encan

Hi

ok, im going to build another piper j3. this time a smaller one. its again from herr engineering. its the electric version and it has 35.52 inch wingspan.

i had once also converted the airboss with the same span and put one of the new babe bees on it. it was too fast.

so i am guessing if i can slow it down a litle bit this time it will be better. How can i slow it down?

what is the bigest prop size i can use without hurting the engine. i have a muffler for the babe bee. is it a good idea to use it??

EC

XJet

To slow down your plane you want less thrust. You can get this by either using a prop that is too big, or a prop that's too small, or by fitting the prop on backwards.

In the olden days (who remembers them) we'd always test-fly our free-flight sports models with the prop fitted backwards to reduce the thrust. Worked just fine.

BMatthews

The backwards prop works great. Maybe TOO great at reducing the thrust. Testing will tell.

You can slow down the Baby Bee just fine and control the amount of power you get at the same time. The trick is to lower the nitro content and add some extra plug gaskets to reduce the compression ratio.

For starters get the right plug. There's three of them in circulation. A high compression trumpet one with sort of a squish band around the edge. A simple cone shape middle compression ratio. And the hemispherical low compression head. The first one comes with TeeDee's. The second with things like Black Widows and stuff where they want a bit more power. And the last one probably came with your Baby Bee. The low compression head is the one you want if you're running large props.

From there be sure you're using two or three head gaskets to lift the plug a bit further. Two or three gaskets for 7 inch props and three for anything in the 8 inch size range. Stick to pitches like 7x2, 7x3,7x4 and 8x3,8x4 and 8x5. In reality you'll probably want a BIT of zip so I suspect you'll end up using the 7x3. But to keep things mild for starters I'd start with the 8x4.

Air leaks around the needle valve are critical on the slow runners. I use a washer on the needlevalve with a short length of large silicone tubing to replace the spring. The washer keeps the tubing from working over the small shoulder on the needle stem. Cut the tubing just long enough that it touches and starts compressing at about 6 turns out from closed to prevent too much tension on the needle for adjusting.

You can then run on 5 to 15% nitro. 5 to 10 being the best. It won't like the hot stuff with the large prop and will overheat badly if you try.

Lynn S

try the backward prop

encan

if i put the prop backwards is it gonna increase the rpm causing shorter runs?? if so i would rather put a bigger rop and reduce the rpm. But then i need lower nitro fuel as Mr Bmatthews said.

Lynn S

I didn't mean to post after what Bruce said, I would go with his advice.

BMatthews

It's been a while since I did the backwards prop thing but I don't think it runs much faster OR slower. So the run time won't change more than a few seconds. But going for the bigger props so the RPM is reduced will certainly make your run times longer. With an 8x5 thinned down prop on the small sized Baby Bee tank I get about a 3:50 to 4:00 minute run with my 1/2A Texaco model.

mclintock

My Feedback: (3)

Does a cox engine in the low nitro, low-compression-via-gaskets state turning a large prop suffer any increased wear?
Or does the reduced rpm sort of cancel things out?

BMatthews

I've used the same engine in two models since 1986. It's probably got about 200 flights on it between the two models. It still starts and runs like a Timex watch, you know, "takes a lickin' and keeps on tickin'"

DICKEYBIRD

Install an exhaust throttle sleeve (assuming you can locate one) and a 3rd micro servo if you can afford the weight. This will allow you to slow your Babe Bee down for cruising yet power up for the climbs.