G'day all,

 

I'm still doing my research on building my first UAV and so far the Skywalker looks like a pretty good candidate but I'll be operating in high wind areas (like the tops of hills) in unrestricted airspace. I was wondering if anyone has played around with a variable sweep wing such as those found on the F-111 and F-14.

 

My initial thought was to have the wing sweep automatically based on forward airspeed thus reducing drag (and lift) when heading into the wind and improving upwind and cruise performance.

 

I would replace the pusher prop with 2x 40mm ducts mounted to the side of the fuse and move the battery to an external compartment on the belly. The freed up space would be used to mount the pivot and servo for the wing sweep.

 

My payload would be a gymbaled camera and transmitter.

 

The mechanism for sweeping the wing is quite simple so I don't see any issues there but I'm concerned that I've overlooked something and being conservative (read poor) I really don't want to make any more mistakes than I have to. (I'm quite comfortable making the required amount)

 

Your thoughts and suggestions would be greatly appreciated.

 

Thanks

 

Mick

 

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Easiest way to do variable sweep is to incorporate an oblique wing concept (see NASA).

Variable-geometry wings are used to reduce the onset of transonic drag rise above Mach 0.8 or so. I will be extremely impressed if you are planning on flying that fast!

 

At low speeds (below 0.75 Mach), sweeping the wings back only makes it more difficult to fly because you are changing the lift location.

oh well that's dissapointing! looks like i'm going to have to replace the EDF with model rocket motors :)

 

Thanks for the info Gents. Although I really wanted to build a sweeper, there is a certain relief in scratching that off my list.

Just to clarify, sweeping the wing back won't lower the upwind drag? If that be the case, how would I improve the upwind performance of the aircraft in windy conditions?

Use a bigger motor. :)

Sweeping the wings back should actually increase the drag since you would be reducing the aspect ratio, so induced drag would increase.

 

hmmmm, It will reduce drag a little because I imagine by sweeping it some wing will be hidden inside the fuselage, and a smaller wing area will reduce drag a bit (lift too).

The wing efficiency will also be reduced though, a swept wing promotes flow across the wing which is effectively useless.

 

My verdict? variable-sweep wings will be fun to build and look shweeeet but have little or no performance gains at the low speeds of a model.

 

And by better upwind performance, you're basically just asking for a faster aircraft...put a smaller wing + flaps for low speed or just add MOAR POWAH

Would a delta wing be better in these conditions?

One of the other problems with variable sweep is the CG shift.

I believe it is a valid concept. The swept wings will have lower drag and if a portion of them is hiding inside the fuselage the wing loading will also increase thus reducing sensitivity to the wind. Of course as mentioned L/D will have a penalty in low speeds but that shouldn't be much of a problem.

An oblique wing appears to be simple structurally but it faces huge aeroelastic loads and like all fwd sweep aircraft it should be a very sophisticated design to fly safe or more than once.

The swept wing in a low speed aircraft was a core element of Lockheed's VARIOUS UCAV vtol concept, not for tolerating wind gusts but for trimming the aerodynamic center of pressure because of the very big V-tail (necessary for sufficient control in the transition phase) and the vtol gear.

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