PAINLESS

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Been a while since I've posted here. Putting together a .46 size mustang with a TT .61 pro with a macs header/pipe and wondering what prop to start with. I have zero experience with 60 size engines. Planning to use 20-30% nitro and some port timing mods. Shooting for 17k on the ground. Any advice would be appreciated, thanks.

Pylonracr

Prop selection depends largely on airframe drag and the speed you intend to fly it. It sound like you are going for top speed here. Has the engine been broken in? I am not sure how a TT61 will like turning 17k as I have never had one, but I would probably start with a few props in the 9x9 to 10x10 range and see how the engine likes them. Keep in mind the pipe needs to be tuned for each prop, so plan on a gallon or so of fuel and the better part of a day.
Keep us posted on your results.
Scott

PAINLESS

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Plane and engine are on order. Hope to start testing in january. I plan to do some warbird racing in the silver class and need to get about 120 mph out of it to be competitive. I have done some port mods on both a TT 40 and 46, they respond very well to the dremel tool.

MJD

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At 17k you should be able to get 120mph with 7" of pitch on an APC sport prop, though you might need 8" depending what you can squeeze out of that engine for rpm. As it is a .46 size airframe, I'd try a 10-8 - the 10" diameter will give you more traction out of turns, if the top end is competitive. But if you want instead to push more on straight line speed a the expense of some disc area, then I'd investigate higher pitch 9" props. Might want a bit more carb bore for the higher rpm, but YMMV - I don't know what the TT has stock.

Pattern planes were rocketing around at 115-120 mph on 11x7.5 - 11x8 props on piped .60's not that long ago.

Rough rule of thumb is your limiting pitch speed will be around rpm in thousands times pitch in inches. APC props have true pitches higher than the designation so often this works in your favour. So for quick math - 17k x 7" = 119mph. 15k with 8" pitch = 120mph. (IF the horsepower is there to fly at those speeds!) This gets you in the ballpark.

I'm leaning towards spinning a 10-8 fast as you can. Unless 9" diameter works well, which will need experimentation to find out.

Using excessive pitch for the available hp and resultant airspeed can waste hp by operating the prop blades at a less efficient L/D.

PAINLESS

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I have ordered a 10x10 and 11x9 to start with. Currently I have a 40 size mustang with a TT46, header and tuned pipe running 15% fuel. I get 16.5 rpm with a 10x7. I increased exhaust duration by 10 degrees. Guesstimates are 120 to 125 mph by guys at the field. It weighs 4 lbs 12 oz without fuel. I have a 9x8 I want to try on it.

Years ago I flew some control line 80 mph combat and thru some testing found the master airscrew props we were using to be about 80% efficient but the engine gains rpm once launched so it was a wash as far as calculating mph.

PAINLESS

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A scale plane like this typically has a large diameter spinner, on the above plane it's 2.75". Subtract that from the 10" diameter prop and your left with 7.25" effective diameter. Is that the right way to think about that?

speedracerntrixie

My Feedback: (29)

That airplane is pretty draggy so I would stick with 11 " props. I ran a Rossi 61 powered pylon racer for a couple years and tried quite a few prop sizes and pipe lengths. In the end what worked best was a 8" header and full length pipe and APC 11x11 spinning at 12K on the ground. The engine was set up that it would not come on the pipe until the airplane got close to around 100 mph. My suggestion for your setup would be to increase exhaust duraration to 175-180 degrees, high quality bearings long header and an 11x9 APC. If this setup falls short of 13K start trimming the header. Once you see a jump in RPM switch to a 11x10 and see if you can get that above 12K.

MJD

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Perhaps, but the disc area close to the hub isn't doing much work anyway. Reynold's numbers increase as you move outward. You could also imagine that reducing the turbulence/drag around the hub area and fairing the power system into the fuselage to reduce drag coefficient is of more importance than using the inner portion of the prop.

On that note, that poses an interesting question. Has anyone ever quantitatively (it took me three trys to spel that rite..) measured an increase in rpm (or the opposite) between no spinner and a balanced spinner of sensible size? Hmm.

The percentage of pitch speed you achieve, or efficiency as you termed it, is so-o-o dependent on the airframe. So you might achieve 50% on one aircraft while achieving 85% on another. It is not unheard of that the airspeed on highly powered aircraft exceed these theoretical numbers (in fact, this has been specifically reported with TT .46's and 10-7 props.. by airraptor IIRC). This could easily be from unloading in the air, but also because there is more than one way to measure pitch - the pitch from the lower surface datum line is always lower than the aerodynamic pitch for example. Roughly the same deal as the zero lift angle of an airfoil is often negative in relation to the chord line.

In the end anyone will tell you - use these napkin calcs to get you in the ballpark, then experiment to find the best combo. There are a lot of variables at work simultaneously.

I'd still put about $3.50 (but no more..) that a 10-8 or maybe 10-9 might be the quickest. If the drag is low enough then the higher pitches will win out. It will be interesting to see.

