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Take off RPM

1

1Oldguy

Active Member
What RPM's should I be seeing on take off? I just started flying my new 6 with an IO 320 airboat mod engine, fixed pitch wooden prop. Right now best I can do is 2,240 rpm on take off. Goes like stink when I get some altitude. I know I'm over pitched and the prop is going back for rework. Just need some ballpark figures on what I should expect when dialed in. Thanks!:confused:
 
A little more info is needed to make an informed comparison. What kind of prop/ what pitch do you have? At what point in the takeoff is the 2240? Static, start of t/o roll, after liftoff, etc. What kind of RPM do you get wide open, leaned for best power at 8000 feet? These are all important data points.

As a point of reference, 2240 static is about right for a fixed pitch RV, if that is what you are seeing.
 
Sounds like you're in the ballpark, Joe

....our -6A turned 2200 with Catto's three blade and turned 2760 wide open for a near 200 MPH true.

Best,
 
same here..

2200 -2250 is about what I get with my wood cruise prop & 150 HP engine.

I'm overpitched but I am going to withhold final judgement until I get my
wheel fairing on.

Dave
-9A flying
 
1Oldguy said:
What RPM's should I be seeing on take off? I just started flying my new 6 with an IO 320 airboat mod engine, fixed pitch wooden prop. Right now best I can do is 2,240 rpm on take off. Goes like stink when I get some altitude. I know I'm over pitched and the prop is going back for rework. Just need some ballpark figures on what I should expect when dialed in. Thanks!:confused:
Click to expand...

Don't touch that prop!

You are most likely right in the middle of the sweet spot, the ideal combination of climb and cruise performance. I see 2270 at rotation with my RV-6 (O-320, FP Sensenich).

The low takeoff rpm is a little startling to someone whose experience is with fixed-pitch 172's, etc. There is a huge gap between stall speed and cruise on our planes, and the steep pitch is necessary to prevent the engine from overspeeding at max cruise. Makes our planes go fast, too! :)
 
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Some propellers have an unusual characteristic that the rpm will be higher at the start of the take-off run, and as they pick up speed, the rpm will drop 50-150 rpm, then be back at the same rpm as when they started. This is due to the inner part of the blade being past stall AOA and not generating as much lift. This means its induced "drag", a measure of its torque load, is not as much and so the rpm goes up. Then when the forward speed increases, the blade will come un-stalled, absorbing more engine power.
 
1Oldguy said:
What RPM's should I be seeing on take off? I just started flying my new 6 with an IO 320 airboat mod engine, fixed pitch wooden prop. Right now best I can do is 2,240 rpm on take off. Goes like stink when I get some altitude. I know I'm over pitched and the prop is going back for rework. Just need some ballpark figures on what I should expect when dialed in. Thanks!:confused:
Click to expand...

Joe,

Tell these guys a bit more about your airplane. What top speed you are seeing? Also, they should know about the interesting build background. This is one special airplane. Trust me! Post some pics!

Congrats on the first flight!

The other Joe
 
elippse said:
Some propellers have an unusual characteristic that the rpm will be higher at the start of the take-off run, and as they pick up speed, the rpm will drop 50-150 rpm, then be back at the same rpm as when they started. This is due to the inner part of the blade being past stall AOA and not generating as much lift. This means its induced "drag", a measure of its torque load, is not as much and so the rpm goes up. Then when the forward speed increases, the blade will come un-stalled, absorbing more engine power.
Click to expand...

Thanks for that bit of information.

It explains what is going with my prop in that it turns nearly 2200 static and stays that way up to about 110 knots. I never understood why rpm isn't increasing as speed builds up on the roll.

What you say makes perfectly good sense.
 
Light bulb.... <BG>
This explains what I've been seeing... Happens pretty quick though...

DM
RV-4 MT F/P Prop.

elippse said:
Some propellers have an unusual characteristic that the rpm will be higher at the start of the take-off run, and as they pick up speed, the rpm will drop 50-150 rpm, then be back at the same rpm as when they started. This is due to the inner part of the blade being past stall AOA and not generating as much lift. This means its induced "drag", a measure of its torque load, is not as much and so the rpm goes up. Then when the forward speed increases, the blade will come un-stalled, absorbing more engine power.
Click to expand...
 
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