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Author Topic: Prop recommendation for peanut Cessna Skymaster  (Read 443 times)
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strat-o
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« on: August 26, 2018, 12:19:16 AM »

Hi, I am building a peanut rubber-powered Skymaster and hadn't though much about the propellers until now.  I measured the clearance for the rear propeller and was a bit surprised at how narrow the space is.  It looks like the prop will have to be under 3" in diameter (probably 2 3/4").  So I'm starting to think about going with matched 3-bladed propellers front and rear.  One specific reason for wanting to go with matched blades is I'm planning to run a single loop driving both props.

My performance goals are modest as this is a fun flyer and not for competition.  I'd like to be able to get it to gain altitude from launch and maybe get 10 - 15 second flights.  I'm shooting for 7-10 grams weight.

My thinking on the multi-blade approach is to try to get the most bite out of the air as I can and hopefully slow the motor run since both props are pulling off winds from same the motor.

Does it sound like I'm on the right track here to meet my goals?

Thanks,

Marlin
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USch
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« Reply #1 on: August 26, 2018, 04:44:17 AM »

Not a real expert on this argument but just my 2-pence worth.

A 2 3/4" propeller even at peanut scale size doesn't sound like a much efficient power train. Even more so as you would have to work with a similar prop size up front following your idea to have them spinning from the same rubber motor.

So why not have a fake propeller on the rear and a normal 5,5" or 6" prop up front? Also the launch would be much easier than a Peanut with 2 propellers to hold and launch without getting caught in the tailplane  Wink

Urs
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mescal1
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« Reply #2 on: August 26, 2018, 08:01:39 AM »

For fun flying, the 2 3/4" prop will give you short quick flights unless you increase the pitch and width.  I'd play around with it.  If you can end up with decent flights using the short props, you can always follows Urs advice later for longer flights.
Mike
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strat-o
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« Reply #3 on: August 26, 2018, 03:27:38 PM »

Thanks Urs and Mike.  That gives me a good perspective.  I will probably go with a dummy prop in the rear that is scale and windmills.  If it is scale it shouldn't add too much drag.  I'll give it a coarse nearly feathered pitch so it rotates but not too rapidly which should help cut down on drag.

Marlin
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tom arnold
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« Reply #4 on: August 26, 2018, 04:21:14 PM »

For whatever it is worth, here is my adventures with a 24"span push-pull Do335. I tried every combination of props and motors possible and they all worked....to varying degrees. With 2 props and 2 motors, I found the CG to be moved quite a ways rearward. If you draw out the position of the 2 motors, it becomes apparent that the "doubled motors" are behind the CG and combined with the necessary free wheeler, I needed to add a painful amount of nose ballast. It flew fine though and I launched it by pinning the rear prop and attaching a long string to the pin and my right wrist. When I launched it, the string played out and pulled the pin out a number of feet from my outstretched arm and it was like an afterburner kicked in. Very cool and a lot of fun to watch.

With both props powered by a common motor, the CG moved forward a small bit and an equally small bit of nose ballast could be removed to keep things in balance. It was launched the same as above and, you are quite right, the motor run was only half as long but there was no climbing the rear hook problem, for obvious reasons.

Then I flew it with only the front prop powered and a free-wheeler on the back. It was a VERY draggy arrangement and really soaked up the power even with a high pitched rear prop.

Finally, and the way I flew it for years with great flight times was with the front prop powered and the rear prop blades removed with only a dummy spinner. I also found that in addition to needing less nose weight, in the Do335's case, I could use a shorter motor (1.25 X hook-to-peg distance vs 2.25 times) for the best flight times.

Whether any of this can be of any use to you with the Skymaster, I don't know but it will look cool in the air no matter what you do!

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strat-o
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« Reply #5 on: September 19, 2018, 11:23:31 PM »

Tom, responding a little late here but appreciate your advice.  I think I'll go with a simple single prop on the front and just put a dummy spinner on the rear.  I think push-pulls are neat.  I built a Do 335 but never completed it.  At some point I might go with an electric push-pull which I think would solve some of the problems presented with a rubber powered model.  One thing I just noticed is the model numbers Do-335, C-336 and C-337.  Coincidence?  I think not!

Marlin
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flydean1
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« Reply #6 on: September 20, 2018, 11:16:51 PM »

On rear props, I remember reading a flight test of a Cessna 337 concerning the noise of the rear prop due to exhaust gasses blowing through it.  "Everyone in town knows when you are leaving town".

Had a guy who I trained for  his multi engine rating.  The day after  his checkride he and some friends went down to Opa Locka, FL to retrieve his Cessna 336 (a Skymaster with fixed gear) and ferry it back to Lakeland to continue it's rehab.  Well, it seems the nose engine failed just after takeoff on rwy 9, and wouldn't feather.  At exactly 70 kts with the stall warning screaming he staggered along looking for a place to put it down.  Turning left over a 6-lane street he decided to not try that due to heavy traffic.  Seeing his chance to turn again over a used car lot, he now was going west.  BTW he was below the tops of most of the buildings.  The tower had lost sight of him and was watching for smoke to tell the fire crew where to go.

One more left turn and he was headed back toward the field, only one more obstacle, a power line.  Waiting until the last second he horsed back on the yoke and got the nose wheel and one main over the line.  The other snagged the line which then blacked out much of that part of Miami.  The tower and his friends were looking in horror to the east expecting the smoke plume when he plunked down in the grass just across the runway he had taken off from!

The experts opined that he was saved by the fact that the rear prop was slightly more efficient than the front.

He called me at home that night.  He said he could hear me talking to him all the way around back to landing.

That said, you may want to put the dummy spinner on the front and power the aft prop.

Keep us posted.
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strat-o
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« Reply #7 on: September 20, 2018, 11:41:55 PM »

I would put a rear prop on if I could but the limited clearance of the booms makes for an impractical prop diameter.  Hopefully I'll be able to put together a build thread on it before long.

Marlin
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