Max Engine RPM

[cgruby]

I have a type III with a Subaru EA81 engine and a 1.80:1 reduction drive. The engine is rated at 80 HP at 5,000 RPM. The max RPM I've seen is 4,000. Since I didn't build the airplane, I have no knowledge as to how this came to be. The question is, should I just simply adjust the prop until I get the 5,000, or are their other things that I should consider?

Thanks, I'll standby for your comments.

[Dave S]

Hi Chuck,

Interesting deal.....First off, I cannot claim to be any kind of expert on the EA81; but, the prop pitch does have a dramatic influence on any engine's performance; and, general principals apply often.

Might be good to let the list know what prop you have on the engine....and prop model...for instance, Warp has a square tip and a tapered tip which behave a little differently and they come in different diameters (blade length).

Might be worthwhile to do a couple preliminary things before adjusting the prop.....1) measure the existing prop pitch with the appropriate protractor so you know what your starting point is. 2) Be sure the engine tune is up to par....no restrictions due to filters which are not clean, plugs clean & timing and correct, throttle cable & carb adjusted so the throttle opens fully, correct fuel mixture, carb heat not locked on, compression...basically check anything that could result in a power reduction if it is not 100% ....

I am sure others on the list have experience with EA81's & if people know what prop you have it would help with advice folks may offer.:)

Sincerely,

Dave S
Kitfox 7 Trigear
912ULS Warp Tapered tip

St Paul, MN

[cgruby]

Hi Dave. Thanks for the "to do" list. I've pretty much done all those things. It's interesting to note that this prop is not the one the builder originally installed. It seems the second owner ground looped the A/C and wiped out the gear and prop. This prop manufactured by IVOPROP Corp. I'll get all the other poop tomorrow and post that as well.

Cheers,

[avidflyer]

Might want to double check the tach/RPM also. Not all of these gauges are accurate. Sometimes things are not what they seem :-) Jim Chuk

[cgruby]

Hi folks,

An update to my prop/RPM question:

The prop's an IVO, 68 1/4" in diameter, two blades. The pitch measured at the 75% point is 16 degrees. Maximum RPM during T/O is 4,000 ( I haven't checked in level flight).

The engine is a Subaru E81, rated at 80 HP at 5,000 RPM.

Note: I talked to the original owner and builder Jack Bally about the above question. He said that the engine repair manual rates the engine at 72 HP (not 80) at 4,800 RPM ()not 5,000). He orginally pitched the prop to yield 4,200 RPM on T/O and cruised at 3,800. His original prop was a 3 bladed Warp Drive.

I think based on the above, I'm pretty close to where I ought to be, so as far as I'm concerned, I'm going to let this one rest and work on some other concerns.

Thanks everybody for your help,

[896tr]

Chuck, at those rpm your prop is only turning 2333 rpm on take off and 2111 at cruise. You might be wasting some performance that way.
I could be incorrect.
Steve

[Danzer1]

Definitely losing performance. At 4000 at TO = 2222 prop rpm and with that engine likely around 60hp. Prop is at 59% of mach and ideally you would want to run it at least 75% and sometimes as high as 90% depending on the prop and climb vs cruise performance considerations.

With a 16 deg pitch on a 68.25 inch prop you could gain a little by increasing the pitch to say 18 deg, but personally I'd want more umph on takeoff - if anything just for some margin. You are also probably running about 50lbs heavier than a Rotax 912 which won't help performance.

There were also many different variations of the E81, you will hear numbers all over the board. If yours was an early single carb model, out of a Brat - likely factory spec was 73hp @ 4800.

Greg

[cgruby]

Hi Greg,

A couple of things:
1. Could you give me the formula for mach speed versus RPM for given prop lengths?

2.Are the performance percentages a percentage of the mach one speed?

3. I favor having more power for takeoff as opposed to cruise. As a goal, I would like to target for 65% efficiency and see what that yields.

Thanks,

[cgruby]

Chuck, at those rpm your prop is only turning 2333 rpm on take off and 2111 at cruise. You might be wasting some performance that way.
I could be incorrect.
Steve

Thanks Steve. I'm definitely going to seek a little higher RPM for T/O.

