Van's Air Force
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Slow Prop RPM WOT
- Thread starter maxmirot
- Start date
Carl Froehlich
Well Known Member
Need more details. What flight conditions such as altitude, speed, fuel flow, loading, engine timing verified, etc.
Carl
Carl
Like Carl says, We need a LOT more information. If this is static RPM, it's too high. If it is cruise it is low. I usually shoot for 2750 WOT @ 7500' MSL.
BTW; I hope you meant 180 HP!
PerfTech
Well Known Member
Altitude 7000 to 8500 typically.
I am seeing TAS of 145 to 155ish.
2 people and full tank or 1/2 tanks with just me.
I get a bit faster lightly loaded and climb performance is noticeably better
Timing last checked at 25 degrees before TDC ( mags )
I have no way to determine momentary fuel burn and yes 180 HP
Assuming you really mean TAS = true air speed, this is slow, even for ‘just’ 2500 rpm.
I’d suggest:
1. Find the make/model of the prop, and confirm with Sterba that it’s recommended for an O-360.
2. Assuming it is, find out why you aren’t making rated power. How are you leaning in cruise?
I’d suggest:
1. Find the make/model of the prop, and confirm with Sterba that it’s recommended for an O-360.
2. Assuming it is, find out why you aren’t making rated power. How are you leaning in cruise?
PilotjohnS
Well Known Member
Read it here
I read it here a while back that fuel flow when lean of peak is proportional to power. So what is your fuel flow lean of peak? It doesnt matter how lean, just that you are lean of peak.
I read it here a while back that fuel flow when lean of peak is proportional to power. So what is your fuel flow lean of peak? It doesnt matter how lean, just that you are lean of peak.
1) This is not a static test. This in flight
2) Yes TAS =True Air Speed
3) No other reason for not making rated power.
Checked carb mixture and throttle cable have full travel.
WOT at peak power, lean and rich, all tried.
Running smooth all the time , it just reaches 2500 wide open and goes no further.
I have not called Sterba.
(I am not that interest in a keeping a wood prop in the long run)
I am assuming this is a prop issue having rule out fuel air and spark.
Agree or no?
Caveman
Well Known Member
Not necessarily,
Perform a full throttle static run up and report back here with the rpm results. (Be careful and tie the tail down). This will help us determine if your engine is making rated power.
Is your reported TAS in knots or mph?
Go here and read:
https://www.kitplanes.com/flight-testing-finding-tas-from-gps-data/
Then fly the test and report your results back here.
We are here to try and help.
Thanks,
Joe
I spoke with the builder/former owner about the situation.
He told me originally the new motor WOT in flight it was doing 2850 rpm with a new Sterba prop.
So he asked Ed Sterba for his recommendation.
Ed recommended changing the pitch to 82 degrees.
It now does just 2500 rpm in flight at WOT after changing to a 82 degree pitch.
Looks like they over did the pitch change.
That's why I am looking at a Whirl Wind ground adjustable prop.
So yes, this is due to the prop pitch
Max
He told me originally the new motor WOT in flight it was doing 2850 rpm with a new Sterba prop.
So he asked Ed Sterba for his recommendation.
Ed recommended changing the pitch to 82 degrees.
It now does just 2500 rpm in flight at WOT after changing to a 82 degree pitch.
Looks like they over did the pitch change.
That's why I am looking at a Whirl Wind ground adjustable prop.
So yes, this is due to the prop pitch
Max
Larco
Well Known Member
From watching two RV's with same scenarios. Prop needs needs a finer pitch so to get 2700-2750 RPM WOT at approx 7500 feet. If the prop is pitched correctly it will climb better and cruise faster. The engine needs proper RPM to develop proper power. It only took 2 years to convince on of the RV owners to get the prop repitched but the hugs were worth the wait.
This is an acceptable solution. When I modified the engine on my -6, We weren't sure of the exact HP and Sensenich shortened my prop 1/2" and it worked perfectly.
I will check the wheel fairings. Another RV builder (a repeat offender ) came by to look at my airframe and could not find any obvious issues.
You mentioned the air filter. Put it a new one two weeks ago. The old one was not dirty.
I would argue that the over pitched prop is not hurting much at cruise speed, relative to a finer pitch. Most CS prop users dial back the RPM to less than 2500. The lower RPM and higher MAP, especially at lower altitudes, can give a slight economy improvement without sacrificing speed. However, the RPMs keep dropping as you climb, so cruise speed will drop as you get into the higher altitudes.
The problem with too much pitch is poor take off and climb performance. Also, I don't believe that the Sterba is the most efficient prop out there and that could account for your missing performance relative to more modern designs.
FP props require a compromise on the pitch to balance performance atall flight regimes and cruise altitudes.
Larry
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Carl Froehlich
Well Known Member
I propellers ability (efficiency) to transform power to thrust is far more complex than just pitch. I’ve flown an RV with a fancy composite CS prop that took all the power from the engine but failed to provide the performance of the other CS props I’ve flown behind. This boils down to good airfoil design to match the airplane flight conditions. I would assume this rings true for FP props as well.
Carl
I'm curious what weight you were operating at when you made these tests. My RV-6 will lose 7 or 8 mph in cruise between light and heavy.
Just as a data point, my O-320 powered RV-6 will produce 184-192 mph TAS, at 3000 to 6000 PA, turning 2400 RPM @ WOT, with a Sterba prop, (82" pitch?). Maximum level flight RPM 2475 at 9000 DA. Typical gross wt with me and half fuel is 1275 lbs
Just as a data point, my O-320 powered RV-6 will produce 184-192 mph TAS, at 3000 to 6000 PA, turning 2400 RPM @ WOT, with a Sterba prop, (82" pitch?). Maximum level flight RPM 2475 at 9000 DA. Typical gross wt with me and half fuel is 1275 lbs
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Good info.
I will eventually replace it with something else. Wood is not really a IFR prop material these days.
