Knife-Edge?

[Foxtrot Sport]

During the past few airshows I've been to, I always see at least a few performers claim that their aircraft can fly even without the wings generating lift.  They roll the plane 90 degrees, and say that the only thing holding the plane up is the vertical stabilizer (that is now horizontal mind you).

How in the world do they figure that this 'knife-edge' flight is including the wings not generating lift?  The wings are still going through air, thus creating lift, right?  Of course, one couldn't do this without the correct rudder compensation, but still, the wings still work!

Any clarification?

[Mobius]

They do, but the component of lift they create is horizontal, therefore perpendicular to the ground, and the force of gravity, so the lift generated by the wings isn't doing anything to keep the aircraft in the air.

[Brett_Henderson]

If the plane isn't turning (i.e.  if it can hold that knife-edge straight down a runway), it's not flying. It's more like a rock "flying" on momentum and hanging on the prop.

It would be losing airspeed quickly... or have a disproportionately low airspeed for that power setting... maintaining altitude by thrust and control surface deflection. That aint flying.. thats being a missile..

As the old saying goes..  A barn door will "fly" with enough, applied thrust.

[beaky]

They are only controlling their angle relative to the horizon with rudder, but the  fuselage (which is usually just a little more like an airfoil than a barn door) itself provides some lift.... I've heard more than one airshow announcer describe it that way. The vertical stab probably doesn't have enough surface area to provide enough lift. But it no doubt helps!

Knowing that, and having seen some very low-alt and low-airspeed knife-edge passes, I disagree that it's a matter of ballistics (the "missile" analogy). Thrust is a must, of course, because for level flight in any attitude lift, thrust, weight, and drag must all reach a certain equilibrium, and losing the lift component of the wings against the weight of the plane requires more thrust to re-balance the equation.
But it's true, the lift component of the wings, regardless of the attitude of the plane, is always within a few degrees of perpendicular to the top or bottom surface of the wings, and as long as they're moving through the air forwards, they will produce lift.
 in knife-edge flight, they will produce some lift (now horizontal, relative to the ground)), requiring elevator input to keep the plane on a straight line.
I'd guess that even with the nearly-symmetrical airfoils found on most aerobatic planes, the wing would produce a little more lift towards the top surface, even though it's vertical.
Haven't done it in RL (only loops and rolls), but when I do this in the sim, I find myself adding a little forward stick to stay on the "show centerline".

[Brett_Henderson]

I'll have to "re-disagree"

[Hagar]

High-performance aerobatic types like the Extra 300 & Sukhoi SU-26 have fully symmetrical aerofoil sections set at zero incidence to the thrust line so you can discount any "lift" from the wings in knife-edge. The amount of lift produced by the fuselage is debatable. These aircraft have an extemely high power-to-weight ratio. In a prolonged knife-edge pass the main lift is coming from the sheer power of the engine held at a high "angle of attack" by top rudder.

[Foxtrot Sport]

Well yes, taking into account the amount of elevator compensation, the wings are still holding the plane up aren't they?

Say when a plane is traveling straight and level, and the stick is pushed forward like it would have to be in knife-edge flight, the wings are still producing lift and flying the plane, right?

[H]

Well yes, taking into account the amount of elevator compensation, the wings are still holding the plane up aren't they?
Say when a plane is traveling straight and level, and the stick is pushed forward like it would have to be in knife-edge flight, the wings are still producing lift and flying the plane, right?

Everyone here has mentioned something that has an effect, HighFlyer. However, in knife edge flight, the wings aren't providing the lift that is keeping the plane up: the major contributed lift is actually from the propeller since that is what's providing the thrust. A  football (American type would be the best example) would stay in flight much longer with stabilizers attached (as does a feathered arrow); if it had its own power plant...
Understand that the plane is already in flight and only needs enough lift to sustain flight, not take off.

8)

[Hagar]

Say when a plane is traveling straight and level, and the stick is pushed forward like it would have to be in knife-edge flight, the wings are still producing lift and flying the plane, right?

In a word - No. You seem to have the wrong idea about this. In knife-edge the wings are vertical & producing no conventional lift at all. The elevator now acts as a rudder with the rudder used as an elevator to keep the nose up & maintain straight and level flight. This is known as "top rudder". Without power the aircraft would fall out of the sky.

[Chris_F]

Well yes, taking into account the amount of elevator compensation, the wings are still holding the plane up aren't they?

To elaborate on what Hagar said...

Airfoils (either the wings, or the rudder, or the fuselage in some cases) can only produce lift in a direction perpendicular to the wing.  "Lift" by definition is vertical relative to the earth's surface to counteract gravity.  So "lift" has to occur in an upward direction.

Now, imagine an aircraft with a big arrow pointing out of each wing.  And imagine the sun low on the horizon.  The aircraft flies in front of a giant movie screen and the sun casts a shadow of the aircraft and the two arrows on that screen.  The size of the shadow of those arrows is like the lift produced by the wings.

The plane repeats this, only at a 45 degree bank.  The shadow the arrows cast is now shorter, right?  The arrows themselves (the force the wings produce) are the same size, but the shadow is shorter because it's at an angle.  That's because part of the force the wings produce is lifting the plane, and part is turning the plane.

Now the plane flies a knife edge.  The shadows are gone.  But the force remains.  So the wings still produce force, but no lift.

[Ivan]

with 450hp up front the only limits are newton and the pilot capabilities

[Scorpiоn]

Unless you're flying a C-5A.

[SkyNoz]

hmmm, the wings take the place of the vertical stab and the Rudder becomes the elevator to keep the aircraft airborne.....

[Alphajet_Enthusiast]

Saw a knife edge pass just yesterday... Either it was the Patrouille de Suisse (you can see the same display at RIAT in a few weeks) or a Swiss or Belgian solo display can't remember.... in any case it was awesome!  :)