Boeing Stearman Model 75

[aleatorylamp]

Hello Ivan,
Oh well... such is life.
Some are better and/or quicker than others in finding the information needed at a given moment - obviously because some know more than others about the subjects in question, and can thus more easily identify useful from useless information within the quagmire of Internet.

Even Pilots´ Handbooks for similar aircraft supply different and/or inconsistent information, (did the Lycoming engines in question have 220 or 225 HP??) as in the case of Army Corps and Navy Stearmans, or information which is only similar to that required, as in the case of the Orion.

Also, Engine Manuals ommit certain details. For example, there isn´t a military designation for the 240 Hp Continental W670-M, but the Navy had quite a few, but neither do they say how many, nor on what planes. ...and not to mention the 280 Hp Lycoming-engined Stearmans, buried in oblivion largely due to the FAA, although 2 or 3 historical texts do mention 255 units as having them! So, to say I´m perhaps simply not looking is quite unjust, to say the least.

Anyway, turning back to propellers: After the more thorough method of testing and tuning that you proposed, which yielded almost the same results as I´d got before, now I am surprised to hear that the Stearman propeller tuning is still not good. Oh, well, such is life... I don´t know any more to do anything further, but I do know where the problem lies.

In real life, 2100 RPM on the Lycoming gave 220 or 225 Hp (we will never know the exact number, will we?), and 2200 RPM, gave 237 Hp. In the Sim, this seems impossible to achieve, at least for me. The positions on the curve are so close that a 100 RPM difference for 12 or 17 Hp is impossible without incorporating a stupid, big step - so again, ...such is life!

My question on how to calculate the point where the propeller curve reaches zero was meant in general.
I have already said that I mentioned the Stearman example only as a check-reference.
You suggested J=0.8 because your calculation results ranged from J=0.7 to J=0.9, and included some working assumptions - and, now why was the J=0.8 out of context? ...and I do apologize for misquoting something or other on the Stearman propeller curve.

Anyway, my aim behind getting this information (that you so generously supplied, thank you very much) is to use for the Lockheed Electra-10´s two-pitch position propeller, which is quite a lot smaller than the Condor´s one you so effectively made.

This business is tedious enough as it is, more so if one adds the CFS and AF99 limitations.

Anyway, it seems that all my questions have been causing you some degree of bother for quite some time already, and I am sorry to hear that. So, the solution is really very simple: I will just reduce the number of questions and interventions to the barest possible, absolute minimum. The last thing I want to do is to be a pain.

It´s a hobby, and it´s supposed to be fun, so why turn it into such a harsh discipline in so many aspects?

Cheers,
Aleatorylamp

[Ivan]

Hello Aleatorylamp,

The misquote I mentioned is here:

As it happens, for the Stearman-75, I am getting consistent differences
of 10 mph and 0.05 in the "J" factors. You had calculated a recommended
"J" factor of 0.8 for the Stearman.

This would add another 0.13 to the "J" factor difference, making the
difference 0.18, i.e. 3.6 times larger, which would account for the
working assumptions you were making.

The recommendation of Efficiency dropping to Zero at J=0.8 was for a Propeller Pitch of 10 degrees.
If that is still what you are using, then Great!
If not, then this Quote is out of context and is not what I recommended.
I have not worked out the rest of the math, so I do not know what the 0.13 or 0.18 or 3.6 are referring to.
I do not have your flight model, so I really have no idea what might be causing your 10 MPH issue or even what the issue is for that matter.

Your comments about Engines on the Stearman would have made more sense to me a couple weeks ago than now because as I said before, I never was particularly interested in the aeroplane and after I found what either you or I was looking for, I didn't archive it for later use.
Normally I keep a Data Repository and Data Sheet on projects that I am working on but didn't see the point of doing that here.

That is why it is so difficult giving a diagnosis when you are describing that something is wrong.
Without gathering the data and either building the flight model myself or at least poking around with data in a spreadsheet, I can't really get a feel for the specifics of what your flight model may be doing.

I suppose directing YOU to try an Internet search was a lazy approach, but the alternative was that *I* would do the Internet search myself and just tell you what I found which seems a bit silly.

- Ivan.

[aleatorylamp]

Hello Ivan,
OK, thank you very much, and pardon my frustration, so it wasn´t irrelevant at all, but quite important!

