prop suggestion?
 

I have a 3.5 lb, p-51 racer, 43" wingspan. I run a magnum .32 and a 9X7 apc prop. performance is good, but I think there's more to be had. I've tached the engine as high as 13270 rpm. The manual says the upper limit of the engine is 13000 rpm. What prop would be worth trying to improve performance for racing?

RE: prop suggestion?
 

I think 13,000 is too low. Start by going down to a 9-6. I think stock you would like to see around 16,000 or so. Make sure there is no baffle in the muffler. What I would do is get an Ultrathrust pipe or mouse can muffler (cheaper) and go down to an 8-7 or maybe one of the QM40 props that APC sells. This will get you in the 18,000 to 19,000 range. This smaller diameter should work just fine with your smallish airframe. Good luck and have fun experimenting.

RE: prop suggestion?
 

try a 8x10 prop. You can mod the engine but otherwise you shouldn't be looking at top rpm but top hp, beyond that torque decreases too quick (i.e. high rpm only achieved with a small prop) and you're better off swinging a prop with higher pitch at lower rpm.

Clement

RE: prop suggestion?
 

8X10 is way too much. A 8X8 will be best and a 8X7 will be a good second. Both APC sport props. The ultrathrust is good but expensive. A Macs Peace Pipe is not quite as good but a lot cheaper. A full tuned pipe is best, not cheap and will not look good on your P51. What kit?

The pic is a Tower combat P51, OS .32 and a Macs peace pipe. 125mph on radar...

RE: prop suggestion?
 

What he said.

RE: prop suggestion?
 

Model Tech. Fun fighters reno racers edition. Arf.

RE: prop suggestion?
 

WAY too much prop.... The 9x7 is better suited for .40 and .46 size engines.

The manual I have for the Mag .32/.36 touts a practicle rpm band of 2000-16,000

You will be much better off shooting for that 15 - 16,000 rpm mark, or even a bit higher.

APC 8x7, 8x8, 9x5 or 9x6, MAS 9x4 or 9x6
APC D-1 series 8.5x7.0 worth trying too :)

Bob

RE: prop suggestion?
 

Great minds think alike.....:)

RE: prop suggestion?
 

fhm:

I would jack up the prop and put a Jett 35 on it, then you really have a motor scooter. ENJOY

RE: prop suggestion?
 

If you want to add some power for not a lot of money, contact these guys and get a mouse can combat muffler. If you prop to the pipe properly it will give you a big boost. http://www.airscharnell.com/

RE: prop suggestion?
 

yup, those work pretty darn good on .25 fighters.

RE: prop suggestion?
 

Huh? You don't understand my Chinglish or have different opinion?

I think over-revving an engine only results in excessive wear and doesn't help in achieving top speed, rather use a prop with higher pitch much like going to 5th gear. The result of highest product of rpm x torque (i.e. hp) can achieve higher speed with a high pitch prop than over-revving a low pitch prop. Of course if you can pipe or mod an engine to rev faster without excessive decrease in torque then you have a new high in hp. The original poster is pushing his engine to rev beyond the manual's stated maximum (13000) and I think max hp occurs somewhere around 12000, and I believe 8x10@12000 may go faster than 9x7@13270.

Clement

RE: prop suggestion?
 

Depending on how the engine is stroked, it needs to operate at a certain rpm to be in it's power band. I think 15,000 to 16,000 is where that is. Even though the original post stated it was 13,000 I felt that was low for an engine of this displacement.
Pitch is less efficient thrust than diameter. (especially if hand launching) It takes a very strong engine to pull over sqaure props. Also I have a ton of experience running this displacement and these are the sizes of prop that have always worked best for me.

RE: prop suggestion?
 

Actually, over propping an engine screws up a great deal of design parameters, and creates a higher risk of over-heating an engine ..... and a hot/lean run does far, far more wear and damage to an engine than running at high rpm at correct operating temperaturs.

An engine can be designed/altered to run at a lower rpm - typically you want a smaller carb inlet (tighter venturi) that matches fuel/air flow to the expected massflow into the engine. Just as an example, I have an older Webra .61 engine set up for scale use ..... includes a ST40 carb on the front, and that combo happily turns a 14x6 prop around 10,000 rpm. If i tried that prop with the stock big-bore 7mm carb, the engine would be impossible to needle, and would tend to go lean the instant the nose was pointed skyward (poor fuel draw). And Id never see that rpm in any stable fashion.

Another brife example. When breaking in my Thunder Tiger .40 and .46 engines --dead stock, stock muffler ---- I use the same proceedure I use for all of my ABC engines. Target rpm, low prop load, slightly rich. For a TT.40 pro, I target 17,000 rpm on the test stand ---- this reached with a clipped 9x6 prop, and I adjust the mixture so it hits that number -- resulting mixture is slightly rich. After the first 3-4 runs (15 min) I will briefly take it to peak rpm with that prop (usually hits 17,500 rpm) then immediately take it back down to 17K. Subsequently, the engine is now properly broken in for use with a standard full blade APC 9x6 prop ---- and will hit 16,500+ rpm ground peak, which is my target for 424 racing. The engine will unload to 17k+ in flight.

RE: prop suggestion?
 

I concur about overpropping but if the engine can reach normal top end rpm range then it's not overpropped. I'm not familiar with the particular engine so I would rely on manufacturer's literature bacause engines of similar displacement can have vastly different design and output range; unless I obtain the figures myself on a test stand which I don't have.

Clement

RE: prop suggestion?
 

I think what some of use are trying to note here...

... there is a wealth of bench and flight and historical performance data on the Mag 32 and Mag 36 engines specifically .... and general engine experience.... which we are trying to share.

The point of someone posting a question similar to this original post in this forum I would imagine would be to harvest some of that data and experience, in the form of a prop recommendation. Avoid re-inventing the wheel or making similar mistakes other can caution against, while at the same time optimizing the performance for the intended application.

The original information on the manufacturer specifying a 13,000 rpm target in the manual was not accurate.

RE: prop suggestion?
 

Also I doubt anyone has run a 10 pitch prop on this engine with any success.

RE: prop suggestion?
 

13,000 from a .32 as the advertised max would really kill sales,.32s and .36s are sold to performance buyers.
17-18,000 rpm with a 8x7 or 8x8 would make that little Mustang pretty impressive.

RE: prop suggestion?
 

I appreciate that and is also intrigued to learn about the engine application of it either being pushed over the limit or has power to spare. Please keep them coming.

Clement

RE: prop suggestion?
 

The rod, crankpin and rear bearing is the weak link in alot of cheap engines. The Magnum might not hold up over the long haul to 18,000 rpm......but the original poster wants to get max speed out of his engine. Don't over look thinning the blades as a way to boost power. I would also use up to 25% oil, castor will help the parts live a longer life.