Thanks Brian, for your thorough and detailed answer. I appreciate the opportunity to learn more about indoor flying from you and Jeff A. And I always figure that a long answer is more likely to include more useful information:)bjt4888 wrote: . . .I start with the Bernie Hunt design spreadsheet . . . Almost all of the airplanes my students built needed about 1 degree less wing incidence for a total decalage angle of about 1.9 degrees. . . Sorry for the very long message.
Brian T.
It is apparent that I need to "hit the books". I know of the stability vs. efficiency tradeoff as you move the cg, but I never looked at the numbers as you describe. Where can I find the Bernie Hunt spreadsheet? I saw your previous post saying that you were unable to post that large a file, something like that, but I've looked online without success.
I guess that some of my confusion is due to the fact that your account of trimming the FF planes is so different from our experience. My students' planes are similar in length, have a flat wing with endplates angled about 15 degrees from vertical. The stab, however, has similar endplates, and is mounted with tissue tubes below the tailboom. The cg is currently about 1.3" (3.3 cm) behind the TE of the wing. We originally had the cg farther back, about 2" behind the TE, but the recovery from ceiling hits was poor; the plane would lose 6-8 feet of altitude before recovering. With the new location, recovery is MUCH better, and the flight times seem about the same.
Here is where things are really different from your account. We have the thrust line parallel to the motorstick, and the wing has LE 1mm higher than TE. About 0.7 degrees positive incidence. Our best results have been had with the LE of the stab 7 mm below the TE. About 6.7 degrees negative, so our total decalage appears to be more than 7 degrees. Maybe a little more or less, if the boom is not exactly parallel to the motorstick. But still, this is way, way more than your students are using with their FF planes. I would think that this would greatly increase drag and adversely affect flying times. Maybe it does, but our best no-touch flight is currently 3:39 under a 19' ceiling, which seems pretty decent.
So I'd like to take a look at Hunt's spreadsheet, if you can point me to it. If you have any ideas as to how these two plane designs can trim so differently, but still get similar times, I'd be most interested. And once again, thanks for bringing your expertise to these discussions!
Dave Drummer