26
$\begingroup$

I would like to know if a prop plane, for example the P-51, could be able to fly backward. Is it possible through some trick like changing blade angle or reverse the prob rotation direction? It would have been hard for piloting, but is this theoretically possible?

$\endgroup$
  • 2
    $\begingroup$ it can't doesn't have that talent to do that, but their are planes that can actually fly backwardsArticle about a plane that can physically fly back words Also flying backwards limits the thrust of an airplane so even if it can do this it wouldn't have a lot of lift and has a high risk of stall. $\endgroup$ – Ethan Aug 20 '15 at 1:31
  • 4
    $\begingroup$ You don't need to apologize about your English a lot of other people on this website don't know much English either so no reason to apologize and your English isn't bad because I understood the whole question. $\endgroup$ – Ethan Aug 20 '15 at 1:34
  • 3
    $\begingroup$ From my Kerbal and RC flight knowledge, if your center of gravity is behind the center of pressure, your aircraft becomes extremely unstable. Normally the CG is in front, so if you rotate the direction of airflow, all things equal, bad things happen. $\endgroup$ – Nick T Aug 20 '15 at 5:20
  • 2
    $\begingroup$ The propeller would not work if rotating in the other direction. Contrary to the wings, the blades would probably stall in any case. But the pitch of the blades could be reversed without changing the rotation direction. $\endgroup$ – mins Aug 20 '15 at 7:29
  • 5
    $\begingroup$ @Ethan: The AN-2 does not fly backwards. It has a minimum forward airspeed that is less than a very strong headwind. This produces a negative (backward) ground speed not airspeed. The aircraft is still moving forwards in the air and its propellor is propelling it forwards through the air, not backwards. $\endgroup$ – RedGrittyBrick Aug 20 '15 at 9:48
41
$\begingroup$

Fly? No.
Wings generally only produce enough lift to keep an aircraft aloft when air is flowing over them in their intended direction. If you reverse the airflow over the wing (by moving backwards in the air, for example) the wing would no longer be producing the necessary lift, and the aircraft would "descend at an extremely rapid rate" until a normal airflow over the wings was restored. (That's the polite way of saying "the darn thing falls out of the sky like a rock!")

Move? Yes. At least on the ground.
This has been demonstrated on more than one occasion by Fat Albert, the C-130 that supports the Blue Angels. While the propellers themselves do not reverse, the blade pitch is changed to produce reverse thrust (slowing the aircraft the same way thrust reversers do on a jet engine, and in this case reversing its direction of travel).


Note that there is one notable exception to the "airplanes can't fly backwards" rule, which the article Ethan linked to alludes to: If the wind velocity exceeds the aircraft's stall speed it's possible for an aircraft to "fly" at its minimum airspeed, but be moving backwards relative to the ground.
This is a bit of a cheat though: The airplane still thinks it's flying forward (the relative velocity of the wind over the wings is in the "normal" direction, and the airspeed is fast enough to create enough lift to sustain flight). It just happens the the airspeed includes a headwind component sufficient to give the aircraft a net "negative" groundspeed.
The hypothetical P-51 in your question would require sustained winds of about 83 knots to make this trick work (we generally call that a hurricane), but something like a Piper Cub can do it at much more reasonable wind speeds.

$\endgroup$
  • 3
    $\begingroup$ Plenty of turboprops have the same ability to reverse blade pitch, reversing thrust. It's a requirement for the larger turboprops using L'Esperance on Saint Martin; there's no taxiway entrance at the threshold on either end and no room to turn, so aircraft that need the full runway to takeoff must powerback from the entrance to the threshold. $\endgroup$ – KeithS Aug 20 '15 at 3:05
  • 8
    $\begingroup$ @KeithS Most turboprops can do it (at least the multis) - Fat Albert just does it with style :-) $\endgroup$ – voretaq7 Aug 20 '15 at 3:37
  • 1
    $\begingroup$ @voretaq7 I guess an alternative question might be: Is it possible to design an aerofoil that can provide lift efficiently in both forward & reverse flow directions? $\endgroup$ – curious_cat Aug 20 '15 at 4:58
  • 4
    $\begingroup$ @curious_cat: That is a good question. Go ahead and post it ;-). $\endgroup$ – Jan Hudec Aug 20 '15 at 6:11
  • 4
    $\begingroup$ +1 for the cheat. The number of times I have to explain to people that the aircraft might be moving backwards relative to the ground but is still flying forwards. $\endgroup$ – Simon Aug 20 '15 at 6:23
18
$\begingroup$

In short, no.

