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High aspect ratio design for long range, optimized for cruise efficiency

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karoliina.t.salminen

Well-Known Member
Joined
Oct 7, 2008
Messages
407
Location
Finland
So I thought that Jay's thread gets hijacked if this thing is continued to be discussed there, so I opened a new thread.

So consider this (if the initial requirements were) and we leave out all hybrid propulsions from this speculation at this time:
- Typical cruise altitude 25000 ft. Service ceiling much higher, but because we have pretty much concluded that it is hard to get permission to fly higher than this, so be it, this is then maybe the real life cruise altitude.
- Indicated airspeed at that altitude at 55% cruise power in the realm of 135 kts IAS (200 kts TAS). Low power is assumed because otherwise the fuel tank size needs to become enormously big.
- 2500 nm range at that speed means 12.5 hour endurance requirement.
- "easy trip" with this kind of plane is 1000 nm (half fuel). Tanks sectioned with individual fuel caps allowing partial fueling when full fuel is not needed.
- Assume turbonormalized diesel cycle engine.
- Payload requirement: 3 * 90 kg (270 kg) + full fuel for 2500 nm range. The extra 90 kg is for carrying rescue equipment and luggage. Some of this weight can be compromised (third person can be diminished to 70 kg), but that's about the lowest limit it can go because life raft and the stuff like that just tends to be not coming without weight.
- Assume higher aspect ratio than current certified planes like DA40 and considerably lower than Jay's motorglider or Stemme.
- Assume airfoil which has relatively high profile L/D at low alpha and assume shiny surface finish resulting from molded composite construction (with CNC made tooling).
- Assume carbon construction instead of glass. "Reference plane" - Diamond DA40NG, with similar engine, is made of glass fiber. I have seen a destroyed wing of a DA40 and another wing under inspection, and I could find no carbon from this plane, all glass, so the plane does not have optimally light structure, but a compromise for manufacturing cost.
- Initial stall speed requirement, same as DA40, 49 kts sea lever. With full fuel for extreme range, the stall speed can be compromised a little from this. 55 kts is about as far as I consider it being acceptable for considering engine failure at start.
- Assume slotted trailing edge high lift devices with 75% span - deviation from a typical sailplane.
- Assume pusher propulsion with constant speed propeller(s)
- Space for two side by side in the cockpit + space for all kinds of pilot accessories needed for a 12.5 hour IFR leg.
- Assume tailed conventional configuration, like Stemme S10, but with shorter wings and lower aspect ratio than Stemme, but not as low as Diamond or Cirrus or Lancair. Not a motorglider, but not a low aspect ratio GA plane either, something in between.

As this has been discussed before on Jay's thread, lets continue in here instead.
So comments on these? Now the wing size, plane size, aspect ratio, engine power required, structure weight, feasibility, achievable mpg?
I will do a new excel for this concept, and will be calculating these things along the way, but it would be your turn for comments, after the initial requirement list. What requirement exactly of the above you would see needing compromising the most?
 
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