Don talks with Steve about Roadkill battery packs and high altitude flying!
From : Don Stackhouse
I'm glad to hear that it went well!
That should be plenty. The width is usually the limiting factor, and we regularly fly ours in a 40' width.
We raise the backboards up when we fly. I decided to try it with the 6 x 50 nicad pack. After some fast taxiing and a short, straight ahead hop I gave it the "gas" and it took off. It flew a lot slower than I imagined it would, but it took a lot of runway to get airborne. I did put on the turbulators, but who knows if they really help?
Sounds like the RPM and prop pitch might be a little marginal for your 4740' msl altitude. With the same voltage you're getting the same RPM's we get at 1000', but your thinner air means you're not pulling as many amps. You also need a higher true airspeed, which the prop doesn't want to give you. It also takes a longer takeoff run to accelerate to the higher airspeed.
You're trying to take off with not enough pitch+ RPM, and at about 86% power, in thinner air. Imagine what would happen if you had a governor failure on your Cessna 180 that left your prop sitting on the low pitch stop while you were trying to take off from one of those mountain airstrips, and then you retarded the throttle to hold the rpm down to 100% as the airspeed increased. You'd probably get a very close look at the terrain at the far end of the runway, right? That's essentially what's happening here from the combined effects that thin air has on the prop and on the power it absorbs from the electric motor.
A seventh cell will boost the RPM and the current draw. These should offset most, if not all of the effects of your altitude.
My only problem now is to add the 7th cell-the takeoff run is about 50' and that is getting close to the far wall.
The increased power should give you a shorter takeoff run, despite the extra weight. It should also allow you to sustain a tighter turn. Both takeoffs and steep turns are essentially climbing maneuvers, and climb is the parameter most affected by a power increase, especially on airplanes with marginal power reserves. For example, if an airplane required half its power just to sustain flight, a 10% power increase would increase climb rate by 20%. If it needed 90% of its power to sustain flight, a 10% increase in power would DOUBLE the climb rate.
One other observation: it seems to fly in a left turn with a 5 or 6 degree right yaw. I am thinking I might re-set the rudder with a little left when the ailerons are neutral. Or perhaps I need a higher ratio of rudder to aileron. It takes very little aileron to hold the turn. Have your prototypes ever done this? I could rig some differential aileron, but as you can imagine that would be a last resort due to hassle factor.
No, don't worry about differential ailerons. This is not an uncommon problem, and it's a rudder trim issue. Just adjust the rudder neutral position until the plane turns the same way in both directions.
If it still tends to yaw to the outside of the turn for both left and right turns, it means you probably need to try to fly faster. Stall warning on these little warbirds is marginal, much like their full-scale counterparts. It comes too late to do much good, you usually won't have enough room indoors for a recovery. Somewhat before the stall the planes get wallowy, the controls get soggy, and they tend to yaw to the outside of the turns. That's your signal to give it more power and to get the nose down a bit more.
If you're still finding the P-51 a handful in your flying area (my guess is that you'll do fine once you get used to it), you might try one of our new A6M2 Zeros. The first 6 warbirds all have similar spans and span loadings, but the wing on the A6M2 is about 2" longer. This makes it much happier when doing tight maneuvers at minimal airspeeds in constricted spaces.
Anyway, she flew and all were astonished to see it. My buddy took about 30 sec of video and we'll play it back at the club meeting in Jan.
I will look for your 7 x 110 NMH packs on your website. That might be just the ticket for flying at this altitude. Run time isn't an issue- too much excitement to fly more than 30 sec!!!
I let Keith Shaw fly the A6M2 with the main gear removed (just loosen the screw and slip the wire strut out from under it, then tighten the screw back down so it doesn't get lost)) at an outdoor e-fly in Ft. Wayne, in about 8-10 knots of wind. With a little help from a few weak thermals, I timed his flight at over 18 minutes, with a fair amount of aerobatics. Some of the other fliers who tried it got typically around 12-14 minutes, with lots of loops and rolls. The Mustang with the 7-cell pack has a little more induced drag, so it should get a little bit less than that, probably about 10-12 minutes if you fly efficiently. For indoor flying that's plenty. Joe likes to do several landings, taxi-backs and takeoffs during a run, including some near the end of the charge. Folks are usually impressed by an enthusiastic and very scale-like takeoff after 8 minutes of flying. However, I think Joe might be doing this at least partly to give his nerves a rest. Flying indoors in a confined space is definitely good for your adrenaline production!
Don Stackhouse
DJ Aerotech
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