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Oct 09, 2019, 11:45 AM
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Discussion

Prop size and type recommendations .


Voodoo Mustang 46 with an OS 55AX. Prop recommendations? Wood? Nylon? Size?

looking for speed here.
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Oct 09, 2019, 11:52 AM
A man with too many toys
APC are the most efficient so that's what I use most of the time.

Are you using a stock muffler, Jett-stream tuned muffler or something else. That does make a difference.

With stock muffler a 11x7 would be a good place to start.

After the engine is completely broken in (2 to 4 gallons of fuel) you should do some experimenting to find the best engine/airframe combination.

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Oct 09, 2019, 12:04 PM
AMA 46133
SeismicCWave's Avatar
Quote:
Originally Posted by paulwgraber
Voodoo Mustang 46 with an OS 55AX. Prop recommendations? Wood? Nylon? Size?

looking for speed here.
APC 11 X 6 to 11 X 7.
Oct 09, 2019, 01:16 PM
Registered User
Thread OP
Stock exhaust. So how does it work : a 11/7 would be 11 degrees by 7”? Are smaller props faster?
Oct 09, 2019, 01:20 PM
A man with too many toys
You will get a lot more speed if you upgrade to a Jett-Stream tuned muffler. I have them on two engines and it really does make a huge difference. Those cheap stock muffler are good at making quite but not power.
http://www.dubjett.com/mufflers2015.html

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Oct 09, 2019, 01:26 PM
A man with too many toys
Quote:
Originally Posted by paulwgraber
Stock exhaust. So how does it work : a 11/7 would be 11 degrees by 7”? Are smaller props faster?
11/7 prop is 11 inches in diameter with 7 inch pitch.

Smaller props are not always faster - it's all about matching the prop to the engine and airframe. It's totally different than for electric airplanes.

In general smaller diameter props are better for speed but sometimes reduce takeoff and climb performance. Larger diameter and lower pitch provide faster acceleration, quicker takeoff and hold airspeed better in climbs.


You really do need to do some flight testing with several props.

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Oct 09, 2019, 03:08 PM
AMA 46133
SeismicCWave's Avatar
Quote:
Originally Posted by paulwgraber
Stock exhaust. So how does it work : a 11/7 would be 11 degrees by 7? Are smaller props faster?
Propellers are expressed in inches not degrees. 11" diameter by 7" theoretical advance per revolution with the fixed pitch.
Oct 09, 2019, 05:53 PM
Registered User
Thread OP
OK. Im going to check out those mufflers. I was looking at tuned pipes before because they were very $$$ and seemed like a lot of work.
Oct 09, 2019, 06:52 PM
------
coriolan's Avatar
Quote:
Voodoo Mustang 46 with an OS 55AX. Prop recommendations? Wood? Nylon? Size?
looking for speed here.
???
Quote:
Stock exhaust. So how does it work : a 11/7 would be 11 degrees by 7”? Are smaller props faster?
Curious how it will end up!
Oct 10, 2019, 07:07 AM
A man with too many toys
What's the wing area and estimated weight?

Will it have retracts?

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Oct 10, 2019, 07:49 AM
AMA 537620
ChillPhatCat's Avatar
Sorry I wrote a book.

So an 11x7 on an OS 55 AX would probably get you to roughly 90-95 mph... do you want more? A 10x8 or 9x9 would also be in the same loading range and should get you past 100 mph.

Also to more directly answer the question about "smaller props". Speed has little to do with the diameter, it's mostly about pitch. As mentioned above the pitch gives how far forward it would move without losses, per revolution... so a higher pitch prop at the same RPM as a lower pitched prop will give you more speed... an 11x7 at 14k RPM will give you more speed than a 12x6 prop at 14k... provided the two props are equally efficient.

The only limitation is if you make the diameter too small, because diameter is what gets you thrust. So that 11x7 at 14k will get you less thrust than the 12x6 at 14k. You need enough thrust under the curve to overcome the drag at top speed... so if you put say, an 8x10 prop on there... you'll get a ton of RPM and a ton of theoretical speed... 132 mph at 14k RPM, but the thrust curve might only be able to propel you to 110 mph... that is, it hits 0 lbs thrust at 110 mph and can no longer accelerate.

All that said, you pick a high pitched prop that the engine doesn't over-rev or cause too much load, and check it against similar props, typically you can trade an inch of diameter for an inch of pitch if the change is not more than 2-3 inches either way and it will load the engine similarly... a good exercise is to just multiply the two numbers together and stay near that total range. 11x7 = 77 sq inches, 10x8 = 80 sq inches, 9x9 = 81 sq inches.
Last edited by ChillPhatCat; Oct 10, 2019 at 08:05 AM.
Oct 10, 2019, 08:02 AM
A man with too many toys
Quote:
Originally Posted by ChillPhatCat
11x7 on an OS 55 AX would probably get you to roughly 90-95 mph... do you want more? 10x8 or 9x9 would also be in the same loading range.
Maybe - weight and drag are a huge factor. A 9x9 might be very hard to get off the ground and would quickly die out in a steep climb. Like I said you need to do a lot of flight testing.


A 10x7 with a jett-stream tuned muffler might be a winner. I still think that an 11x7 would be best place to start with stock muffler.

.
Oct 10, 2019, 08:17 AM
AMA 537620
ChillPhatCat's Avatar
Given the scale, I have not looked it up, but a 46 sized mustang is typically in the 50-55" wingspan range... and normally you're talking about an 10 or 11 inch prop with the recommended engine size.

I think that a 9" could provide enough thrust but I agree, it is not a certainty. A 10" prop will certainly haul the plane around though... I'd say starting with an 11x7 and a 10x8 would be ideal... maybe a 10x7 as you suggest, which could get you enough RPM advantage to beat the 10x8... see how that flies and if either one gets you where you want.
Oct 10, 2019, 08:27 AM
Registered User
Quote:
Originally Posted by RC Man
Maybe - weight and drag are a huge factor. A 9x9 might be very hard to get off the ground and would quickly die out in a steep climb. Like I said you need to do a lot of flight testing.


A 10x7 with a jett-stream tuned muffler might be a winner. I still think that an 11x7 would be best place to start with stock muffler.

.
The Voodoo is a pretty clean air frame for sure, and an 11x7 on a 55ax is a pretty safe bet for sure. However, if one would like to try and go faster with a different prop, ChillPhatCat's advice was spot on.

I would suggest to our OP that the same higher pitch props that will be necessary to go REALLY fast, are going to be the props that are going to use a LOT of runway slowing down. The bigger diameter with less pitch prop designs slow down way better, as the larger diameter acts like a larger brake. This being a clean design is the perfect combo to demonstrate what I'm talking about if you haven't already experienced it.

Point being, maybe start with an 11x7 to get the plane sorted out and get used to it a little, THEN try a faster prop (like a 10x8).
Oct 10, 2019, 08:48 AM
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Thread OP
Great great advice guys. I really appreciate it. I will start with an 11 x 7, get it sorted out then drop a 10 x 8 in. Definitely thinking of getting a Jett exhaust too. So a guy at my local club told me to get a wood prop vs apc. Any thoughts on that?


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