3 blade prop question...here we go again - RC Groups
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Dec 09, 2003, 02:29 PM
"Bible Thumper"
AMTJIM's Avatar

3 blade prop question...here we go again

Okay...a few questions here. I couldn't locate any readily available grish black magnum 3 blade props, but I did find a few sources for their tempest white nylon type. They still look a little small on the Mpx Cargo and they are white. I just noticed MAS has released a new series of of 3 blade props that are 6x4 and 7x4.
1) Do you think a 6v spd 400 could handle the 6x4 on 8 cells for take off and survive throttled back for regular use? Could I drop down to 7 cells? The flying weight looks like it's going to be 5 lbs with bombs included, I thought it was heavy until I saw a larger 5 lb B-24 fly this weekend on 4 spd 400's very nicley at 1/2 throttle.
2) Could the 7x4 be used on the GWS 350 drive that uses the 8x4 2 blade prop, or would it be crap since it was ment for high revs and doesn't has a slow flyer profile. I also may use these direct drive on some car motors for my sonic liner project.
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Dec 09, 2003, 04:32 PM
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I've done a bunch of bench-testing on the old Grish 3-bladers, and I like them a lot, but they seem to have quit the business. I've emailed them and phoned them, but they never fill any order I give them.

The MAS 3-bladers are fairly efficient. Not like APC E's, but good. A 6.0 volt 400 would turn a 6x4 direct, but it will draw 10-12 amps on 8 cells, which will make the motor life short. At 1/2 throttle in the air it would be OK. Maybe use 7.2 volt motors?

Try the 7x4 3-blader on that drive and see. My guess is it will work reasonably well.

Using Keith Shaw's old rule of 50 watts per pound, it's a bit of a surprise to hear of a 5 lb. model flying well on 4 SP400s. They're only good for 80 watts apiece input, and only 50% efficient at best.

Can you afford to buy four of those long-can 400s that Hobby-Lobby sells? The are heavier, but they do a bit better than the run-of-the-mill S400.
Dec 09, 2003, 06:03 PM
"Bible Thumper"
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Seems normal to me, most twin 400 planes weighed in at 2.5~3 lbs or more before Kans and lipos came around.