Some who have followed my blog posts will know that I have been working on my HDwing design series for a return-to-launch glider concept using ArduPilot Mega.  Everything I have revealed so far has been based on relatively conventional flying wing ideas. Till now...

This forward-swept flying wing concept was inspired by the work of Justin Ammon (EdgeRC / birdofprey) that I spotted on www.rcgroups.com  A period of spreadsheet analysis and messing about with various X-plane simulations over te Christmas/New Year break demonstrated to me that, whilst unlikely to be a walk in the park, a forward swept wing could be made to work and would yield some worthwhile advantages over a mainstream flying wing.

A bunch of CAD work and several iterations later, based on an acceptably flying X-plane model, I came up with this design and I am now committed to bringing it to reality.




One of the difficulties with the forward swept design, especially as a pusher, has been to establish a workable centre of gravity position.  This necessitates the use of a protruding fuselage (a deviation from the pure flying wing) but offers an advantage of placing a payload area virtually right on top of the CoG.  Thus the design can accept a wide variation in payload mass and still be (X-plane) flyable.  How this translates into real world usefulness is yet to be seen, but it bodes well.

The drag-based yaw stablity of a conventionally swept wing also no longer works with a forward swept design, so the addition of a vertical stabiliser fin is required.  Bad for stealth, I know!


The nose cone has been sliced off at an angle to yield a removable camera pod as well as a large access aperture for working on the interior of the airframe.  I am designing in a removeable avionics tray so that the majority of the avionics and payload can be mounted on a lightweight demountable chassis to aid bench testing, modification and repair.


The location of the battery at just ahead of the CoG will allow me to increase its capacity with minimal adjustment to balance.   It is also is attached to the avionics tray for ease of complete systems-level bench-testing.

Personally, one of the attractions of the forward swept wing is that, similar to a canard design, all of the wing surface is lifting to produce pitch stability.  The lift distribution isn't the most theoretically efficient, but at higher speeds and lower C_L, this is not such a huge penalty.  The X-plane simulation shows a best L/D of about 19:1, which is not so bad for such a stumpy, low aspect ratio craft.  My earlier HDwing designs didn't get this far and they had the advantage of winglets and a lift distribution closer to the theoretical optimum.

The forward swept wing layout does come with some compromises but also offers compensatory advantages to exploit.  There is a whole host of detail design work to be done, but hopefully a worthwhile UAV airframe will result.  Time will tell.


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  • It's my own design folding prop based on a HK CF blade for want of anything better.  I made up a couple of samples but it needs some further work.

    More info here, Kevin

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  • What kind of prop is that? It looks awesome.

  • Hi Andrew Rabbit.  We are trying to start a research program in a couple of universities here, but we have little experience in aircraft design for high altitudes.  I am very interested in your designs and perhaps we could manufacture and test some adaptations.  Is there a way to contact you?   Regards.

  • PeeBee,

    It's a stalled project, pretty much.  The aerofoil section I originally chose was never going to function and it forced me to do a bunch of research and learning.  The fuselage plug that I made also sits here on my desk as a reminder not to jump too soon!  I have a reworked CAD model with revised aerofoil sections that would do a decent job and the concept still offers some advantages, but I have put my limited resources elsewhere for now, not that there's much to show for it yet.

  • Its been a long time since the last post here... A great looking idea!  What has become of it since?

  • Hi Mike. There is an earlier blog post where the issues of thermal management were discussed. It turns out to be less complicated in practice than first imagined
  • Andrew,

      Have you made considerations for cold-soaked batteries?  I used to test missiles and we had special issues with even mil-spec batteries at -65F (-54C).  You are going much higher.  Did you consider temperature?  How about temperature on your servos?  Motors?  Hinges?  Might be able to put a warmer on the balloon side, but still might need something for the descent to keep things warm.  Good Luck!

  • In 1983, AeroVironment obtained funding from an unspecified US government agency to secretly investigate a UAV concept designated "High Altitude Solar" or HALSOL. The HALSOL prototype first flew in June 1983.

    1983.....1983 and we still cant use it. I'm looking into amorphous solar cells but i only just want to supplement or run extras on board

  • I wanted to add back into the blog two posts that were made when this blog was accidentally published too early.  The first was:

    bGatti - You might change the diagonal to avoid landing on two parts. If the "cockpit" piece sloped \ instead of /, this smaller and more easily replaceable piece would likely absorb the balance of bad landings.

    and the second was:

    MC Andrew, It is very nice visually appealing design, very good animation and fantastic music selection! Is it legal to use (quad band) GSM on high altitude? I think there are some rules and regulations where  (how high) can we use GSM.  What about the rules/regulation about the size/weight, on-board avionics selection for the HAB operations?  Is it legal (how to make it legal) in Australia to launch HAB with such a payload?

    @bGatti, I guess a bad, nose-first landing will take out the nose-cone, but a good (auto?) landing will actually only land on the main fuselage, at least on level ground.  I take your point though and will consider the strength of the nose-cone intersection under a 'sub-optimal' arrival!

    @MC, you raise two very interesting and important questions. Firstly, I know that the FCC have a ban on mobile/cellular communications use whilst airborne, but I don't know of anything similar in Australia.  I have done some searching for regulations about this but I wouldn't claim to have any definitive answer in this area.

    Regarding the operation of the whole rig in Australian airspace, the regulations that apply are in CASR101 under free balloons and, presumably model aircraft - it will always be an amateur/hobby project, at least as a HAB payload.

    The free balloon regulations are pretty easy to navigate and people have already sent HAB's aloft with payloads of similar and larger masses, presumably without getting into trouble.  As far as the model aircraft aspect is concerned, there are a few greyer areas and I read that OCTA, as long as I have line-of-sight, I can (claim) all is OK.  One thing I must check is whether class G, or anything considered OCTA goes beyond FL100 - I should know this, being a (lapsed) PPL, but...  Ultimately, I may choose to fly in an ambiguous geographical area and leave it up to the authorities to prove any flights actually occured in their jurisdiction...

    In any case, if and when the flight actually occurs, I will be doing my damnedest to ensure no conflict with manned aircraft.

  • Thanks Max!  I doubt you could do solar power successfully without an odd looking aircraft - well, more odd than this one!  The Helios is probably the best example of what actually works.

    Also, on the forward swept wing, most of the wing area is behind the CoG, so adding solar cells is putting mass in the most sensitive area.  Getting the CoG forward far enough has been something of a challenge without adding to it.

    Actually, as my brother pointed out to me over the weekend, the Helios holds one of the records for sustained straight-and-level flight at 96,863 feet!  Proof that you can still fly when you get up there! :) 

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