I was thinking one day about the Bohannon B1. It is basically a modified RV (Harmon rocket something) with very high power to weight ratio and that's it. This plane climbed to something like 14 km.

So consider this (high excess power) case hypothetically:
- Airplane with high aspect ratio (low span loading) with high power engines with high power to weight ratio. Example: Chevrolet LS9 (600 hp).
- If the plane can maintain level flight with minimal power. 35000 ft we have remaining power 0.2 * 600 = 120 hp.
- Diamond flies nicely with 120 hp, actually 90 hp is quite sufficient for it for normal cruise speed. With lower span loading much less should keep the plane level.

So now the naysay would be "nah, LS9 can not sustain 600 hp continuous without breaking". However, 120 hp is hardly 600 hp continuous even if the engine is at full throttle and giving all it can at the altitude. It is still stressed only for the 20 percent power.

Same engine, with single stage turbocharger, it should be possible to extend this quite a bit further. With two stage turbocharger even higher altitude should be possible, 70000 ft might be feasible given that the other challenges that come with the altitude are solved somehow.

So you could have a 1200 hp airplane with 240 hp used at altitude for cruise (in case of twin). This should give a quite generous cruise speed at the altitude given that the props are big enough (disc loading low enough).

Airfoil



KS400 wing at altitude 20 km, speed = 155 kts



Here is the dat-file. Download it here: KS400.dat

Works from Re 500 000 up.

More simulations to follow later.

If you are into flying wings like me, this PowerPoint slideset might interest you. It compares the basics of conventional airliner and flying wing airliner. Much bigger planes in other words, than my interest area. However, some of the pros and cons findings for each configuration also apply for the small version. Not all though as the starting point does not have engine nacelles and engines sticking out of the wing. http://www.engbrasil.eng.br/index_arquivos/ap23.pdf I haven't done yet comparison for the wetted area of a flying wing compared to a sailplane like structure. Logic tells that the flying wing in this size category might have more wetted area. But I am not sure. I need to design both and then measure the wetted area of both and compare. I am not a big fan of wing twist and the amount of wing twist on PUL-10 causes me shivers (wing tip twisted 10 degrees). That can't be good for cruise, simply can not. Ten degrees is insane amount of twist - on cruise the tips are on negative angle of attack and cause a lots of negative lift. The wing tips act as rather poor tails this way - it is very short coupled and if you have tail deflected that much on that close, the wing center section will need to lift also the negative lift of the tips which will make the plane to perform poorer. I am quite sure that a flying wing should be made stable without that much twist. I have a related idea for a flying wing: - one problem with flying wing is that flaps can not be used - what if you had small trim tails that look like the ones in SpaceShipOne. When flaps would be down, the trim tail, would cause opposing pitching moment to negate the pitching moment of the flap - The elevator control otherwise would be like on a flying wing, with elevons. - I haven't tried this out yet but it can be tested with RC model.

I am progressing forwards from concentrating on aerodynamics to also fabrication and optimizing the fabrication process. Been doing hand layups for a quite long time now, but I need to start doing shape accurate parts. Testing aerodynamics requires very high accuracy.

I have been doing several molds lately. One was almost successful, but it was a lot of work and it still had non-sharp edges.

So what I am trying to do is a wing mold with CNC fabrication. I have a CNC mill that can carve blue styrofoam, wood, MDF, Corecell etc. and is large enough for producing a half (lower or upper) of a wing section (with the limitation of the length, have to glue a big mold from ~1.2 meter pieces together).

I would need a rapid prototyping technique to produce shape accurate molds with glossy or at least almost glossy surface. So that manual work would be minimal. This is especially pronounced in case of making wing molds.

The problem I am facing is this:
- if I carve the plug to blue styrofoam, and then paint it and polish it, there is the downside:
- the styrofoam can be only painted with a paint that does not have solvent in it, and the only paint that will fill the surface is solvent free epoxy primer. The problem with that is that it also makes sharp edges round, and especially in a small scale (RC scale) the roundness becomes way too big to be acceptable. The edges where the lower and upper half meet, should be also absolutely accurate and sharp. Doesn't happen with this technique.

Has any reader used molding epoxy? I saw some picture of a mold being filled with a molding epoxy and then milled with CNC again to shiny surface directly (?). Would that be viable option for my use? As it would be for rapid prototyping and for fabricating many wings (and not just one pair), it should be somewhat reasonably cost effective. Making the mold from wood is not completely inexpensive either - requires a very thick perfect wood block (or MDF block). I could not afford consider replacing the styrofoam with a huge solid mold plastic block (that is used in industry for prototyping shapes with CNC), because the same volume is much more expensive, would be possibly fine for a CNC model of a small device, but for making a mold for large wing the cost hikes out of the roof very quickly. Styrofoam is cheap and very easy for the machine to carve, but that's the best part of it, otherwise it is really poor material.

Any first hand experiences on this?

I referred to the AR-drone in previous article about flying car. We produced a short video about Ar-drone flying:
http://www.vimeo.com/16147472

iPad provides control input (which direction one wants to go) and the computer inside the AR-drone provides artificial stability (so it is very easy to fly unlike RC-helicopters).

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):

http://www.icas.org/ICAS_ARCHIVE_CD1998-2019/ICAS2000/PAPERS/ICA0344.PDF

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?

The Joby Motors seems to have good enough KV-values for running slow turning props (especially JM2S and JM2):
http://www.jobymotors.com/public/views/pages/products.php

Electric Lazair uses these motors. Standard windings are available for up to 700 volt system!