Prop -v- RPM Mystery

Geico266

Geico266

Touchdown! Greaser!
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Husker Nation, NE
Display Name
Geico
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KCEK
Mystery might be overstating it, but now that I have your curiosity I would appreciate some diagnostic help.

I have an RV-3 with an 0-235-C1, 108HP @ 2,750 RPM. Basically, unknown engine hours, around 600 on the top end as best I can guess. Engine is off a wrecked plane, lost logs. The engine was "gone though" when it was installed about 400 hours ago, & 10 years, but this was by a good, knowledgeable, homebuilder, not an engine shop. Compressions are good high 70's, new Slick mags, harnesses, & plugs, RPM drop on mag check is 75 RPM both sides, rebuilt carb 75 hours ago.

Okay, the prop was a Sterba 68" x 66". The engine would spin 2,150 RPM static, 2030 RPM take off, 2,650 max straight & level, speed 158 MPH GPS checked 4 points.

I sent the prop back to Sterba to re pitch it to 62" thinking more RPM = more HP. Red line is 2,800 RPM, and I could only get 2,650. So re-pitching it to 62" was the plan.

Here is the "mystery" I got the prop back (did not touch the engine) and now I get LESS RPM than I did before.:confused: Now I get 2,100 RPM static, 1970 RPM take off, max is 2,500 RPM, but I gained 5 MPH :yikes: It is colder now than before re-pitching, but it was 50F yesterday with only a 10 RPM improvement.

How can this be? Finer pitch should equal faster RPM. Going from 66" to 62" I should have seen a 400 RPM increase. What am I not seeing here? Engine problems? Pilot error? :nono: ...Geico has entered the twilight zone?

Any thoughts greatly appreciated
 
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This may be a dumb question, but I thought your RPM would be higher on take-off than during a static run-up. Or is that just with constant-speed props?

As for the weird stuff, I vote for Twilight Zone. :)
 
CJones said:
This may be a dumb question, but I thought your RPM would be higher on take-off than during a static run-up. Or is that just with constant-speed props?

As for the weird stuff, I vote for Twilight Zone. :)
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:rofl: The Twilight zone it is!


RPM on static is a tad higher due to the prop cavitating in place, I've been told. It is "biting" clean air and working harder on take off.
 
Assuming no significant changes to blade shape, finer pitch should result in higher RPM under the same conditions. So either the conditions were different (colder air temps?), the tach readings before/after have different errors, or the effective pitch is greater now than it was before the change. Did you have the prop pitch checked at multiple stations? For a given specified pitch the actual blade pitch varies significantly from hub to tip and if someone had changed the pitch before your prop may not have followed the manufacturer's pitch profile.

For instance if the tips had less pitch than they should have relative to the mid blade pitch, the effective pitch might have increased when the prop was recently altered.

This is all assuming that the prop currently matches the specs for the pitch you requested. It's certainly possible that that's not true and the prop shop made a mistake of some sort.
 
I vote for something changed. Since the prop was what you sent out and had changed, I'd vote that the prop changed. Did they give you a before and after reading of what the prop pitch was and where they measured it? I think it's the prop. They probably messed something up.

Alternately, Twilight Zone sounds like a good option to me. :yes:
 
lancefisher said:
Assuming no significant changes to blade shape, finer pitch should result in higher RPM under the same conditions. So either the conditions were different (colder air temps?), the tach readings before/after have different errors, or the effective pitch is greater now than it was before the change. Did you have the prop pitch checked at multiple stations? For a given specified pitch the actual blade pitch varies significantly from hub to tip and if someone had changed the pitch before your prop may not have followed the manufacturer's pitch profile.

For instance if the tips had less pitch than they should have relative to the mid blade pitch, the effective pitch might have increased when the prop was recently altered.

This is all assuming that the prop currently matches the specs for the pitch you requested. It's certainly possible that that's not true and the prop shop made a mistake of some sort.
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Well, I called Ed Sterba and he is perplexed also, but he said this has happened in the past. His suggestion now is wait until the weather is warmer and try it, or cut 1" off the end of the prop & rebalance. I think I'm gonna just cut the 1" off and see what I get. The red line for the 0-235 is 2,800 RPM and I can only get 2,500 right now. The 1" missing off the ends should only add a couple of hundred RPM. (I think)

Any thoughts on cutting down the prop?
 
