Push - Pull propellers revisited

[Tom Harper]

An inline twin engine model with one tractor and one pusher propeller has numerous advantages. But, posts on this forum say it is the worst possible layout for thrust. When I lean back and consider it, I come to the same conclusion. The pusher prop operates in a headwind that decreases the blade angle. Looks like only the tractor contributes most of the thrust.

However, many designers have used this configuration successfully. The Donier 335 and the Cessna O-2A proved the value of the concept. So, what's the problem with our intuition? Do you pay an acceptable efficiency price for the tandem set up? Does it provide the same thrust as two props side by side? Or, are the Donier and Cessna really bad airplanes?

[Brandano]

Just use a coarser pitch on the rear prop.
The advantages are in the one engine out behavior, and in the fact you can have counter-rotating props without the need for different motors.

[macboffin]

Tom, both props are operating in a headwind! Just will be a bit quicker for the rear than the front one.Which pitch change can handle if significant.
That change would be greater for something like the 60 ft long German heavy fighter than for, say, the Dornier "Wal" with two engines virtually back to back. Twenties and thirties large aircraft frequently had "back to back" pairs of engines, although that was mainly because they didn't have single engines with high enough power ratings. There are advantages for planes with a prop each end , one being reduced drag, the rear prop isn't blowing turbulent air, (at a faster speed than the aircraft speed, remember) down the length of the fuselage and over the wing roots and tail group. (The Cessna P-P was 32 knots faster on rear engine only than on front only, and could reach 4000 ft higher on rear only than front only.) Torque forces balance out, and the spiral flow component which wants rudder deflection to cancel out at high power settings at low speeds, (ask any warbird pilot about that!) is also lower. Another advantage over a single bigger engine is shorter lighter undercarriage, since two smaller props instead of one big one. There is also the real advantage for certain duration/range orientated designs of using both engines for T/O/ climb, and cruising for range duration on the ( lower drag ) rear engine only. That worked pretty well for Rutan's non-stop round the world flight, and was used between the Wars for Dornier's ( and others) long range seaplanes and flying boats.

[macboffin]

Another example in the same vein.

[Tom Harper]

Macboffin,

Thanks!

I notice that in your tandem designs you place the rear wing below the front wing. Urban legend would have it that downwash makes that a bad arrangement. I doesn't seem to bother your designs.

[macboffin]

Urban legend says a lot ; It also says that biplanes work better if the top wing is forward of the bottom wing. So stretch the biplane a bit more between the wings, dihedral the rear and sweep it forwards, anhedral the front and sweep it back, so the tips meet in the same plane, (then you have monoplane drag, biplane lift) . Flaps on both wings, (a crank in the rear wing enables a longer front flap without flap tip flow vortex upsetting the aileron flow when the flaps are deflected.best tailplane type for pitch/yaw control, a Vee at 90% included angle.( DARPA and NASA tests costing over $1.2 mill confirm all that.) Each wing struts the other, so a lighter airframe (less material) for the same area/loadings. The combination goes to a higher angle of attack before stall than a monoplane or biplane, so higherC/L, and since the overall size is only 60% of a conventional configuration for same area/ load it makes a more difficult target, and is easier to transport and hangar. My first patent was in 1969 since when the configuration has generated many millions of dollars ; mostly applied to surveillance and other UAVs.Not saying that to boast, but to show this is not another crackpot theory.

[Brandano]

Urban legend occasionally gets things wrong too: http://en.wikipedia.org/wiki/Beechcraft_Staggerwing
In any case keeping the wing outside the turbulent flow of the stalled stabilizer improves stall recovery

[MarkusN]

I notice that in your tandem designs you place the rear wing below the front wing. Urban legend would have it that downwash makes that a bad arrangement. I doesn't seem to bother your designs.
Downwash is a field. It extends well above and below the wing causing it. You usually cannot escape it by changing elevation of the trailing wing.

BTW, there's also upwash in front of a wing. Nobody seems to be concerned about that...

[macboffin]

Some are concerned. As has often been said, an aeroplane is a bundle of compromises flying in close formation. One therefore considers the whole bundle and the various interacting flows around that bundle, and their changes with speed, air density, temperature, humidity, transient and longer lasting effects of surface changes of airfoils, effects of engine vibrations on flow attachments, Reynold's numbers, comparisons of wind tunnel results with actual flight testing , etc etc etc.........
Some merely hack out some foam and tape on motor, elevons etc, servos, tape wiring to surfaces, and "suck it and see". After modifications etc this may result in a reasonable flying device which will cavort around the sky while the battery lasts. This is not design from first priciples, this is practical empirical experimentation. Frequently effective, especially if starting from an example or by making a similar shape. NOT a good way to go for a program aimed at resulting in an aerial device costing a lot of money, (often tax money at the end of the day!) and a lot of time.

[macboffin]

Urban legend occasionally gets things wrong too: http://en.wikipedia.org/wiki/Beechcraft_Staggerwing
In any case keeping the wing outside the turbulent flow of the stalled stabilizer improves stall recovery Are you talking about a Canard type here? Would have to be if stalled stabilizer could put turbulent flow over main wing?
(Submitted tongue in cheek!)

[macboffin]

Reference the Beech "Staggerwing" I had always understood that the primary reason for the forwards low wing was to house the retractable main gear. Where else could it have gone?