[cgruby]

Might want to double check the tach/RPM also. Not all of these gauges are accurate. Sometimes things are not what they seem :-) Jim Chuk

Thanks Jim. It seems like I remember a trick where you could run the engine at night looking through the prop at a florescent light and adjusting the RPM until the prop stops. This will give you a multiple of 60 RPM, for instance, you should be able to stop the prop at 3660 RPM. Any difference in the tach reading from that value, would be the error. And then there's the trick of flying behind another airplane with a know accurate tach and having him set it upon an agreed upon value and adjusting your rpm to stop the prop. I've done both. Thanks for the reminder, I'll check it out.

[Danzer1]

Chuck,


1. Could you give me the formula for mach speed versus RPM for given prop lengths?

Warp has 2 pretty good calculators here: http://www.warpdriveprops.com/propspd2.html

They are generic, not specific to Warp propellers and a "rule of thumb" only. I do not know of anyone that publishes specific calculators for any specific prop.


2.Are the performance percentages a percentage of the mach one speed?

No, but there is certainly a correlation, 65% of tip speed does not equal 65% efficient. That is specific to each prop design, hp/torque/rpm and pitch and can only be determined by testing the configuration.

You can get some pretty good ideas though by changing the diameter, pitch and/or rpm in the calculators shown to see the theoretical change in performance.

As others have mentioned, the 1st thing I would verify is rpm readout accuracy. I would also check to see that you are getting full range on your throttle cable at the carb and go from there.

Greg

[cgruby]

I went over to the Warp Drive Web site and checked out their calculator. It looks like if I pitch to gain 5000 rpm at T/O, I'll really take it on the chin for cruise speed. I'll suppose a compromise is in order.

[Danzer1]

Yes Chuck, it will always be a compromise unless you have an in flight adjustable pitch prop!

I feel compelled to add though as you've mentioned 5000 rpm. You are totally in uncharted territory. Reasoning:

There were at least 4 companies doing Subaru conversions with various redrives, none are currently in business. None of them and to my knowledge - no one (including Subaru) has ever published a horsepower/torque chart for that engine.

Torque is the true measure of the real ability (grunt) to turn the propeller. No one knows where that (what rpm) occurs on this engine or if it can effectively do it with longevity at high rpm and within an (unknown) max MOI (moment of inertia).

MOI is most critical to the redrive - basically it is the measure of stress put on the shaft and bearings based on where the mass weight of the propeller occurs.

We don't know what redrive you have (other than 1.80/1), or if that manufacturer ever specified a max rpm and max MOI.

Further, Ivo does not publish MOI's for their props - they simply say they are "flexible" - that's fine for the prop, but says nothing about the stress put on the redrive. You could calculate the MOI from here: http://www.ul.props.fr/MOI/rotax_MOI.pdf it is from Rotax, but is the same for all props. Useless info though unless you know the limits of the engine and redrive combo.

I know there are plenty of Subaru installations out there, some also in Avids and Zeniths primarily - you might try some of those forums too. Maybe someone with an installation similar could chime in.

Good luck, but be careful at high rpm given all the unknowns here.

Just curious: After the previous owners prop strike, was the engine and redrive completely torn down and inspected?

Greg

[cgruby]

OK here’s what we’ve got: According to Warp Drive’s calculation:
5000 RPM yields 563 mph tip speed (73% of mach 1) (speed of sound 767 mph).
The prop was at 16 degrees, I adjusted it to 14 degrees. We’ll see what the next flight brings.

[cgruby]

Today I went out to fly and damaged my rudder getting the plane off the trailer, so I'm out for a while. Grrrrrrrrrrrrr!!!!

[cgruby]

I got the rudder repaired, and it turns out it was due. The tubing had several rust holes in it and evidence water had stood at times 2 inches deep. I cut out the old and spliced in new, recovered and repainted. This time I put in a couple of drain grommets.



My original problem was that my engine only turned 4,000 RPM on T/O. Feeling I should be turning 5,000, I checked the prop pitch and found it to be 16 degrees, so I turned the adjustment 1 turn ccw and came up with 14 degrees. I went out and flew today and now see 4,400 RPM on T/O. I couldn't tell from a pilot's standpoint that there was any change in performance, so I probably won't adjust it any more than that.


Thanks everybody,

[kmach]

Ivoprops are easy to adjust ,so why not try some different settings. There will be a sweet spot to your liking at a certain setting.

[cgruby]

Thanks Kevin, good advice.