# Then, Biplanes Can't Fly Upside Down!

I read a good argument that, by the common dogma on lift, a bi-plane should not be able to create lift when flying upside down…because the so-common wing profile we always see would be inverted, and lift would not be induced.

But, do bi-planes fly for long periods upside down, or is it “falling with style” as Woody (in Toy Story) would say! - Jinx

The only major problem with flying upside down is getting fuel to the engine.

Excellent previous thread on the subject:

http://boards.straightdope.com/sdmb/showthread.php?t=310889&highlight=flight+upside+engine+lift

Basically anything can fly with enough thrust, regardless of lift.

So, if a biplane can’t fly upside down it’s an engine issue, not a biplane one.

Short summary: the Bernouli effect (when faster moving air has less pressure) is not the chief reason airplaces can fly. Most of the lift is the result of air being defelcted downward of fthe tail end of the wing. Newton’s third law (or is it first?) then tells us that every action has an equal and opposite reaction, so the air, in turn, pushes back up on the wings.

Therefore, many planes can fly upside-down for long periods of time, because the shape of the wing still allows lots of air to be defelcted towards the ground.

Why would this problem be limited to biplanes? Biplanes use the same kind of wing sections used in single-wing planes, but they use two of them to double the span. They still generate lift by putting an airfoil at angle of attack. When flying normally, the angle of attack is set by the way the wing is mounted on the fuselage and the way the flaps are set. When inverted, these mechanisms don’t work (the fuselage mount provides negative angle of attack) but the control surfaces can be used to put the entire fuselage at an angle of attack so the wings still create lift. I’m not a pilot, so I don’t know the proper control settings, but I don’t see any aerodynamic reason you couldn’t do it. The main problem with doing this in an old biplane might be that (if) they were underpowered to begin with, they might not have the power to sustain the inefficient flight while inverted, but that has little to do with the aerodynamics.

But you can use gravity for thrust - no need for an engine (gliders do this regularly). Assuming still (as opposed to rising) air, you need the engine in order to maintain or gain altitude.

If you look at a lot of ultralight wings many of them are only the top side of the so-called airfoil shape. IOW they are definitely flying by the “barn door” principle…

It wouldn’t - it’s the same story with a monoplane.

Well, they actually use two wings to (more or less) double the wing area without increasing span.

No, it’s pretty much always set by the elevator, both in normal and inverted flight.

Biplanes can’t fly inverted? This will come as news to:

Rob Holland
Sean D. Tucker
Sean D. Tucker (He does it a lot!)
Jeff Dawson
Kirk Dawson (Do you suppose they’re related?)
This guy (flying a model biplane!)
The U.S. Post Office

Depending on the engine type, the type of oil pump is also an issue for maintaining inverted flight. If the oil is pumped from the sump to the top of the engine and expects gravity to pull it down through the engine inverted flight is an issue. Cite? Vague memories from reading Flight magazine or flight manuals from 16 years ago.

Staff report and discussion from This Thread

Oil is a problem Most radial engines have a small sump at the bottom. This sump is continually scavenged by a pump that returns the oil to a tank where the oil pressure pump picks it up and supplies oil to the engine. When the engine is inverted the oil isn’t returned to the sump and is not scavenged. Furthermore the oil pump in most engines is at the bottom of the storage tank and thus will not pick up oil to lubricate the engine. Fortunately, carburated engines starve for fuel almost immediately and so the lack of lubrication isn’t all that serious. And unless the pilot has trained a lot for it and is in good physical condition he, or she, also has systems that don’t work all that well inverted.

But the airplane, bi-, tri-, or monoplane flies just fine.

I have spent a bit of time flying upside down in a biplane. It was a Pitts Special, had a symetrical aerofoil, a fuel tank with two pick up points (one top and one bottom), and an inverted oil system.

The fuel went to an engine driven pump and was injected into the engine, so that solves the fuel delivery problems. The oil was picked up either from the top or bottom of the engine (wet sump) and had a simple mechanism consisting of a couple of balls that floated in a junction in the oil hoses. When flying upside down they would drop in such a way that would allow the oil to flow from the top of the engine to the oil pump. When flying the right way up, they would drop the other way.

It’s been a while, but I recall that inverted flight was not limited in any way. The only flaw in the system is that it wouldn’t handle knife edge flight (that’s the aircraft flying on its side) very well. I think it was time limited to about 30 seconds.

All things being equal, the symmetrical airfoil would allow it to fly upside down just as well as it did the right way up. All things are not equal though and in inverted flight it suffered from more adverse yaw requiring more rudder input in turns. It also had more drag inverted and flew slower.

Yeah, if nothing else, you could just push the stick down far enough that the elevator will correct for any inverted lift, depending on various other factors of the plane’s design.

If the plane uses gravity-feed fuel lines (ie: the gas tanks are on the top, and gravity pulls fuel down to the engine) then you won’t be able to fly inverted for long before you’ll run out of gas with full fuel tanks. Of course, this would apply to any fluids in the engine (fuel, oil, coolant) that might rely on gravity to circulate through the engine. If pumps are used to circulate the fluids, then this isn’t much of an issue.

I have heard, in a couple of places, that some helos can fly inverted. Is this true? BS?

Simply put, the primary principal involved in keeping the plane elevated is the angle of attack.

Think about when you were a kid and you stuck your hand out the car window at highway speed. Angle your palm in the finger-up position…hand goes up. Fingers down, hand goes down.

There are certainly helicopters that are able to do aerobatic manoeuvres involving positive g inverted flight, e.g., loops and rolls. I have seen a BK117 do a loop. I’m not sure if any are able to fly straight and level inverted, i.e., with negative g.

Third Law.

Second Law: “Force equals mass times acceleration”.

First Law: “An object changes velocity only if something makes it.”

Loops and some kinds of rolls may not be the same as invereted flight as it’s possible to keep positive G loading though the maneuver. I’ve seen a film clip the interior of Bob Hoover’s Shrike Commander during a barrel roll with a glass of water in the cockpit in a fixed holder. The water never sloshes or spills despite clearly seeing the plane do a 360º roll relative to the ground shown through the windscreen. This is the same type of roll that Tex Johnston did with the Dash 80, the 707 prototype.

The question was rhetorical.

Semantics. The point is they have about the same lifting surface (i.e. span) without having to have the structural support for a wing twice as long.

I deal mostly with theory and wind tunnels, but in every real aircraft I recall examining, the wing was mounted on the fuselage with a positive angle of attack. Sometimes it’s subtle because it’s caused by the assymmetrical airfoil design, but it still has a positive angle which makes inverted flight less efficient. Obviously designs differ and planes which are intended to fly inverted will minimize or eliminate this. Of course the various control surfaces can be used to vary the angle and, especially in the case of flaps, their effect may be much greater than the angle caused by the fixed design, but that doesn’t eliminate the fact that there is a permanent positive angle of attack in the basic construction that must be offset when flying inverted. That is the point I was trying to make.