, the British Vulcan bomber had a cranked delta wingshape and a very small radar cross section, but since it was a pretty standard delta shape, it was stable and fast. A group in the UK is trying to restore one of these beauties to flying condition and have got quite a long way on a shoestring budget.

A bit off topic there. Sorry about that!

John

 

[bwain]

With regard to the initial question of engine placement, there is a simple thermodynamic question to be considered. Do you drag the airplane through the messy air that the prop makes, or do you drive the prop through the mess the airframe makes? It turns out that propellers, especially high powered ones, do not last long in the turbulent, buffeting airflow that often exists at thebackof an airplane during some flight segments. Nor do they survive ice shedding off the airframe for very long.

Consequently, you can't have open gear bay doors or wing flaps in the airstream ahead of a prop. Even if the prop lasts, the vibrations entering the drive train usually tear it up in time.

NASA research on laminar wings in propwash shows that the laminar flow is slightly disturbed in the region of the actual prop wake passage, but the overall drag increment is not large. Much of the laminar benefit is retained.

The Cessna push-pull twin, "Skymaster", is said to have noticeably better performance in cruise and climb when flying with the aft engine alone as compared to the forward engine only. First, note that this was an entry level twin and single-engine performance is only barely acceptable in either case; but, yes, it does a little better on the aft engine. Why? The blunt upsweep of the aft engine cowl has an impossibly high pressure recovery profile that the air cannot follow (i.e.,poor streamlining, which was done to avoid a long shaft). This causes considerable turbulence that causes high base drag when the aft prop is stopped; but with the aft prop actively pushing, the lower pressure ahead of the aft prop draws air smoothly around the cowl and eliminates most of the turbulence there.

The Skymaster does nothing to settle the question of which is best, fwd or aft props; because the bluntish aft body of the Skymaster is there only because of the aft prop. If the aft prop weren't there, the aft body would be faired to eliminate the base drag. A fair comparison would require that the airplane have a faired nose built for an aft prop performance test, and a faired aft body built for a fwd prop test. No one has done that.