DamBo

Hey Bob,

I have SJ 60LX that put on Hanger 9 P51 Mustang. I had to make a 3/8" spacer to keep the muffler off of the side of the plane. I flew it with a 10x6 . It seemed to pull real strong, unlimited vertical, but did not seem to have a real high air speed.
I have just put on a 9x9 it ground tachs at 18800 so I backed it down to 17800. I noticed that it does not seem to unload in the air. The vertical is still unlimited, but airspeed does not seem to be there. Is the best thing to do here, to keep going down in props, 9x8,9x7,9x6 until I find the sweet spot?

bob27s

My Feedback: (19)

Hey...

thanks for writing......

Is this the PTS mustang? or the bigger scale one?

Neither of those aircraft are particuarly quick. The small one was intentinally draggy, even cleaned up a bit of all the trainer stuff. With gear down, its not going to really move. So what you want to find is a balance of performance.

Spacers on the LX are not usually a good thing... but not terrible either. But I always avoid them on that particular engine. A SJ-50 tends to accomodate exhaust extention better for some reason.

18,800 on the 9x9 tells me the system is working well. Your rpm target is great. No problem there.

But since the exhaust is a bit longer, its not really going to jump on the pipe and hit another note, unless the plane is REALLY moving. It is already deep staged on the muffler.

Its sorta like setting up a race car for a short track, as opposed to a larger speedway. Same power, just used differently.

Two things......
try the smaller props. A 9x8 sometimes is the right number.

try an APC 10x7 prop also. My suggestion here is to set up with a 10x6 first .... leave the needle pretty much alone. And just switch to the 10x7.

Some folks flying larger warbirds tell me the 10x7 seems to have a good balance of performance racing, especially corners. Tends to be a little faster in straigh line, and runs about the same rpm in flight at the 10x6.

DamBo

OK

I'll try the 10x6 to 10x7 change and see how it does.

Thanks Bob

DamBo

That's it speed is up vertical is still there 10x7

Thanks

bob27s

My Feedback: (19)

Great

send me a photo or two if you can.....

and if you have a tach available and the opportunity, see what your launch peak rpm ended up at.

Bob

DamBo

OK

Bob

I forgot, to tell you it is the 40 size PTS.
I still need to lose the over sized tires.
Do you think that the little silicone diverters on the muffler affect performance any?

What about a 10x8 or an 11x6 ?

bob27s

My Feedback: (19)

Nothing larger than a 10x7 at very most on the LX engine. Its time for rpm. Not getting to rpm will make it misbehave and overheat. An 11x4 will work, but the hub is too thin and it will shed a blade much over 17,000 rpm

The diverters DO affect performance, if they are too small in inside diameter. We use the "60-90" size great planes diverters when needed. The 40 size inside diameter is way too small, and affects the pressue wave in the muffler.

Bob

DamBo

Here are few pictures of the tach readings.
This is with the 10x7. The first one is with the muffler extention only, then with a large diverter 90-120 size and finally with the 40-60 size one that I am flying with.

bob27s

My Feedback: (19)

rpms are low for the engine ..... need to drop the prop size for a while. You noted the 9x9 was up way over 18,500 and launching at 17,500+ - that is right on the money. The 10x7 should be over 17,000.

Im a bit confused though..... ground rpm should not vary more than 100-200 rpm whether it has the diverter on it or not... only difference tends to be whether it ends up running hot when it unloads.

Without any diverter is best. The big diverter should not make any rpm difference at all. Small diameter will drop rpm a few.