Or as speedracer says, maybe 11" props work the best!

speedracerntrixie

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MJ the reason I say to stick with 11 inch props is that I currently race the format that Painless is talking about. Lots of guys are running the World P-51. The popular setups are the .46 mustang with a YS 70 and 12" APC. The guys flying the .60 Mustangs are running race wings and YS 115 spinning 14x14 APC. The course is two pylons 700 feet apart. To be competitive in silver class the airplane needs to be around 130 mph. The airplanes are pretty draggy and loose a lot of speed in the turns, the larger dia props will help accelerate back up to speed after the turn at a loss of some strait away speed.

Im im currently working on a new race plane myself. A Macchi 202 with 515 sq in at 8% thickness! retracts, 6 lbs and powered with a YS 115 WS spinning a CF 13X15

PAINLESS

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Speedracer, (one of my favorite cartoons as a kid, I'm dating myself) any idea what kind of mph you were getting with that Rossi combo?
I recently converted a glow plane to electric and spend way to much money trying to find the right prop and in the end still wasn't happy with it, was hoping to avoid that situation.
Ideally the only way to know for sure is to get a radar gun on the plane, will have to pursue that. I also have an on board tach I will try to mount, it records highest rpm reached in a flight.
MJD, yes there are a lot of variables at work. Anyone who has raced anything will tell you that all the little things add up. I enjoy tinkering and experimenting as much as anything else.

PAINLESS

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I raced a dago red w/ a 91 4 stroke 8 or 9 years ago in the silver class. it was swra back then and since then they have lowered the break out times. I got a first place at speedworld and some third places at Tucson. I ran a 12x10 and guessing the plane did about 105 mph on 30% fuel.

speedracerntrixie

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The 61 powered airplane ran about 150 mph. Not quite fast enough to run gold. I did end up clocking a 1:27 with it on a practice flight. The Macchis are my own designs and quite a bit cleaner then the WM Mustangs though. I would think that your setup running an 11x9 in the 13K range will do well in silver. My silver airplane was powered with a Rossi 45 spinning an APC 9x7.5. Best time with that one was a 1:39. I broke out more times then not but it's all fun. You may want to spend some time looking through the scale racing threads here on RCU. Lots of good info there.

PAINLESS

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wasn't that rossi 45 powered plane for sale on rc groups a few weeks ago? I had asked you about prop size and rpm. Did it sell?
That yellow plane pictured is a house of balsa 40 size mustang with custom foam wing and a lot of beefing up. No retracts though. It's way faster than my 424 q500.

speedracerntrixie

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Yes on of the guys I beat with it snapped it up the same day. It was mainly designed for a race series here in the SF Bay Area where we are limited to a max displacement of .46. Imagine that same airplane 5% larger with retracts and a YS 115

MJD

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I'm with you on that, and certainly defer to those that fly exactly the same stuff. I didn't see where he said which model it was other than a ".46 size Mustang". My thought process was based on "hmm.. .46 airframe, 10" prop disc area should do the trick, how much hp are we really getting from that motor, and I have had some good results in the past with .40-.46 aircraft on piped .61's set up for 10-8 and 10-9 props, etc.". I might have been using cleaner airframes than the WM P51..

PS that Macchi sounds like a riot!

speedracerntrixie

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MJD, if the goal was strait line speed I would be right there with you on the 10X9 or 10X10. I did assume it is a WM mustang .46 due to them being so popular with the race guys. Really draggy airplane, mainly the wing. Its a really thick blunt airfoil.

PAINLESS

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It is a WM p51 that I'm getting. I can generate airfoil patterns and cut foam if I need to, that's what I did on that yellow mustang.

speedracerntrixie

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Let me know when you are going to cut some foam wings. If you are game I have an airfoil that has been working really well for me. I would be happy to send it to you.

PAINLESS

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Is it a standard type airfoil like a selig? Do you have a number for it? I use solid works and down load point files from a web site that I found. I can scale them in chord and thickness separately if needed.

speedracerntrixie

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it's an airfoil I designed for warbird racing using our speeds and loadings with attention to maintaining energy in the turns.

PAINLESS

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I'd be curious to see it, can pm it to me?

MJD

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Capiche - 11" is making a whole lot more sense then. 10" might not be enough rubber on the asphalt.

speedracerntrixie

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Painless, I don't have a plot of the airfoil. I can scan it but not sure if it would come across accurately. Best thing to do would be to mail out a couple templates.

PAINLESS

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I'm not really planning to make wing unless it's the only way to get a little more speed out of it. The plane showed up yesterday, expecting the engine monday.

Pylonracr

Oh Sure
I do not post for a week and this thread explodes. MJD and Speedracer are correct. I was under the mistaken assumption that you were going for all out speed. For pylon racing you need to make the obvious compromise between straight line speed and acceleration off the corners.
I would probably follow Speedrader's lead here as he has the experience in your racing class.
There is a wealth of information and people willing to help here on RCU, one only needs to listen.
Good luck with your racing.

Scott