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A method to produce homebuilt aircraft by using existing components differently

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karoliina.t.salminen

Well-Known Member
Joined
Oct 7, 2008
Messages
407
Location
Finland
We once had a discussion couple of years ago with couple of aircraft builders at SIL Talvipäivät (equivalent of EAA Finland, and a yearly gathering of experimental builders and enthusiasts, much smaller scale than EAA though) about my specifications for the personal HALE-cruiser/personal globetrotting machine. This is hypothetical speculation and should not be taken as a promise of any kind to do any of this.

- We know that any wing at positive angle of attack will provide lift.
- We know that aircraft will fly with any wing provided that the CG location is correct and there is enough tail volume
- There is no such thing that a certain wing will work on this aircraft or this aircraft class only. The wing will work always, but the question is about optimality of the such design. Obviously optimality gets sacrificed when not using components designed for purpose.
- But this idea also mitigates (in configuration) some of the problems associated for using components not designed for purpose, ie. stall is mitigated by ensuring that rear wing will never stall and forward wing stalls always first.

So I presented my specifications on what I want. And we discussed about a crazy idea how to make a plane that meets
this criteria quickly with skipping design and fabrication of certain major components. The resulting plane would not be optimal
and it would not be as good as purposely designed for the purpose, but we thought out that:

- Buy two sailplanes, salvage their wings.
- Build a fuselage only
- Join the two wings of the sailplanes with the new fuselage like in Rutan Proteus, ending up a tandem wing with
each wing carrying roughly 50% of the load (or the forward wing a little more than the rear wing).
- Reduce the length of the forward wing a bit and replace flaps with the canard-type elevator mechanism.
- The rear wing's ailerons would be reused as ailerons
- Add vertical surfaces with booms like Proteus
- Add landing gear (like in Proteus, rear gears in the roots of the tail booms and the front gear in the fuselage)
- Reuse water ballast tanks as fuel tanks
- The fuel would be located unoptimally in relation of the CG, but it would not be hard to make a software that would
pump the fuel evenly from the tanks to ensure that the CG remains in the same location through the flight envelope.
- Add engine/engines and props

I was sceptical at the time about the idea, but I have been thinking it that actually it could make sense.
The sailplane wings have one not so good thing though, they have the optimum L/D optimised to high Cl/angle of attack,
which is typically unoptimal for a motor plane. However, the HALE thing would account for that, the plane
could fly at high altitude with relatively high angle of attack much nearer to optimal L/D with low IAS, but the
altitude would make the low IAS to reasonable true airspeed and actually the plane would cover distances without
going so fast in IAS.

This is not my plan and this is not exactly what I am going to do (at least have not decided anything like this currently),
but this was a speculation. Plan A is self-designed system from grounds up but this was discussed as a potential shortcut ticket.
So I would like to hear your opinion if it was good speculation or bad speculation or indifferent speculation.
We discussed this circa 8 years ago and did some rough estimation that it would probably just meet my specifications
despite of all the compromises made. Two sailplane wings with a low drag fuselage would still be low drag and especially low
induced drag vs. the more low aspect ratio typical planes. Since this would be a high altitude craft, the two wings would make
its wing loading low which would be advantageous for high altitude flight. The low wing loading would be also advantageous for
low landing and takeoff speed, and for optimising the cruise power as low as possible (ie it would be cruised at very low percentage
power, like 20% power vs a typical GA plane like DA40 that we cruise at 40-50% power as the Lycosaurus is unsuitable for lower power
setting as the SFC curve gets worse and 5.5 gallons per hour is the lowest we can get on it in cruise and also the DA40 is unsuitable
from aerodynamics standpoint for lower power cruise than that as the induced drag rises as the nose gets too much up, hence
suitable engines would be car or motorcycle engines with a hybrid system for transmission (ie not need to match prop rotation speeds
and engine power and engine rpm, and variable rpm for the props for the high altitude flight (ie high rpm at high altitude, low rpm at low altitude)).

And then one more thing to add, how to route the controls from the cockpit to the control surfaces: easy. There would be no pushrods, but just cablings and there would be servo motors on each control surface actually turning them. The stability would be augmented with software and it would be possible to move the CG a little bit aft (to a neutral position at least) during cruise for better efficiency by pumping more fuel from the forward wing than the rear wing, and end up later with a CG that is again positively stable for landing. Like sailplanes can release water ballast, in emergency, the fuel would have to be released quickly and the design would have such CG location that the plane with empty tanks would be at positively stable CG location.

Typical water ballast capacity is 190 litres per wing. Two wings = 380 litres. The 380 litres should last for 5500 km range. That would call for 6.9 litres per 100 km efficiency. I think better could be reached quite easily at high altitude high L/D cruise, exceeding Toyota Prius mpg would be the target. This is not so far fetched target as the DA40 already is more efficient in cruise than an SUV.

The tandem wing configuration makes it possible to use sailplane wings intended for lighter aircraft, to be used
in a heavier aircraft because the load is shared between the two wings. The resulting aircraft would not be optimal
in aerodynamics nor optimal in structures probably, but we discussed that reuse of components would allow potentially faster
and more likely completion for otherwise very ambitious project.
 
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