I´ll have to repeat, for clarity but expounding as well:
Testing in most operative flying conditions, with a 16-degree pitch propeller, (66 inches pitch Advance in one revolution), i.e. Normal Vcr, High Vcr, Vno, 100% Power Vmax, and 105% Power Vmax.
There was a consistent +10 mph difference between what the pitch 66 Advance Speed calculation gave, and what the model´s actual speed in the sim was,
for the following RPM settings, (Hp are just a reference):

Hopefully these number will give a feel, like you mention you need, but without having the inconvenience of making a flight model or a spreadsheet run... sorry about putting you out.

1900 rpm, 156 Hp, actual speed: 190.4 mph, J= 0.620. Zero point calculation: 118.74 mph, J=0.673
2000 rpm, 181 Hp, actual speed: 115.7 mph, J= 0.623. Zero point calculation: 124.99 mph, J=0.673
2077 rpm, 202 Hp, actual speed: 120.4 mph, J= 0.625. Zero point calculation: 129.69 mph, J=0.673
2100 rpm, 207 Hp, actual speed: 121.9 mph, J= 0.625. Zero point calculation: 130.67 mph, J=0.673
2143 rpm, 220 Hp, actual speed: 124.5 mph, J= 0.626. Zero point calculation: 133.93 mph, J=0.673
2160 rpm, 225 Hp, actual speed: 125.4 mph, J= 0.627. Zero point calculation: 135.00 mph, J=0.674
2200 rpm, 237 Hp, actual speed: 127.9 mph, J= 0.627. Zero point calculation: 137.49 mph, J=0.673

A) 1900 RPM: desired for 202 Hp, but this power isn´t possible even at 2000 rpm.
B) 2100 RPM: desired for 220 and/or 225 Hp but impossible to achieve in the sim.

Summing up: Using the J factor formula, there was a consistent +0.50 J difference in all these flying conditions.
The J factors that came out in the calculation were all around 0.673, and the factors in actual flight ranged from 0.620 to 0.627, so the difference was averaging 0.05.

The rest was then a mistake. I was wrongly expecting the correct "zero efficiency value" to be at J=0.8,
and I thought that the difference would account for the working assumptions you had mentioned.
So, the whole paragraph on "the 0.13 or 0.18 or 3.6 times" I was referring to can safely be discarded as erroneous, obviously...

This would however not altogether invalidate the results giving +10 mph and +0.05 J factor difference that I got in the calculations, compared to the actual in-flight aeroplane speeds.
I thought it seemed logical that pushing the plane 10 mph faster in all these conditions, would make the Advance Speed equal to the Aircraft Speed, thus reaching the (theoretical) zero efficiency point, but maybe I am wrong...

Sorry... So I am led to believe that the results of the calculation are actually the zero efficiency point, MINUS the necessary working assumptions, which have to be done as yet. So, if the curve dropping down to zero at 0.8 is completely wrong, obviously the aircraft´s real flight behaviour will change. ...or will it?

Now I have to add a working assumption to J factor 0.673 and test for results. As zero at J=0.8 is bad, I could try 0.9. I wouldn´t use 1.0 because that seems to be for a larger propeller. My working assumption would then be adding 0.23 to the J factor calculation result.

Update: OK, I just tried the propeller with the graph going down to zero at J=0.9, and unfortunately, there was no performance difference. The 220 and 225 Hp positions and corresponding RPM, as well as max 105% 237 Hp at 2200 RPM positions all remain unchanged.
I very much doubt that anything can be done to improve the propeller.
I don´t get a bad feeling out of it at all!

OK, that´s it! I´ve decided the propeller and the engine are doing fine, and I´m stopping work on it.

I really must get to building the plane - the delay with the propeller has taken FAR too long and hasn´t made any tangible improvement for a long time. Thus, the job qualifies as "as good as it gets", which is excellent!

Anyway, full technical information on the propeller is available here on this post, and it works fine!
I don´t think I have to bother you with it any longer, Ivan, the torture has ended!
Possible improvements I´m afraid will only be very marginal, so there´s nothing to worry about.
Much more important now, is to get the visual model underway!

Cheers, and thanks for the support all the way along this very long propeller whittling process!

Hereby I declare that this is the END of my participation on this thread.

Three Cheers,


Aleatorylamp

[Ivan]

Hello Aleatorylamp,

A) 1900 RPM: desired for 202 Hp, but this power isn´t possible even at 2000 rpm.
B) 2100 RPM: desired for 220 and/or 225 Hp but impossible to achieve in the sim.

What have you tried to achieve the correct power levels?
I don't know why you are testing anything else if you haven't gotten the Engine power correct with a Constant Speed propeller yet.

I am not familiar with the program you are using to check Advance Ratio but I am guessing that you are not using it for its correct purpose.
You do realise that Advance Ratio for Zero Efficiency does not change unless the shape of the propeller changes.

