Tuesday, October 23, 2012

Propeller placement article (external link)

There are many considerations where to put a propellers in a small aircraft or RPV. The common place to put them is at the nose. The biggest reason and driver for this placement is that it is advantageous for CG location. However, from aerodynamic standpoint that is not very optimal. There was discussion at HBA Forums about propeller placement and this document was linked (it studies difference of prop placed in pusher and tractor configuration):


Optimal place is behind the wing, a bit above the wing centerline (only small part of the prop circle goes below the wing). This placement has the typical CG challenges with it. And it will require either pylon on the wing, or a pylon on the fuselage (assuming a single fuselage). There is then the question about the effect of the body to the prop located near the fuselage behind the trailing edge of the wing. There might be unfavorable flow due to the effects of the wing-fuselage joint that this study did not take in account.

According to the article, it was possible to increase quite significantly the Clmax of the wing with the rear placement of the propeller due to the suction effect to the wing. This leads to interesting thought about a line-thruster - multiple small electric motors turning multiple relatively small props behind the trailing edge of the wing, providing suction to the whole wing surface, or at least large part of it. Interesting question then would be that would a varying thrust angle be beneficial, should the pylons be actually mounted on the flaps? Downside of this is that this may lead to flap mechanism that is not very lightweight as the flaps have to take all the torque and push from the motors. Normal flap mechanisms would not like that.

Any comments on this?


Jon C said...

I think it would be possible to put several small propellers along the trailing edge, but they would need to be farther out than how close the flaps would extend. Probably something like a bunch of smaller rear-wing nacelles similar in concept to the ones conceived for the Boeing B36 except on the top of the wing.

You're right about now wanting to put them on the flap railings - not only would you have issues with the flap mounts having to withstand more forces, but you would also end up with a changing thrust angle as your flaps came down. Unless you're trying to conceive of a competitor for the V-22, I don't see this as a good idea.

Jon C said...

Apologies, it was not the Boeing B36, but the Convair B-36 I was thinking of.

Nathan said...

It's been a long time since you posted; it's good to see you back! :-)

That was a very informative study. Thank you for sharing it. I wonder how much the upstream presence of the wing will effect the propeller. Do you think that the obstruction will make the propeller less efficient by altering the air flow, especially by reducing the pressure in front of the propeller? If so, might the Clift gains be cancelled by the propeller efficiency loss?

I understand that if thrust is reduced then some of the energy would be captured as increased lift. This would allow smaller wings, thereby decreasing drag.

How do you think these effects will combine in real-world use?

Unknown said...

Nathan: thanks. But summer time been busy riding motorcycle (and airplane to some extent) :)

That is an interesting question, the relation with the propeller efficiency loss and the gains. These studies are a bit narrow since they study only one parameter at time and drawing conclusions from them is hard because they don't seem to consider the problem as a combination of these individual things.

However, not sure how much the efficiency loss will be. Since according to literature, the efficiency loss on pusher propeller is small if it is far enough behind trailing edge. And if it is as far as in this study, it is far enough. Also tractor prop loses efficiency due to objects behind it. E.g. tractor propeller in front of wide airplane fuselage is worse than tractor prop in front of a small nacelle. And optimal tractor prop is like in Stemme S10 VT where the continuity behind propeller is gradual and there are no stagnation points immediately behind the prop.

Best person to answer this question would be a aerodynamist like John Roncz etc. However, the answers may vary between persons (as far as I have noticed).

There is one catch on increasing lift this way: when the power is out, it is not helping. And the small stall speed is most needed when doing forced landing. In normal operation, faster landing speed is no problem. I could be coming in approach at 100 knots if there was a large airport ahead, but if it was a forest or field in emergency, 100 knots would no longer be anywhere good and the speed should be less than 50 knots forward speed at the time of impact, to not get too hurt.

John McGinnis said...

In general I like the concept of distributed thrust, but in practice it tends to be inefficient. Certainly you would not create consistent drag reduction through suction as each small blade would only intermittently affect an airspace the size of its its chord width ahead of itself. Regarding articulation, the thrust line change from a flap attachment would probably cause more drag through trim change and sensitivities than lift would be increased by thrust.

Unknown said...

Thanks for sharing the link about proper propeller placement. Placing propellers in a small aircraft or RPV is quite difficult.

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