How does the altitude effect our models?

[GregG]

For a little over a year now I've been flying a self designed wing AKA the Predator Twin (http://www.ecis.com/~cobrajet/predator/predator.html). Airframe #1 has had hundreds of flights and several different pilots have had a turn flying it. It's worn out three sets of motors with as many flights as it has had if that makes it easier to understand. The old covering was really getting ragged looking, the back spar had been broken for some time, and it had developed a radio glitch here and there so it was decided to tear it all down and build a new one.

The 1st flight on the new and uncovered airframe went well. It flew straight out from the hand toss and needed little trim adjustment. The radio glitch turned out to be the RX antenna had broken leaving it with an effective length of ~8". Soldering that back together made things as good as new once again. I spent a few hr. Friday night covering and trimming it out to look decent.

The day after the 1st flight I made a trip from my house (ele. 30') up to the Spring Fling in Reno Nevada (ele. 4800'). For some unknown reason I just couldn't get the Predator to fly. It wanted to consistantly come off a handlaunch and do a slow roll to the left into the ground. The only time it acted this way it the past was from the CG change after recovering it. I decided to move the battery pack forward and try again. After moving the battery three times to the farthest forward position it could be and still be within the airframe AND taping a small weight to the nose it managed one short dissapointing flight. Something was definately wrong here. Eddie P, a commercial pilot mentioned that perhaps the altitude difference was enough to make the difference in flight between at home and Reno. That seemed to be the best answer to me. After flying the slow flyer Sopwith Pup (Snoopy) and it also having a rather anemic flight, it looked to me like the altitude did have a great effect.

Yesterday I joined the regular Sunday flying gang back at home and gave the Predator another try. Surprise of surprises, it flew great once again!! I even managed to move the battery pack back to its original position and remove the nose weight.

To tell you the truth, I've never seen these effects on my own airplanes before. It really took me by surprise. Has anyone else had any similar experiances?

[Mike Taylor]

I had a Pop-fly act that way. I live near the ocean at 750' ASL and it flew okay here - not super, but at least OK - it would climb, loop, etc. When I tried it at +7000' in Giant Forest, it would just barely maintain controlled flight and never got over launch height on a fresh charge. Also, my old Cessna 150L would fly fine with a passenger and luggage at low altitude, but was a dog flying over the Sierras, and I worried that it would not be able to cross from east to west; once I hit the slope wave after the top it did fly alot better... Its service ceiling was 12,500', and you couldn't get any higher without the slope lift.

[Ollie]

As you know, the air density decreases with altitude. Wings produce lift by accelerating an air mass downward. Propellers produce thrust by accelerating an air mass through their disc. When the air density is less, the volume of air that has to be accelerated is greater. To produce a given force the air has to be moved faster when the density is lower. That's why the stalling speed is higher for lower air density. That's why the thrust is less for a given prop RPM, diameter and pitch when the air density is lower. Using a higher pitch prop at higher altitudes will help some. Using a more powerful motor or bigger engine will help a lot.

See:
http://wahiduddin.net/calc/calc_da.htm

[Dick Huang]

GregG,
See "high altitude flying" thread. I suggest you get MotoCalc and analyze the performance between 30' above SL and 4800' above SL. Try a new prop that gives 30 feet rate of climb when flying at 4800 feet.
Dick Huang:)

[GregG]

Thanks for all the replies. I just can't get over the shock of this as I wasn't prepared for that difference at all. Next time I'll use my head (and a calc program) before going uphill to fly. ;)

[Sail 'n Soar]

The MotoCalc suggestion is a good one, but for future reference, if the CG is in the correct spot at sea level, it is right at altitude - unless you fly at altitude stalled or approaching transonic - only kidding! The big problem is that you were not launching your Predator fast enough, coupled with reduced thrust. Between sea level and 5,000' the air density drops about 14%, and the stall speed increases about 8% (squart root of the air density ratio). Your 30' to 4,800' will be some slightly smaller increase. Your "usual" hand launch, which works at sea level, probably wasn't fast enough to be above stall speed at altitude and you didn't have enough thrust to accelerate through stall.

However, with direct drive props, your prop speed was still probably greater than ~2.5* stall even with the 8% increase. I'd guess that if you had a higher initial flight speed/ more energetic hand launch your Predator would have flown OK, albeit with a lower rate of climb. Your URL didn't provide enough info to run the numbers.

[Daemon]

Since I fly at 5000-7000 feet all the time (and occasionally up at 12,500 feet), I gotta agree about the CG comment. If the CG is right, then it's right at all altitudes. As mentioned above you probably threw it into a stall, and after moving the CG way too far forward, when you finally did get it to fly, it flew badly because of the now misplaced CG.

One thing you are likely to find is that at sea level the CG placement is probably a little less critical because your motor will pull you out of trouble, and your stall speed is lower. At altitude you don't have that margin or safety, so if the CG is off, then it'll behave badly. If I'd been in your shoes I'd have starting moving the CG *back* a little and probably launched off a slope into a little breeze somewhere to give a little assist.

ian

[BEC]

Concur - changing the CG is not necessary.

From the Ezone archives: http://rcgroups.com/links/index.php?sid=498277416&t=article&cat=215&id=3829