First, the wing of an aircraft is designed to produce lift in only one direction. Airflow moving backwards over the airfoil would not be directed around it properly; the air coming across what's supposed to be the trailing edge would be divided too cleanly (so it could stall too easily) and wouldn't be accelerated as quickly by the gentler slope on what's supposed to be the back side of the wing, thus reducing lift. The leading edge, now the trailing edge, would increase drag and further reduce lift as the boundary layer would separate too soon along its curve. In other words, a wing moving backward produces very little lift and much more drag, both bad for an airplane trying to stay in the air.

In addition, most propeller-driven aircraft have their wing chords angled slightly upward from the thrust vector of the engine, which provides a nonzero angle of attack in level flight. This provides more lift at the cost of slightly higher drag, and allows a plane to maintain altitude more easily at cruising speeds with the nose level. In reverse "flight", this would end up a negative angle of attack, reducing lift even further.

Lastly, the horizontal stabilizer is designed to provide downforce in forward flight to counteract a slightly forward center of gravity (this basic design causes desirable stall behavior, causing the plane to nose down to restore normal airflow). This is accomplished in low wings with a slight downward angle of the horizontal stabilizer (or a slight up-angle to canards), and in high-wings by using the downwash from the wing to push on the tail. Moving backward, there's no downwash to balance the weight at the nose, and a downward cant would actively push the tail up as the wind moved past it, in either case flipping the plane into a nose-down attitude (also a desirable recovery behavior if you find yourself hanging by your prop).

In a stellar engineering choice by aircraft designers, they orient the curve of the wings and trim the horizontal stabilizers to produce lift and balancing trim force when the aircraft is moving in the direction its occupant(s) would call "forward", i.e. the direction the pilot's seat is facing.

There are a few aircraft, notably late Soviet designs like the MiG-29 and Su-27, that were engineered for desirable "post-stall" behavior. These aircraft are capable of remaining stable and controllable in extreme angles of attack (exceeding 90° off-chord) and are the best examples of an aircraft that can "fly backward", at least for a couple of seconds. The maneuvers involved include the tailslide (pull in to the vertical, stall nose-up, and fall back to earth tail-first, then pull back on the stick to kick your tail out behind you and drop the nose to recover) and the cobra (from full throttle, cut the engine and pitch up hard to intentionally stall the aircraft and rotate nose-up, then center the stick to allow the plane to nose-down). Most counterpart U.S. aircraft are incapable of these maneuvers as they're engineered to avoid stalling, following the "energy management" theory of Western combat maneuvering (where stalling, regardless of airspeed, means you're out of energy to maneuver, as you either have insufficient forward airspeed to maintain your turn, or you've just turned your plane's wings into air brakes).

$\endgroup$
  • 2
    $\begingroup$ The MiG-29 and the Su-27's post-stall behavior doesn't quite count as "flying backwards" so much as it "falling tail-first with some control effectiveness" (so much like being blown backward by a strong headwind I would say it probably doesn't count in the spirit of the question, since TBBT asked about using reverse thrust & presumably sustained flight). It is however impressive to watch! $\endgroup$ – voretaq7 Aug 20 '15 at 6:14
  • $\begingroup$ In WW I, some German pilots perfected a technique to fly briefly backwards by stepping hard on one rudder pedal. The low directional stability and the impulse from the initial push would spin the aircraft through 360° around the vertical axis. But like the Su-27, this is less "flying" than a ballistic movement. $\endgroup$ – Peter Kämpf Aug 20 '15 at 6:25
  • 1
    $\begingroup$ The first point of this answer is based on a naive (incorrect) assumptions about how wings work. Read What really allows airplanes to fly?. It is in fact very possible, for example to fly a plane with a symmetric aerofoil. $\endgroup$ – Nathan Cooper Aug 20 '15 at 8:45
  • $\begingroup$ @NathanCooper: You both talk of different directions. While Keith means the direction in the plane of the wing, your direction is up or down. $\endgroup$ – Peter Kämpf Aug 20 '15 at 12:21
6
$\begingroup$

When, as a youngster, I made model planes, I tried this. The result is very unstable and usually the tail lifts on launch causing the aircraft to flip over. I think that, with the use of computer control to counteract the inherent instability and a specially made propeller it might be possible but the development time and expense means that no-one would ever try to do it..

The problem is that the tail assembly of a standard configuration acts like a weather vane. It naturally wants to turn away from the direction of airflow.

$\endgroup$
6
$\begingroup$

In theory, yes, inefficient, very very unstable, with the control surfaces facing the airflow rather than behind it so a big risk of the airflow snatching the controls and forcing them over to full travel - with very bad results. The aeroplane would want to flick round and fly in the opposite direction because of the way its designed to fly forwards.

$\endgroup$
  • 1
    $\begingroup$ +1 for the theoretical 'yes'. The OP didn't specify theory vs practicality and you are the only answer that went there. $\endgroup$ – Steve H Aug 21 '15 at 1:36
4
$\begingroup$

There is a relation between the thrust provided by the engine and the lift created by the profile of the wings. Usually this relation is not the same backwards, so it will not fly backwards.

