Stuka Stunt Main Forum · 12 of 12 known posts recovered
Last weekend I sat poised over a slab of 3/16 balsa to build (repair, actually) a new stab/elev for one of my U-Key 40s based on the responses to Xain's thread. Then I decided to ask y'all for advice.
At 500 square flapless and something like 25%, what would you think is a pretty good total ratio, and a pretty good stab to elev ratio?
From the responses, seems like 125 squares split 50/50 between stab and elev is in the ballpark. Agree?
Larry F
Humor..God's gift to us to keep us from making sausage out of each other daily.
All the smart guys (including you) seemed to go around 25% on flapless. The stock U-Key 40 is shy of that by a wide margin. (I have an Excel virus, so I can't do arithmetic for now).
How did you arrive at 22%?
Larry F
You don't need as much tail volume on a flapless airplane as you do with flaps. 25+ is the standard for flapped airplanes these days. You don't have to overcome as much adverse pitching moment without flaps. I don't think even the Doctor is 25%. The Skyray 35 is a lot less. The Doctor (to me) has a sort of exponential response when balanced per plan, moderately soft around neutral, and more sensitive as the controls are deflected. The Skyray, balanced at 15%, is pretty strongly regressive, quick around neutral and falls off at the ends.
Moving the CG around changes this relationship. The general rule is that the CG position controls the neutral sensitivity, and the controls speed (handle spacing, total elevator throw) controls the sensitivity at larger control deflections. The more aft the CG is, the more sensitive it is around neutral, and the more regressive the controls are. This is very easy to control with elevator-only airplanes, and a good trimming characteristics. Flapped airplanes have to many other things going on to make this rule generally true.
The larger tail of the Doctor allows a larger range of possible CG's, so you can dail in what you want. The Skyray is much mroe sensitive to the CG position, and there isn't a huge range of possible CG, and thus isn't as easy to dial up the response you want. The Doctor is a much superior airplane, just from reduced wing loading and better airfoil.
If I was going to optimize my Skyray, I would probably shorten the tail moment an inch or so, and then enlarge the tail enough to get the same tail volume. The split between the elevator/stabilizer is about 25/75.
Brett
Hard to say without knowing how it flew with the original, and it's size. Things to consider
- was it limited by rotational rate (could you use full up without stalling the airplane?)
- was it twitchy at neutral, or soft?
- was the control response progressive (more sensitive at higher deflections) or regressive (softer at higher deflections)
- did the CG have to be too far forward to make sure it was stable?
Brett
And now we're going to "fix" it?
Brett
There is no doubt that my flying skills are somewhat less from what they were in the 60s. I don't mind that as much as you might think, but it does make design issues a little more important. As to your specific points:
Was it limited by rotational rate? (could you use full up without stalling the airplane)?
Yes, full up did not stall the controls nor the wing.
Was it twitchy at neutral, or soft?
Twitchy
Was the control response regressive or progressive?
Much softer at higher deflections. Big control responses at major "saves" had no response.
CG location is a little farther back than normal, but within limits. I haven't changed the CG, but have flown with where it is.
Larry F
Two minor corrections Scott.
First, Paul flew "only" a 513.5, I kept the score sheets & added them up Sunday night after the COLD contest. I still have'em too, somewhere....
Second, I'm pretty sure it wasn't a doughnut, I believe it was, in fact, a butterhorn. I think he finished it somewhere around the triangles, so it wasn't too much of a factor after that, other than it seems to me that he was still licking his fingers partway through the square eights.
I feel privileged to have watched the flight. It isn't burned into my brain like the first time I saw The Bomber pull out inverted 6 feet in front of my nose, but it was still pretty cool 
Btw, it was Loren Andersons Ukey 35 & it had a stock Fox .35 in the nose & a 4 ounce plastic uniflow tank. After the flight Loren asked Paul what needed to be changed & Paul told him to move the CG back a tad. That was the only change he suggested.
Nils
While kit bashing my Skyray I added stab and elevator area but kept the proportions (Stab to Elevator) the same as they were. I also added small fixed flaps, samller than what a Flight Streak has. I think that probably equals out to about what Brett said he would do to modify his Skyray. My modifications are to help use a more aft CG than the standard 15-17% and keep the control system response relatively slow around neutral for better level flight tracking. Hope it works O.K.
Jim Pollock