Staggerwing Loop

[Don Cooper]

I have a scratch-built Beech Staggerwing - span 25.5", weight a little under 5 oz including everything but battery. I usually fly it with 7-cell nicads which weigh about 2.8 oz. Motor is IPS-B with 10-8 (I arrived at this after experimenting with IPS-S2, -A and -B with various props). My problem is I can very seldom get it to loop. If I give it up and a very small amount of rudder, it performs a snappy, relatively small radius cross between a loop and wingover. If I give it up with a lot of rudder it performs a fast rotation, relatively tight barrel roll. But if I try to do a pure loop it will occassionally perform one, but the vast majority of the time it will go vertical for a few feet, then hang on the prop for 2 or 3 seconds, then fall off on one wing or the other. I have tried moving the C.G. back until it gets too twitchy for my taste, and increasing the elevator deflection, again until it gets too twitchy for me, with no noticeable effect on the loop characteristics. I have a hard time believing it is a power problem, partially because of the way it will go vertical, then hang on the prop, and partially because it is lighter, faster and has a much better max climb rate then my GWS Cub which loops readily after a shallow dive.

The only possibilities I can think of are that the elevator stalls or is blanketed and doesn't provide the pitching moment necessary to bend it over, or the wing stalls and doesn't provide the lift vector necessary to bend it over the top. Any thoughts/ comments/ suggestions?

[Dave Robelen]

Hi Don,
While the various surfaces are probably stalling rather early, My guess is that you are short of "Pitch speed". A combination of RPM's and the pitch. If you divide the RPM by 60 to get RPS, and then multiply by the pitch of the prop in feet, you will have the pitch speed in feet/second. Multiply by .682 to get MPH. One quick check in flight is to drop the nose in a dive. If the model seems reluctunt to accellerate, then probably you need either more RPM or pitch (or a combination of both). It takes adequate airspeed to fly through a loop, and it sounds like you have plenty of thrust, but the prop is peaking out too early for true aerobatics. If you are running GWS props, they can be re-pitched rather easily with a hot air gun heating the hub area and twisting. A gage to match the two blades can be made from card stock. I would be inclined to put one size smaller prop on and then add pitch.
Let us know how it goes, Dave

[Don Cooper]

Dave,
Thanks for the suggestion. I will try it.

Don

[Don Cooper]

Hello Dave,
There were finally enough calm days in a row for me to do some systematic testing and get some repeatable results. I have not yet tried customizing a prop but have verified that the Beech picks up speed in a dive in a hurry with the IPS-B/10-8 combination I am using. I have also verified that in a narrow range of elevator application I can get nice big round loops. Too little elevator and the IPS runs out of power before it can pull the plane over the top. Too much elevator and I get the 'vertical and hang on the prop' mode . An interesting characteristic is that if I start out with low elevator deflection, then at about the vertical point give it more elevator, the radius of the loop increases significantly, with large elevator deflection giving the straight up and hang on the prop mode. This indicates to me that the elevator is stalling for larger deflections, and the range of deflections between elevator stalling and giving too large a loop radius is small. Does that make sense?


Don

[Dave Robelen]

Hi Don,
It sounds like either the elevator is stalling, or the wings. One way to sort that one out is a slow, steady pull on the stick while reducing throttle so that the bird maintains altitude but steadily increases angle of attack. If the elevator is stalling, the model will stay level, and at some point the elevator will "run out of steam" so that further deflection would have no effect. If the wings are stalling, I would expect the model to get touchy in roll, and maybe drop a wing before the elevator reaches the limit. There are probably some interesting things going on with the downwash behing the negative staggered wings that could be involved with this phenomenon.
Cheers, Dave