What's the difference between F3J and AMA Thermal Duration & a short story... - RC Groups
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Oct 17, 2001, 03:05 AM
Registered User

What's the difference between F3J and AMA Thermal Duration & a short story...


I have a few questions about pure sailplanes, and thought I'd ask here since there doesn't seem to much activity on RC Soaring.com, or relevent conversation on RCSE. Just what is the difference between a plane optimized for AMA Thermal Duration contests, and those used in F3J? And more relevent to a situation I find myself in, how much elevator/flap mix should I require in my first full-house plane, an Airtronics Legend, that I flew for the first time today...FINALLY, I only finished it last May!!! I guess that's it as far as questions go...until I get a new tx that will do crow, and other fun things!

But...what a blast it was to fly today! Even with the hazy, anti-thermal weather. I brought an electric that I had installed my first brushless motor in, a MPX Milan, and the aforementioned Airtronics Legend. For the electric, I had the requisite three eight cell batt packs...the new sub-c 1700 cells procured through Diversity Modal Aircraft, although I didn't even make it through one. With this motor system this electric 2-meter sailplane (UHU wing, NSP '8-10 cell' electric fuse, and own design tail feathers) flies at MUCH less wieght and with MUCH more power than it did previously with it's brushed Velkom motor/gearbox system. Just a few seconds of motor run puts it up so high, and it thermals so easily now, that I got bored with it before I ran the first pack down. I was a bit nervous about flying the Legend for the first time, realizing that such a big plane doesn't tell you so much from a hand launch as compared with smaller models. I gave it just one hand toss (ok, I really had to HEAVE it) to check the balance, and then put it up on the high start. A little adjustment on the stab and it flew pretty good, then great once I got used to it. With it being hazy, and with about an 8mph breeze, I didn't find any thermal activity I could do anything with, but even then it seemed to have incredible hang time compared with smaller planes. I was even surprised how manuverable it was. It resisted tip stalling very well. Definitely need to play with flap to elevator mixing though...with flaps fully deployed it is a bit of a balancing act that, once I find the right mix, I shouldn't have to worry about too much. In the wind with the flaps down, I could almost fly backwards! Without flaps, it just goes on forever...I never even got to try the Milan before the weather took a turn for the worse.
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Oct 17, 2001, 09:53 AM
Sailplane Nut
NickW's Avatar

Flap to Elevator Mix

As far as mixing your flaps to your elevator goes. Start with about a 1/4" deflection at 90 degrees of flap. If your flaps dont go down 90 degrees, then dont use that much down. Try this right after you launch at good height!

You will also find if you ask some other sailplane pilots that we all like this mix to do different things. I like there to be enough down elevator at full flap to point the nose down at about 40 degrees. This way I dont stall my sailplane at full flap. I also like to make my landing approaches from high steep angles, and this type of flap setup works great for that. I also prefer having to pull the nose up if I am moving too fast, rather than having to push it down to pick up more speed at the last minute.

There are guys I fly with who use less down elevator in their flap mix. They rig their elevator compensation for pretty much straight and level flight. I dont like this approach as it makes landing in the wind difficult as you are opening yourself up for great stall potential. But if you like long and low to the ground approaches this may be the way to go. As long as you have the skill to push the nose down should you need to.

The point is, like anything else with sailplanes there is no clear cut answer, rather just a range of options. Start with your elevator compensation somewhere in the middle of all of this. Start getting in to a groove in your landings. Fly a similar pattern every time, and keep changing those flap settings till you find the angle you like to approach at.

Heres a quick tip though. If you can land a short approach pattern as opposed to a really long one, you will be able to land at any field you compete at. Some fields and wind directions leave you with very short approaches, so it pays to be good at these.

By the way, when a sailplane has its flaps down, it does this weird slow motion wing waggle just before it stalls. If you see that waggle then get the nose down, and pull up your flaps till you have flying speed again!

Good Luck

Nick Wisdom
Orlando, FL
Oct 17, 2001, 02:13 PM
Registered User
Lenny970's Avatar
There is a lot of overlap between AMA TD type sailplanes and F3J sailplanes. They are both designed to launch well, thermal well, and land easily. In general, an F3J sailplane will be stronger in order to survive the two man tow. An F3J ship may have a faster airfoil. The required 10 minute flight time is longer than most TD events, often requiring long searches for lift. The 100 point spot of the F3J task is 2 meters in diameter, so the plane doesn't have to be particularly well suited to landing.
In contrast, the 100 point spot in a typical AMA TD contest is only 2 inches in diameter. Much harder to hit, so the TD ship will have large flaps and be more optimized toward landing accuracy. Since it doesn't need the strength for the hand tow, it can be built lighter.
This is a rough generalization, and like I said there is a lot of overlap between the two classes. Many planes fall somewhere between the two extremes and are used quite effectively in both.
Nick did a good job of addressing the flap/elevator issue. Note that nobody can tell you the "correct" amount of mixing to use. It is dependent upon many variables and your personal preferences. You can only find the right amount by flight testing.

Good luck,
Oct 18, 2001, 02:33 AM
Registered User
After writing the original post it occurred to me that it probably would be impossible to give me an absolute answer about how much down elevator to put in with flaps. I'll start with 1/4" as mentioned and go from there. In those first few flights I did notice the 'wing-waggle' before it would go into a stall with the flaps down. I had a few close calls, but managed to save it every time. Keeping the nose down forty degrees is definitely more stable than trying to fly straight and level...I can agree with already! As for the differences between F3J and AMA Thermal Duration, it's no wonder the U.S is the only country flying the AMA's program. Thank you Lenny and Nick for responding to my questions.
Oct 18, 2001, 02:20 PM
Old Guy
Ron Cichowski's Avatar
Elevator/Flap mix as with most mixes trial and error.

If in doubt I'll start out without any mix and fly the aircraft with both sticks. Make note of your elevator stick position with flaps deployed (when the plane is performing as you wish). When you land move the elevator stick to the position you just used to get the results you wanted. Mark the elevator position at the aircraft with the stick in the position you remembered. Now add mix until deploying the flaps brings the elevator into the marked position.

Then "touch up" the adjustment by test flying.