Hi Jim,

I hope Larry is fully recovered from his Diabetic/foot/leg episode!

As to designing for density altitude, I would think the low drag, large wing area light weight design parameters would come into play.  Less dense air would seem to need a wing that produces more lift with out increasing drag. ? ? ? ? ? ?

Bill <><

Hi JOhn,

I think that is what I was trying to refer to.

Tell me more about fuel...how much nitro...should be added per ft above sea level...also fuel burn times. Should intakes be increased in size...or tank sizes enlarged...etc?

The only time that I can remember flying at high Altitudes was years ago (1965)when I took my old Nobler up to the Parking lot at the Whistler ski area parking lot in summer after we flew in the "30th Annual Canadian Internats.
 Ben Madsen and I took his little trailer, my Nobler, His Shark 45 with a souped up Veco 45. My tired old Fox sucked...and Ben's Veco ran pretty ratty our 10% K&B fuel.
I have slides of that serious lack of judgement test flights. Anyone who has seen that photo of Ben's funny "snyle!" as we were proceeding to unpack these toys for a practice flight on that closed overflow parking lot.
I don't remember the altitude....except that I GOT ONE  of a sun burn.

Good subject.

I, too, built an "ultralight" ship for the Reno Nats and ended up storing it in the attic.  It didn't have a name on it but if I were to have named it after trying to make it fly I would have called it the Challenged Cormorant -- it flapped its wings like one of those "birds in name only" trying to take off.  Too light, too flexible and too bad ...  it turned into an overnight attic queen.

If I were to start from scratch today (based on that experience) I'd try to use some other device than a super light wing loading.  What comes immediately to mind is the use of a higher aspect ratio ... say somewhere between 5.5 and 6.0 to one.  As the aspect ratio of a lifting surface increases (the wing span gets longer for a given area) the lift it produces at a given angle of attack is increased.  This is why (in extremis) sailplanes have aspect ratios as high as 30 to one.  Not only do you get more lift at a lower angle of attack but you get the lift with less drag as well (which adds up to not needing as much "poop" from the old Fox).

Using a higher aspect ratio will also allow you to avoid using large chord flaps which can be huge drag producers as well as increasing the control forces necessary to deflect them (requiring more line tension for control inputs, which is something that could tend to decrease a bit due to lower power output.)

You don't want to go overboard on the aspect ratio increase because the lower drag they produce also can result in a more aggressive "wind up" in consecutive maneuvers when flying in winds (which are often an issue at high altitude locations).

I'd also go with a slightly higher aspect ratio tail for much the same reasons.  Again, modest increases should be the ticket.  I've been using around a 4.5 AR on my sea level ships (somewhat lower than the norm for many designers) and would likely go back to about five or  even 5.5 to one. Don't get crazy with long spanned tails as rigidity of the surface is more important than any theoretical increase in efficiency.

The biggest part of the solution for high altitude flying (and what we pretty much all did in Reno that year) is to liberally inject your powerplant with nitro methane.  A good rule of thumb is to juice the fuel to the point that fuel consumption is consistent with what you'd be getting at sea level.  If you've been burning four oz of 5% for a 6.5 minute run at sea level you might find it necessary to go to at least fifteen and maybe even 20% at a mile high location, especially when it is hot.  You can also expect to run more pitch on the prop because the air is thinner and there won't be as many "chunks" of air per rev.  To get the same thrust you'll need the additional pitch.  Again, modesty is the watchword.


Ted Fancher

I believe Ted's on the right path here.  Imagine that? 

Here in Salt Lake City, where we fly at 4600 feet ASL, we've long ago gone to adding a rib bay to each wing half to increase the area. works every time..

John Miller

Ted is correct about the high aspect ratio`s. I heard that several people at the Reno Nats. had problems due to the light weight planes. Ridgid is the key word hear. I think in the long run I would have gone to a thicker Airfoil. You can get more lift without extra drag. Drag will increase but the thicker airfoil will help more the the extra drag will hurt. I did alot of test back in the seventies and into the eighties on thick and thin airfoils. All the test where done on Combat planes. Thicker I went the better they would turn. I had to go to a two inch airfoil before speed started to fall off. But,even then it was only 3 to 4 mph. With the tighter turn planes also had to be stiffed up to stop flex and in some cases I broke the wing half. You just need to fined that happy medium.

Gordy