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sounds to me like the prop shop twisted your prop in the wrong direction.
 
Geico266 said:
Well, I called Ed Sterba and he is perplexed also, but he said this has happened in the past. His suggestion now is wait until the weather is warmer and try it, or cut 1" off the end of the prop & rebalance. I think I'm gonna just cut the 1" off and see what I get. The red line for the 0-235 is 2,800 RPM and I can only get 2,500 right now. The 1" missing off the ends should only add a couple of hundred RPM. (I think)

Any thoughts on cutting down the prop?
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I wouldn't do that right now, for one thing you really ought to figure out why the RPM is lower before making any changes and trimming the tips is awfully difficult to undo. It sure would be a shame to shorten the blades and then find that you've got an engine problem (restricted exhaust, bad cam etc). Also I believe that unless your tip speed is transonic, decreasing the blade length is likely to just make the prop less efficient and produce the exact same thrust at a higher RPM.

I strongly recommend profiling the pitch of your prop from hub to tip and comparing that against design specs before you change anything. Did you find out if Sterba profiled your prop before changing it's pitch? Any chance you can borrow another prop that's currently on the same engine type?
 
lancefisher said:
Did you find out if Sterba profiled your prop before changing it's pitch? Any chance you can borrow another prop that's currently on the same engine type?
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That is a good idea, but an RV-3 with an 0-235 is scarce and hen's teeth. It's hard even find data on it, let alone another prop, but I'll start asking around.

I've always wanted to pull the cylinders off and freshen up the top end, and take a peek at the cam. Since the logs are "MIA" it would be of benifit to the plane to start over. On the other hand it might be just fine and I'm waiting a good overhaul on a good strong engine. :confused:

For the cost of the overhaul (that might not be needed) I could buy a new Sensinch ground adjustable prop.

http://www.aircraftspruce.com/catalog/appages/sensenich.php

What to do, what to do.
 
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Geico266 said:
That is a good idea, but an RV-3 with an 0-235 is scarce and hen's teeth. It's hard even find data on it, let alone another prop, but I'll start asking around.
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It wouldn't need to be another RV-3, just something with an O-235. At full power, RPM vs airspeed should be pretty close if you climb or dive a little to achieve the same airspeed at the same DA. For starters you can certainly compare static RPM (watch out for the effects of wind).
 
Geico266 said:
Okay, the prop was a Sterba 68" x 66". The engine would spin 2,150 RPM static, 2030 RPM take off, 2,650 max straight & level, speed 158 MPH GPS checked 4 points.
Click to expand...

Do you know for sure that it was pitched to 66 as the starting point?

I sent the prop back to Sterba to re pitch it to 62" thinking more RPM = more HP. Red line is 2,800 RPM, and I could only get 2,650. So re-pitching it to 62" was the plan.

Here is the "mystery" I got the prop back (did not touch the engine) and now I get LESS RPM than I did before.:confused: Now I get 2,100 RPM static, 1970 RPM take off, max is 2,500 RPM, but I gained 5 MPH :yikes: It is colder now than before re-pitching, but it was 50F yesterday with only a 10 RPM improvement.
Click to expand...

Engine is running slower but you have more speed? Sure the prop didn't have problems in the "before" condition?
 
Trapper John said:
You've probably already checked this too, but is the throttle opening 100%?


Trapper John
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It doesn't seem likely that any engine problem would result in lower RPM and higher airspeed by itself.
 
Did you make the TAS calculations for the colder air? The ASI will read higher in the colder, denser air.

And maybe that's also why your RPMs are a little lower, too?

Dan
 
Geico266 said:
:rofl: The Twilight zone it is!


RPM on static is a tad higher due to the prop cavitating in place, I've been told. It is "biting" clean air and working harder on take off.
Click to expand...

The inner area of a prop is stalled in the static condition, especially a higher-pitched prop. As forward motion increases the prop's blades' angle of attack decreases and the prop will load up as it generated more thrust. My prop does this, too.

Dan
 
Capt. Geoffrey Thorpe said:
Do you know for sure that it was pitched to 66 as the starting point?
Click to expand...

