# Could propeller wash provide sufficient lift to take off - even in theory?

I know this seems a crazy thing to ask but it is not the same as lifting oneself by the bootstraps. If you try to lift yourself that way, there are two equal and opposite forces, yet there are experiments with muscle powered planes that work.

So, provided the plane was stabilised somehow, and the wings were designed for the purpose, could propellor wash theoretically lift the plane? If not why not?

Note

I understand that a normal plane would start to move forward. Please assume that the plane is restrained from the rear by a cable and that the experiment takes place in the absence of wind.

• It certainly could, think of a wind tunnel. However I guess the propeller would have to have the same span as the wings. so it's not very practical. Aug 31, 2020 at 20:09
• Related Aug 31, 2020 at 20:16
• That’s the whole idea behind Channel Wings. Sep 1, 2020 at 7:51
• gyros fly more or less this way. Sep 1, 2020 at 10:12
• muscle powered planes - don't need experiments; birds have been proving it works for longer than humans have existed Sep 2, 2020 at 12:01

Yes. The wing doesn't care what is causing air to flow past it. Headwind, propwash, 747 wake, gopher sneezes.

If the airplane is restrained from forward motion, and the propwash over most of the wing is fast enough, let's say more than the airplane's stall speed, then the wing must generate enough lift to support the airplane, causing it to take off.

It's not so much a matter of designing the wing specially, as making the airplane tolerate the many large props needed, and the weight of the engines. That much power is too much for efficient cruise. But it's easy to implement with a one pound model airplane.

• (+1) The AoA must also be factored in: airflow faster than stall speed isn't sufficient to ensure liftoff. Sep 1, 2020 at 8:31
• Follow-on question: how many gophers sneezing in sync would be required to make a 747 lift off? Sep 1, 2020 at 8:46
• Human sneezes clock in at about 4.5 m/s. This article reports that ordinary breathing could power about a microwatt. Unfortunately I have no idea how to convert that into the number of human sneezes necessary, let alone gopher sneezes. Maybe this should be a new question. Sep 1, 2020 at 13:49
• So human breathing, about 4 sec/breath, is a microwatt? Sustained sneezing is about as fast. Gophers breathe 3x faster, but their lungs are 300x smaller. So one steadily sneezing gopher yields 10 nanowatts. A 747 launch needs 90 megawatts, or 900 quadrillion sneezing gophers, covering a square about a thousand miles on a side. Sep 1, 2020 at 15:55
• @CamilleGoudeseune Man... Not even a mole of gophers. Sep 1, 2020 at 17:22

In principle yes, but why would you do it? For vertical take-off this would be grossly inefficient.

Lift is produced by deflecting air downwards.

This becomes easier as more air is available for deflection since the amount of deflection needed for a given lift can be reduced. However, when all air movement is provided by the propeller, why reduce efficiency by blowing it horizontally over a wing, where it creates friction? Wouldn't it be better to direct this airflow directly downwards so no further deflection is needed?

Any deflection will incur losses, so taking the detour over the wing complicates things needlessly. That does not mean this hasn't been tried –as qq jkztd points out in the comments, the Ryan VZ-3 used this concept, albeit with poor results.

Ryan 92 VZ-3, rebuilt after the crash (picture source).

For a theoretical approach the propeller diameter would need to be as large as the half span of the aircraft, with both propellers mounted at mid span. Now the landing gear also needs to be longer than quarter span in order to let them spin freely. Next, the wing needs flaps which are able to deflect the flow of air by 90° which will involve some active blowing. This becomes quite complex quickly, so a design like the one below looks more promising:

Vertol Model 76 VZ-2 (picture source) was the first tilt wing design which successfully transitioned between vertical and horizontal flight and was built for the same purpose as the VZ-3. While not any more visually appealing than the VZ-3, it had a longer career: It flew first (in 1957 as opposed to 1959 for the VZ-3) and was used until 1965 while the VZ-3 was retired in 1961.

• It has been tried, it was Ryan VZ-3 vertiplane Sep 1, 2020 at 5:29
• Interestingly - thrust per power increases with rotor area - whether a single rotor or N smaller rotors. Hence the large rotor area on anything that wants to utilise raw rotor lift. Sep 1, 2020 at 12:05
• @qqjkztd: Thank you for the information! Admittedly, the VZ-2 was a more practical design and better at vertical take-offs. The VZ-3 would always create some forward thrust so a purely vertical take-off would be impossible. Sep 1, 2020 at 20:30

Restricting the question to approximately horizontal airflow ... it's been tried.

The Custer Channel Wing was one that tried ... it didn't quite make it, but was claimed to be able to fly at 8 to 11 mph. That may have been optimistic but the CCW5 (pictured : image from linked Wiki) apparently flew as slow as 35mph.

And it keeps being tried... this time in conjunction with the Coanda Effect ... results not reported so far.

• WOW - what a great answer, best here Sep 1, 2020 at 16:17

Absolutely. Here's what it looks like:

It's a turbojet here (this is an F-35 engine) but from a physics standpoint there's no reason you couldn't do it with a propeller.

So long as you have some means of directing a sufficient portion of the thrust downward, you can lift off. An airfoil would also redirect the thrust slightly downward, but far less efficiently than a nozzle.

This wouldn't be even remotely practical with a normal engine and airfoil. The physics works though, if your engine can produce enough thrust to get the required downward component.

• @fectin I understand the difference, and I think you missed the point. Lift is a form of thrust in the sense that both are forces. The question proposes a scenario where an airfoil converts horizontal-ish engine thrust into lifting thrust. The pictured design is a much more efficient way to do that. But the physics, from a standpoint of arrangement of forces, is exactly the same. Sep 1, 2020 at 14:39
• @fectin Thanks, those are great examples - that's actually almost precisely the point I was hoping to make for the OP: thinking about it in that way makes it easy to come up with scenarios one can use to prove to themselves that it's possible. Sep 1, 2020 at 15:13
• My mistake: that last comment does help! I understand your point better now, but it didn't come through in your answer. Sep 1, 2020 at 15:16
• While a turbofan not a turbojet, the F35 is actually not an example (or at least little more than a helicopter is), as it incorporates a vertical-axis shaft driven lift fan front of the engine, in addition to vectoring the flow out the back. The example you probably want is the Kestrel/Harrier system which is a (multiply) thrust vectored turbofan. Sep 2, 2020 at 13:22

Presumably you are referring to using wings fixed to the airframe with props driving air over them.

If instead you move the wing through the air to create airflow over it you "cut out the middleman" and end up with a helicopter or typical multirotor craft.

Similarly an ornithopter moves the wing through the air to attain lift.

• Downvotes expected. That's life. Still seemed useful to add. Sep 1, 2020 at 12:02