|Mar 03, 2002, 09:55 PM|
Need More Power
I need your help!!! As many of you looked at my recent building project posted earlier on this board, I was out testing to see how it would perform. The initial test flight showed I had too much down thrust built into the motor. After correction, the model flies better but still not what I was after. A bit more looking and I decided to try "drooping" the airlerons a bit (I think they may have been reflexed (up) a bit. Wow, now it shot up and was really flying on the wing. BUT... I still did not have the power to get out of trouble in a 2-5 mph breeze. I bet it would really perform at sea level but I'm at 5000' here in the desert southwest where calm days are uncommon!!!
Wing (including airlerons) 24" span - 168 square inches
(without ailerons) - 132 sqare inches
Weight - 6.6 ounces ready to fly
Power GWS (EPU6) 5.4:1 gear ratio turning an 8x6 prop
(I did get better performance when bending more pitch into the prop but this is a temporary fix at best)
Battery, Qualcom Lithium (only weighs 2 ounces producing 8.4 volts initially)
OK so what do I need? A few hundred additional rpm out of the motor would work! Same RPM with more prop would also work! Some of you might say shed a few grams on the weight, which I could easily do but then it would be unflyable 90% of the time here in the breeze! I am already thinking about a better airfoil, but verticle performance is not improved by airfoil, only power!
OK let's here it---what exotic things can you come up with?
|Mar 03, 2002, 11:26 PM|
In my opinion, getting rid of weight will only help, not hurt. Your problem is not having the airplane thrown around by wind, it's getting it to power out of trouble, and reduction in weight helps that.
That said, have you run an ammeter inline with the motor/esc? See what you're pulling static and see if you can't creep up to something like the GWS 9x7 without ticking off the lithiums. Just an idea :-)
I've found that I can't fly my Tiger Moth in the 5 mph stuff either. I have to wait for calm days or before dark sets in, then I go up and have a flight or two in the relatively calm air.
|Mar 04, 2002, 12:37 AM|
Thanks for the advice. I will attempt to squeeze some weight out of it on the second prototype. I'm thinking thinner wing skins, and a few other weight saving ideas. It survived multiple impacts with the ground today in the testing with very little or no damage so I know that I over built it!!!
That said, I experimented with motor/prop/battery combo's tonight to see what was really happening. 1st, with the Qualcom battery, EPU6 and an 8x6 the best I could get was 3800 rpm. I then tried my 7 cell 150mAh NiCd pack. Same 3800 rpm. I then stuck a 270mAh pack 7cell on it. This got a responce of 4100 rpm. Only problem is the pack is .65 ounce heavier than the Qualcom pack. Then, I found my EPU3 (currently on my son's trainer) and tacked it with the Qualcom pack and 8x6. 4400 rpm was registered!!! Looks like my son is going to be looking for a new motor soon!!! Actually, I'm sure his will fly great with the EPU 6 and his is not nearly as sesitive to the weight of the 270mAh packs.
So, looks like I'm waiting for another calm day...
|Mar 04, 2002, 05:56 AM|
LittleMoose: The EPU6 (IPS DX-A) is rated for a 10x4.7 or 9x7 prop, at under 2A for a 7.2V supply. The 7 cell NiMh (270-300mah) will give virtually identical power to the Qualcom 830's. Is this the battery you're using?
Sounds like you are way under-propped.
The EPU3 (DX-1S) is rated for 7x6 and 8x4.3... The 8x6 is slightly over, but will probably work. You are likely at about 2.5-3A with it. This combo will probably be comparable to a 10x8 on the DX-A, which is a good setup, but shortens motor life considerably. Add a heatsink!
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