That is what was engraved on the prop. It was 66" and Ed Sterba changed it to 62"


Engine is running slower but you have more speed? Sure the prop didn't have problems in the "before" condition?
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Could have, but I do not know what that could have been. You would think Sterba would have caught that.

Trapper John said:
You've probably already checked this too, but is the throttle opening 100%?

Trapper John
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Easy enough to check. Pretty sure it is, nothing changed, and I did not touch the engine. I'll look for air leaks also. But I have more speed. :confused:

Dan Thomas said:
Did you make the TAS calculations for the colder air? The ASI will read higher in the colder, denser air.

And maybe that's also why your RPMs are a little lower, too?

Dan
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Good point, but I calculate speed based on GPS readings. That is not affect by temps.

gkainz said:
and if that doesn't work, then what? Glue it back on?
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JB Weld?? :rofl::rofl:

I'd buy a new prop. There are some ground adjustable ones that would really be "the cat's meow" for this plane. A constant speed prop would really be cool, but I'm only working with 108 HP @ 2,800 RPM.
 
tonycondon said:
sounds to me like the prop shop twisted your prop in the wrong direction.
Click to expand...

It's wood. Ed Sterba (OEM) repitched it. He sands away 1 degree on the back side of the prop (at different stations) leaving the front of the prop (airfoil side) alone. He would thin out the airfoil side if we had to remove more than 1" off the tips.

It was interesting in that if you cut off more than 2-3 inches and do not reshape the airfoil it can actually lower the RPM also.
 
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Geico266 said:
Easy enough to check. Pretty sure it is, nothing changed, and I did not touch the engine. I'll look for air leaks also. But I have more speed. :confused:


Good point, but I calculate speed based on GPS readings. That is not affect by temps.
Click to expand...

Well, if you are going faster, you have more thrust, so you must be producing more power.

I suppose the power increase could be from the colder (denser) air...


Trapper John
 
lancefisher said:
I strongly recommend profiling the pitch of your prop from hub to tip and comparing that against design specs before you change anything. Did you find out if Sterba profiled your prop before changing it's pitch? Any chance you can borrow another prop that's currently on the same engine type?
Click to expand...

I think you may be onto something here. I don't have the equipment to check the prop, but something changed. I think the airfoil of the prop was changed by re-pitching and this is the difference. The re-pitiching is done by removing material on the backside of the prop and not disturbing the front. In this case this was not a good thing to do.

Update. Well for fun I pulled and cleaned the plugs, checked throttle, carb heat, checked the tach, put the plane back togeather and went flying. No improvement.

I'm gonna live with it for a few months then decide if shorting the prop is the way to go.

Thanks for all of your input. I greatly apprciate it.
 
Something is not right with the prop I had repitched.

Before repitching it was 68" X 66"

2650 RPM X 66" pitch X .000947 = 165 MPH. * (Confirmed Speed with 4 directions GPS.)

After Repitching

2500 RPM X 69" (suppose to be 62") X .000947 = 163 MPH*

If it was re-pitched to 62"as requested

2500 RPM X 62" X .000947 = 146 MPH (I'm going 163 MPH*)

* I was going 163 MPH, checked with GPS (add all 4 compass directions (GPS ground speed MPH) straight & level / 4 )

What am I missing?
 
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I don't know man but if it's really bothering you I'll fly your plane for a while and you can have my Fly Baby.
 
timwinters said:
A properly re-pitched prop maybe?
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The more I think about it, the more I agree with you. It has to have been pitched the wrong way. I think it is time to buy a ground adjustable Sensenich.

2 blade or 3 blade?

wby0nder said:
I don't know man but if it's really bothering you I'll fly your plane for a while and you can have my Fly Baby.
Click to expand...





Deal! :rofl:

You are right though, I should not be complaining. She still is a freakin hoot to fly. I can go 165 MPH on 7 GPH, 135 on 4.5 GPH. The fun factor is off the scale. Quick turns, aerobatic, very responsive, well behaved landings. The RV-3 just does want you want a plane to do. EXCEPT turn the correct RPM! :rofl:


Here is the beast if anyone cares to look.

