Very low aspect ratio planes?

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Mavigogun

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I wonder- for sharp leading edged very-low-aspect-ratio craft like the Facetmobile, what impact does prop wash have on the stability of trapped vortices at high angles of attack? Does the tractor configuration risk bursting/destabilizing trapped vortices? At specific or across power settings? Would an aft pusher in any way inhibit vortex formation or reduce the potential vortex lift when compared to a prop placed elsewhere? What of over-wing drives?

Mayhaps any landing larger RC tractor models nose-high might contrast powered and unpowered landings- and any noticeable loss of altitude when applying power to a nose-high approach from an unpowered descent. Observations would needs be mindful of limiting power to scale.
 
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Sockmonkey

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I wonder- for sharp leading edged very-low-aspect-ratio craft like the Facetmobile, what impact does prop wash have on the stability of trapped vortices at high angles of attack? Does the tractor configuration risk bursting/destabilizing trapped vortices? At specific or across power settings? Would an aft pusher in any way inhibit vortex formation or reduce the potential vortex lift when compared to a prop placed elsewhere? What of over-wing drives?
Good question. I've been wondering the same thing ever since learning about the facetmobile.
 

Aesquire

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The disruption in laminar flow and other things like wing strake vortices is momentary as the prop wash flows act from a tractor prop. The later is visible in humid air sometimes.
 

danmoser

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I wonder- for sharp leading edged very-low-aspect-ratio craft like the Facetmobile, what impact does prop wash have on the stability of trapped vortices at high angles of attack? Does the tractor configuration risk bursting/destabilizing trapped vortices? At specific or across power settings? Would an aft pusher in any way inhibit vortex formation or reduce the potential vortex lift when compared to a prop placed elsewhere? What of over-wing drives?

Mayhaps any landing larger RC tractor models nose-high might contrast powered and unpowered landings- and any noticeable loss of altitude when applying power to a nose-high approach from an unpowered descent. Observations would needs be mindful of limiting power to scale.
I've done some research on this question.. the tractor arrangement (nose mounted prop blowing aft) apparently produces a vortex-reinforcing effect because sustained vortices needs core axial flow velocity. It is well-known that vortex stability and intensity is enhanced by blowing a jet of air straight through the middle of the vortex. Flight experiments have been done on modified jet aircraft, such as those trying to land shorter and slower on aircraft carriers, successfully demonstrating this phenomenon. To my knowledge, these vortex blowing devices have not been widely implemented, presumably because the vortex amplifying effect is not great enough to justify the additional mechanical complexity. LE vortex flaps combined with vectored thrust have apparently been the favored solution for jet fighters.

Plenty of room for experimentation in this area. I never saw reports of the Facetmobile attempting to demonstrate high AoA powered maneuvers, but it may be able to do so. RC models have demonstrated this phenomenon to the extreme.. they seem to be able to smoothly transition from horizontal flight to vertical hovering, if thrust-to-weight ratio > 1 , of course .. the shallow CL vs. AoA slope of low aspect ratio wings seems to blend directly into vectored thrust... not clear as to when powered vortex lift ends and vectored thrust begins.
 
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rotax618

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This is a video of a low aspect faceted model I designed many years ago. This was the maiden flight on a very blustery day, and most of the landings were dead stick because we couldn’t keep the engine running.
You can see the very acceptable L/D, the very sharp LE, the faceted airfoil and the vortex lift at very slow landing speed.
 

Mavigogun

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I never saw reports of the Facetmobile attempting to demonstrate high AoA powered maneuvers, but it may be able to do so.
I guess “high” has to be quantified; I believe Wainfan reported in the Contact! interview that FMX4 buffeted over 35 deg pitch-up, but controls remained responsive.

This is a video of a low aspect faceted model I designed many years ago.
Thanx, Rotax- though I had watched that video about 20 times before- including in slow-mo. If you still have it, I’d very much like to see it descend at a 35 deg up attitude with the engine off.
 

rotax618

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We still have the model but no one has bothered to video any of the subsequent flights, there is more footage and better quality video on a hard drive somewhere amongst my stuff, when I posted the edited and compressed video on youtube we only had dial up internet so it wasn’t possible to upload the full video, remember this was 13 years ago. I will attempt to find it and post the whole video.
The model’s performance was pretty amazing, I couldn’t fly RC but my colleague won several RC contests flying it at the local Club.
The design was my attempt to improve on the performance of the FMX4 model we built.
 

Urquiola

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Thanks rotax618: the models you designed and built seem answering, in the positive, my doubt on wether a central fin, leaving wingtip fins fixed just for vortex control, would improve facetmobile.
Another doubt, the answer would be negative, bit I'm not that sure, was if eliminating front propeller, and its wash, by having internal ducted fans, there are a lot of industrial affordable electric fans for sale, acting more on the action-raction principle, could be a good choice. Please comment! Salut +
 

rotax618

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I know that model aircraft EDFs (electric ducted fans) are VERY noisy and not as efficient at producing thrust for a given power input as an open propellor.
I designed and built a couple of pusher light aircraft and can tell you pushers are far more difficult to design than pushers, they are always noisier and getting clean air into the prop is quite involved. In my opinion using a tractor prop is a no-brainer.
The only problem with a tractor prop on a delta is the poor forward and downward vision, it is possible to use transparent covering in the cockpit floor, a more complicated solution is to position the engine behind the pilot and use a shaft driving the PRSU (prop speed reduction unit) and prop in the nose. Having the PRSU away from the engine means that the shaft can be smaller, lighter and more flexible in torsion dampening the resonance on the gears and couplings.
A better solution to building a very low aspect aircraft, and still harness the vortex lift for short, slow landing is to use the Arup or Zimmerman planform, the centre of lift appears to be at about 25% of the root chord, this puts the pilot/passenger well forward with excellent vision in a tractor configuration.
As has been said, an aeroplane is thousands of compromises all flying in formation.
 

cluttonfred

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I wonder how an amphibian-style pylon-mounted engine (tractor or pusher) would work on a low AR design? The engine would be out of the field of view and it might actually help the flow over the wing/fuselage at low speeds, but you'd probably need some sort of horizontal control surface in the prop blast to balance the nose-down pitching moment from the engine.
 

Hephaestus

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I wonder how an amphibian-style pylon-mounted engine (tractor or pusher) would work on a low AR design? The engine would be out of the field of view and it might actually help the flow over the wing/fuselage at low speeds, but you'd probably need some sort of horizontal control surface in the prop blast to balance the nose-down pitching moment from the engine.
Equator-P2-Xcursion-1-1000x667.jpg

:)
 

lr27

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Esquire mentioned laminar flow, but I don't know if that phrase belongs in the same discussion as the Facetmobile.
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Putting the PSRU up front means you don't have to run the drive shaft through the pilot's bellybutton.
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Putting the pilot right up front where he can see also makes him the crush zone,to protect the engine
 
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