My Stechmücke Video Clips

[gjohnson]

I stayed up really late Friday night and finished my Stechmücke (small stinging flying bug, or mosquito). It's essentially a small version of a Kolibri/Stubenfliege. My final AUW was 29.5g on two cells. It uses a 6.6:1 geared LV M20 (E-Charger), a GWS 6x5.0, and the DU radio system. It has a 19-inch span and about a 1.22 ounce/square-foot wing loading. Some still pics are over on the Ezone http://www.rcgroups.com/forums/showthread.php?s=&threadid=31814. I'll put some more detail on my web site when I get a chance. In the meantime I have this one video clip on my web site http://home.attbi.com/~gordonjohnson/ (BTW, this is my new site after moving everything over from the mediaone domain). I hadn't really thought I could get this done in time for Saturday, but made the push, details will have to come as I get the time.

It flies really slow. I tried it in my family room and could get about one circuit before running into a wall. With practice I might be able to do better. Otherwise, the search for a true living-room flyer continues -- but I think I'm on the right track.:)

Regards, Gordon

[gjohnson]

Dave,
I have a couple of questions for you. I need to post some pictures of my Stechmücke here, but just haven't gotten around to it. Using formulas you gave me some months ago, I've computed that the flying speed is about 5.2mph. Also, Dave Lewis has posted that Jean Louis Coural has determined that the optimal combination for a KP-00 (LV M20) is 10:1 gearing with a 23cmx9cm (9x3.5 in) prop. I finished my plane late the night before an indoor fly, so I grabbed a M20 with 6.6:1 gearing off my static test stand and slapped a GWS 6x5.0 prop on it (the largest of the smaller GWS props). I knew that this would probably not be the optimal combination, but it was 1:30 in the morning.

Anyway, using static results for this combination, a static RPM of 2340 with the 6x5.0 prop yields a pitch speed of 16.3mph. You have previously told me that you like to see a pitch speed of about 1.5x flying speed, which in this case would be 5.2 x 1.5 = 7.8mph.

If I simply take Coural's recommendations and scale the RPM down to reflect the change from 6.6:1 gearing to 10:1 gearing, with a 9x3.5 prop I get a static pitch speed of 7.5. This is amazingly close to what your 1.5x requires.

Given the low speed this plane flies at, is this static approximation reasonable? Is scaling the RPM down reasonable? I'll check it on the stand once I've made a prop. I've recently got the prop building jig and related supplies from Anything RC and am about to jump off into prop building land. So, I'm trying to figure out in advance a ballpark range for what size props to build. Is there anything I'm missing here, other than a wind tunnel (I haven't started it yet)? :D

Regards, Gordon

[Dave Robelen]

Hello Gordon,
First, congratulations on building such a slow flying airplane. It sounds like a bunch of fun. Your approach to selecting a prop by scaling sounds quite reasonable considering all of the variables. I have really benefitted from using that pitch speed mesurement as a starting point. Ultimately the combination of the gear effects, motor losses, and battery performance will give you a more refined measurement to work with. None of these estimates are "cast in concrete" but rather useful starting points to get in the air.
Regards, Dave

[gjohnson]

Finally got around to putting a picture up. This is before final mounting the motor and shortening the prop shaft, and final wiring. I should be able to save another gram or so, so I'll post a better picture when I've refined a few things.