Propeller planes like the P-51 has an asymmetric aerodynamic profile that cannot provide the same lift moving backwards.

There is some aerodynamic profiles that are near to be symmetric, but requires a power plant much greater than a propeller. Usually this power plant cannot provide the same thrust backwards than forward.

$\endgroup$
  • 1
    $\begingroup$ It might be worth noting that since the propellor itself is an airfoil, it also won't produce nearly as much thrust when put in reverse. So that would also impede the ability of the aircraft to fly backwards. $\endgroup$ – Jay Carr Aug 20 '15 at 2:08
  • $\begingroup$ @JayCarr: It is the twist, not the airfoil, which makes the propeller so bad in reverse. The direction of airflow over the blades does not change, but the local incidence will be wrong over much of the blade span. $\endgroup$ – Peter Kämpf Aug 20 '15 at 6:20
  • $\begingroup$ @JayCarr: Propellers are reversed by changing the blade pitch, not by turning them in the opposite direction. $\endgroup$ – Jan Hudec Aug 20 '15 at 6:22
  • $\begingroup$ @JanHudec Could the P-51 do that? Eh, no matter, the P-51 was just an example and you're right of course, so... There you go. In my head I was thinking of planes with fixed props for some reason. $\endgroup$ – Jay Carr Aug 20 '15 at 12:43
  • 2
    $\begingroup$ @JayCarr, no, I believe Peter was talking about twist because twist is why changing the pitch won't give you the same efficiency in reverse as you have forward. Because the twist is so that the pitch is higher in the middle where speed is lower, but when you go to negative pitch, the pitch will be lower (probably still positive) in the middle, so only the tips will produce thrust in the beta range. $\endgroup$ – Jan Hudec Aug 20 '15 at 13:14
3
$\begingroup$

If reverse thrust was generated (you couldn't do this simply by reversing the prop) then a plane could fly backwards.

While the wings are shaped to generate lift as they pass through the air in the forward direction, by using angle of attack and enough thrust you can generate lift in any direction. Angle of attack changes the apparent profile of the wing. The same principle allows planes to fly upside down or sideways in knife edge. The simplest example is how a paper plane can still glide with completely flat wings.

$\endgroup$
  • $\begingroup$ Please add for long you could fly backwards, given that all control surfaces will hard over into their stops and stability is negative around all axes. $\endgroup$ – Peter Kämpf Aug 20 '15 at 11:44
  • $\begingroup$ Why would they be? Lift is still generated across the whole surface. Stability would be low, but the question was is it possible, not is it easy. :) $\endgroup$ – JamesRyan Aug 20 '15 at 11:50
  • $\begingroup$ When the cg was ahead of the neutral point, it will be aft of it in reverse flight. The consequence is instability, proportional to the stability in normal flight. Remember that the neutral point is at the quarter chord, measured in flow direction. $\endgroup$ – Peter Kämpf Aug 20 '15 at 12:17
  • $\begingroup$ @PeterKämpf I think the cg is close enough, it works on a model at least. But that is beside the point, my answer is mainly to highlight that the common misconception that wings only work because of their shape (and therefore only in one direction) is false. $\endgroup$ – JamesRyan Aug 20 '15 at 14:12
2
$\begingroup$

No.

Try riding your bicycle backwards down a steep hill if you are not sure what other writers here mean by passive pitch stability and yaw stability.

The reason why the big tail and elevators are well behind the middle of the wings is to make sure that when flying forward, they will tend to straighten any wobble.

Go to the pub and have a look at how darts fly if you want. You can try throwing one of those backwards too.

$\endgroup$
  • $\begingroup$ This assumes that the bicycle and dart analogy extend to airplanes, so it is begging the question. $\endgroup$ – Vortico Aug 21 '15 at 5:31
2
$\begingroup$

You don't need reverse thrust to do this. Just pull the nose way up, and let the speed bleed off until your vertical speed is zero. Then you tailslide. Most planes are designed to make this not very easy to do, but if you move the CG far enough back, by putting bricks in the tail, you can do it.

The trouble with that is, unless your plane is specially made, and you are specially trained, you will have a very difficult time not just sliding all the way to the ground (and voiding the warranty).

$\endgroup$
  • $\begingroup$ It's actually called a tailside and is a standard aerobatic maneuver $\endgroup$ – rbp Aug 20 '15 at 23:27
  • $\begingroup$ @rbp: corrected. $\endgroup$ – Mike Dunlavey Aug 21 '15 at 0:53

protected by Jay Carr Sep 4 '15 at 15:55

Thank you for your interest in this question. Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).

Would you like to answer one of these unanswered questions instead?

Not the answer you're looking for? Browse other questions tagged or ask your own question.