bob27s
Posts: 4736
Joined: 4/9/2002
From: Cleveland,
OH, USA
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quote:
ORIGINAL: TXKflier
Yeah, Bob, but what about an 11x7 on a 90L? Isn't it winding things a bit tighter than necessary? All of the props you listed won't turn quite so many rev's..
Thanks,
Lynn
Not at all...... the Jett engines enjoy rpm. More the merrier. HP = torque x rpm
An 11x7 on the 90L is ok.... but for most applications it is a bit light - you can use more of the engine.
There are a number of folks who prefer to use the APC 11x7 on a regular basis, and have had great success with it.
The 11x7 is my recommended 90L break-in prop, and I also tend to use the light prop for the first few flights, depending on the aircraft.
An 11x7 will turn well over 15,000 rpm on an 90L, and will be very well behaved.... sorta like running with long pipe on it.
So for break-in it is easy to dial in 14,800 and it will run there all day long without breaking much of a sweat.
The standard 90L has the longer muffler (like you have) so the regular 90L has a broad prop selection basis, and broader rpm range.
Anything from an 11x7, 11x8, 11x9, 12x6, 12x7, 12x6 is a great bet. It can also turn as much as a 13x5 or 13x6 set up just a bit differently, but at closer to 12,500 rpm. The performance range on the muffler extends from around 12,500 up to about 15,500 and then tends to flatten out. So propping the 90L for ground peak around 14-14,500 is perfect.
On the 90LX, an 11x7 is fair game.... can turn the prop up over 15,500 and push 16,000 with the right conditions.
The LX muffler is shorter, engine port timing is more agressive, designed for higher rpm. Quite the animal 
Typically we like to prop the LX engines to run right around 15,000 ground peak.
Engine gets there with a 10x10, 11x8, 11x9, 12x6 ------- but nothing bigger is worth trying. Engine does not have the torque.
The timing/porting/crankshaft/carb on the OS91 and Magnum 91 will not support high rpm as the Jett is designed for, but they produce good power in their own right., Both of these engines have excellent torque at a bit lower rpm. Even with a stock muffler, you want to prop these over 12,000 rpm, but below 14,000 rpm. With the jett muffler, you get the best effect in the range I noted ..... and through experience the props noted will get you to that rpm band, and make this particular aircraft fly great.
Worth noting..... with a tuned rpm system...... rpm is the key. Find a prop, ensure it hits the rpm target/range. And also keep in mind that your ground rpm is not where it will be running in flight... it will unload a great deal further. Also worth noting, please do not confuse the pitch of the blade with the forward aircraft velocity. Yes, it can play a significant component. But the airfoil on the blade and how fast that airfoil is moving (rpm) plays a much greater roll in how the dynamic system performs in flight. (example in another thread on this, explains why a sailboat can have a velocity faster than the wind is blowing ....... same reasons). I can demonstrate quite easily how two "dimensionally" identical 9x7 props can vary in forward flight speed, at the same engine rpm, buy a significant margin. (we call the slower ones paint stirrers).
Your magnum setup runs well. You have it right on the money there with that slightly-rich 13,000 rpm on the ground ....... that is exactly what that combinaton needs. You will probably find that an APC 12x6, or 12x8 will be the best all-around match with that power package and the F-18.
The Zinger blades are thicker than the APC props..... good props, but a bit less consistant from prop to prop, so it is hard to evaluate or compair performance. Use what ya have available.
Bob
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Bob Brassell
Jett Engineering - Engine Mfg Support Forum Host
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