Don, I've got several comments to make, so I'm going to quote bits of your message...
"We must be doing something wrong, because to prevent stalling at the suggested CG, we had to lower the wing's leading edge until I couldn't be sure the wing and stabilizer were at different angles. It went from stalling in the glide to just descending in a straight line without pulling up, with just a tiny adjustment. So, we raised the wing's leading edge a bit, and moved the CG forward. Then it glided without stalling and with pitch stability."
You're doing nothing wrong in that regard. I saw no failure of the model to recover from upsets, so the trim is resulting in a stable model. Remember that there is more than one way to fly an airplane, and sometimes you can come up with multiple approaches which all provide similarly high performing results.
"Is it fair to test glide these models? Seems they probably won't have the prop stopped at any time in a powered flight. A stopped prop can't have the same drag as one at low power near the end of a flight."
I've never gotten useful data from test gliding lightweight indoor models. SO models are at the crossover point where you can get some basic stability information from test gliding, but you already have that information, having flown the model extensively. Therefore, don't waste your time.
"Even after it glided OK, we had trouble with stalling under (partial) power until we added a little bit of left rudder to tighten the turn. This was in addition to having some stabilizer tilt already. I ran out of ideas. The thrust line isn't adjustable. The propeller bearing presses into a plastic mount glued into the front of the fuselage stick. If it was adjustable, I'd have tried removing the right thrust and adding a little down."
Here's the meat and potatoes where you can improve flight performance. What you're doing by using the rudder to alleviate a stall is that you're dumping lift from the wing into a horizontal (turning) component. That's literally dumping energy out that you produce with the rubber motor. Never, ever do that on an indoor model with restricted motor weight. It's ok to use the rudder to establish a desired turn diameter, but not to dump lift. The fact that you're having to do this means you've got some pitch trim issues to address. One way is to add some downthrust (cut the motorstick if you have to), but only do this if the stall does not persist to the end of the flight. If the stall persists into the model's cruise and descent, you need to either move the wing back a little, or if you're using ballast to make minimum weight, move some ballast forward to get the CG a little farther forward. As I mentioned in a previous message, you also have excessive stab tilt. I can tell you from extensive experience that the trim adjustments you mention above are robbing time off the clock, probably 10-15 seconds, maybe more, just on the low powered tests you've been performing. It only gets worse as you add more torque.
"We know that flying with a partially wound full motor isn't as good for adjustments as shorter motors wound fully to have the proper torque at the beginning of the flight, But, it was the only choice we seemed to have, if we didn't want to end the testing. Jack had only one short motor. I wondered if he'd make another one there, but he didn't do so."
Partial motors are very useful. I've even used them on outdoor models on occasion. Well worth the effort. I find that they are actually very useful for trim adjustments. For the flight times you're operating in, you don't really need to do partial motors, though, unless you don't have retrieval equipment if the plane gets stuck on a full motor test.
"Jack is using a 15:1 winder, backing off five turns of the crank, 75 turns in the rubber, regardless of how many turns he flies with. So, he may be over winding and unwinding too much with shorter motors. A motor one fourth as long probably should be over wound and backed off one fourth as much.. "
That's probably too much backoff, especially if you're not reaching the ceiling. Do what's necessary to get there.
"He's using 0.056 rubber, because the 0.052 also supplied wouldn't make it climb on his previous outings. I'm not sure it that's width in inches or grams per inch, or even some other unit. But, if it is the same units, then his plane needs a lot more power to fly than your plane! But, Jack did say his model seemed to climb more steeply early on than those he'd seen on the internet, so maybe the rubber is too thick. His motor is at about 1,950 turns when wound fully. Your must be about 2,600 turns."
Don't worry too much about this. Give the plane the cross section it needs. If you run rubber too thin for the plane, you're just going to land with a bunch of unused turns and a smaller number on the stopwatch.
"We aren't using a torque meter. I haven't asked if Jack has one. I'm pretty sure he'd be using it if he did."
Ok, this is a *very* big deal. You absolutely must have a torque meter to get best performance. You have absolutely no way of predicting climb altitude without one. Rubber motor torque curves change with each usage, and if you follow that torque curve correctly, you get more energy in with each progressive winding (until the motor eventually says its had enough, but 4-5 flights is definitely a reasonable expectation). You also need to stretch the motor to about 5x its relaxed length, wind in 55-60% of the turns at that distance, and then work your way in. That gives you nearly the optimum result. This strategy also demands that you never, ever, under any circumstances, wind the motor on the plane. If you do, kiss the plane goodbye, because you are going to break motors spectacularly if you're actually trying to win.
The torque meter is an easy build. Seriously, there's nothing to the task of building a simple torque meter. You can do it in an hour or less from some basic refuse around the house and a 7" length of .015" piano wire. If you go to indoornewsandviews.com -> downloads -> The Best of INAV, you'll find the information you need.
-Joshua Finn
Received on Tue Jan 24 2017 - 10:12:32 CET
This archive was generated by Yannick on Sat Dec 14 2019 - 19:13:48 CET