RV-3_left.JPG
 
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Two blade. We're getting killed speed-wise with our 3-blade MT on the -7A. :(
 
I found this formula for station angle calculations and I would like to check this prop to see if it was re-pitched properly.​


Blade Angle
Pitch/Circ. = the arc. tang. = angle (Degrees)
To find the angle needed at any blade station for a desired pitch.
Example: 60 pitch, 68 diameter (214" Circ.)
60/214 = .2804, then arc. tang of .2804 = 15.70 Degrees​

Dones anyone know how to figure Arc. tangent?​
 
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Yea I hear 2 blade is fastest.

Thats a cool little ship. I didn't know there was a fastback version like that. I thought they were all bubble canopy.

Just so you know I am RV3 crazed. I LOVE my Fly Baby but I have the speed disease. You ever want to sell her let me know.

More photos! Panel, engine close-ups..
 
RV-3_right.JPG





There are two RV-3 fast backs that I know of, both built by Gene Nelson, AZ. The bird started out life with a Subaru auto conversion that Gene could not keep cool in the AZ heat. He pulled the Subbie and threw on the 0-235 with the Sterba prop. Gene was one of the first guys to use an auto MSD (multiple spark discharge) system to fire the right magnito. Very interesting man who lives in AZ.
 
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What can you carry, fuel, baggage, pilot wt?
 
wby0nder said:
What can you carry, fuel, baggage, pilot wt?
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15 gallons of fuel each wing, 200 pounds-o-pilot is not optional :nono:(I'm on a diet:mad2:) , 30 pounds of baggage behind the seat and behind the pilots head. A very nice feature of the "turtle deck". Panel is pretty plain, but I do have a nice XCOM radio that really get out. 496 with WX & XM so I can listen to the oldies while hunting clouds. :nono:

Economy cruise is 4.5 GPH so you have a range of about 700 miles.

It's basically a fly around the area plane for me (trips under 200 miles) so I want to get the most out of the 0-235 engine, hense this thread. Sport aerobatics permitted +6 - 4 G's. This is actually a RV-3"a" meaning (no nose gear) the 2 wing spar SB mods have been complied with. Seems a few of the early RV-3s lost their wings after 10 G pull up after a low pass, 4Gs is plenty for me. ;)
 
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Geico266 said:
I think the airfoil of the prop was changed by re-pitching and this is the difference. The re-pitiching is done by removing material on the backside of the prop and not disturbing the front.
Click to expand...

That obviously changes the airfoil... I'm trying to figure out why they'd do it that way, as it doesn't seem like it'd be all that effective. In terms of wings, I've heard that Newtonian lift is responsible for a much larger percentage of the lift than Bernoulli lift. I wouldn't think that a prop would be any different, it's just a rotating airfoil. If you're only changing the backside, you're only changing the Bernoulli part of the equation - While the "upper" camber pitch has changed, the "lower" has not and thus Newtonian lift would be the same.

That is, if you mean "backside" to be backside relative to the air. If you mean "backside" as the part that you're looking at when you're sitting in the airplane, then never mind - Though I'm curious to know which it is! :yes:
 
Geico266 said:
That is a good idea, but an RV-3 with an 0-235 is scarce and hen's teeth. It's hard even find data on it, let alone another prop, but I'll start asking around.

I've always wanted to pull the cylinders off and freshen up the top end, and take a peek at the cam. Since the logs are "MIA" it would be of benifit to the plane to start over. On the other hand it might be just fine and I'm waiting a good overhaul on a good strong engine. :confused:

For the cost of the overhaul (that might not be needed) I could buy a new Sensinch ground adjustable prop.

http://www.aircraftspruce.com/catalog/appages/sensenich.php

What to do, what to do.
Click to expand...

That is one of the advantages of experimental, you can pull the engine apart, measure and inspect it all out and put it all together at minimal expense if everything looks good. As for the prop, there's a lot more to a prop than simply pitch which can effect rpm. Tip design for one.
 
flyingcheesehead said:
That is, if you mean "backside" to be backside relative to the air. If you mean "backside" as the part that you're looking at when you're sitting in the airplane, then never mind - Though I'm curious to know which it is! :yes:
Click to expand...

Thanks for your input. When I say back side I mean the side I see sitting in the plane. That is the only side that was re-pitched. Ed Sterba said he took off only 1 degree, and he says he went from 66" of pitch to 62". But the same calculations Sterba says to use to calcjulate pitch says he took it the other way to 69" of pitch based on the increase of speed.
 
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