Advance Ratio of 0.673 for Zero Efficiency sounds like a calculation based on a single blade element and a totally planar blade airfoil which is probably not how things really are.
My calculation gives an Advance Ratio of around 0.9 or so for a 16 degree Blade Angle.

The differences you are getting in your calculated Advance Ratio during testing and the Zero Efficiency Advance Ratio is actually not useful information.
You should use your calculated Advance Ratio to do a lookup into your Propeller Tables to see what the simulator is using for Propeller Efficiency.

This would however not altogether invalidate the results giving +10 mph and +0.05 J factor difference that I got in the calculations, compared to the actual in-flight aeroplane speeds.
I thought it seemed logical that pushing the plane 10 mph faster in all these conditions, would make the Advance Speed equal to the Aircraft Speed, thus reaching the (theoretical) zero efficiency point, but maybe I am wrong...

There is nothing to invalidate because the Data you have tried to collect is meaningless.
The only possible way for your Airspeed and Advance Ratio to match the values calculated by this Program you are using is if:
1. The Airframe / Wing has Zero Drag.
2. The Propeller Tables are in agreement with a Theoretical Zero Camber and single Blade Element Propeller.

I believe you are using a tool that is meant as a "Rule of Thumb" check and do not understand the numbers it is giving you.
I believe your tool is giving you the Geometric Zero Efficiency Advance Ratio without consideration for other "practical" aerodynamic factors which may alter that point of Zero Efficiency.

Update: OK, I just tried the propeller with the graph going down to zero at J=0.9, and unfortunately, there was no performance difference. The 220 and 225 Hp positions and corresponding RPM, as well as max 105% 237 Hp at 2200 RPM positions all remain unchanged.
I very much doubt that anything can be done to improve the propeller.
I don´t get a bad feeling out of it at all!

Your Aeroplane is getting stuck at an Advance Ratio of around 0.625 or so by your own account.
What did your alteration to the Tail End of the Graph to extend it to 0.9 actually do to the place you actually happen to be?
(Probably Nothing which is why nothing changed!)

The Technicians in my Workshop actually have also been working on a Propeller for the BV 141B.
Actually, They are working on the Constant Speed Unit and if you think you have it bad with just one Blade Pitch.....
They finished up the CSU just before I started this post, so it is time to install and test.
(Can't Ground Test this one because it needs some altitude for a proper test.)
I suspect Zero Lift Drag will change again as a result, but such are the problems when things are done in the wrong order.

- Ivan.

[aleatorylamp]

Hello Ivan,
It was getting too hard and taxing on my mind, and I had to disconnect from the Lycoming engine/propeller tuning for a while. A change is as good as a rest.

Meanwhile, I´ve managed to build most of the Stearman-75 Model, with reasonably satisfying results, in 2 versions: One blue/yellow Army Corps version and one yellow Navy one.

I still wanted a better engine though, as the 9-cyl. Lycoming R-680-? was only possible as a usable approximation. Thus, I switched to the 7-cyl. Continental R-670-4, applied all the necessary propeller and engine adjustments, and tested it.

Lo and behold! I expect you will be pleased to hear that after a few tries and slight further adjustments, it worked out perfectly! Performance is almost exactly as per specification!

So, why did it work so easily with the Continental engine?
What was amiss with the other engine? Conflicting information from the two Lycoming engine certificates regarding rated horsepower and RPM that just couldn´t tally. So, this engine can´t be used until these contradictions are cleared, which I doesn´t worry me, though!

Anyway, I just thought you´d be interested to know.
Cheers,
Aleatorylamp

[Ivan]

Hello Aleatorylamp,

Sorry about not getting back to you for a while.
I wanted to concentrate on the BV 141B until it could go into production (Uploaded).
My Son also has been sick and out of school for the entire week with a fever, so online time has been somewhat limited.

I am somewhat surprised that the Continental and Lycoming Engines are so different in the Flight Model.
In theory, there should be very little difference as far as representations of the 220-225 HP military engines are concerned.
RPM, Displacement, Power, Compression are all very similar, so I wonder where the difference would be?
I have done no research on the Continental R-670, so can offer no specific information.
Glad you are happy with the results.

Please note though that the flight performance and correct engine simulation are really not the same thing so I would suggest not letting that be the sole determination of the correctness of the flight model.
On my BV 141B for example, the straight line performance was probably closer to the book when I started than it is now but the engine and propeller models had serious errors.
Other than straight line performance, there were other performance and handling issues that were not so obvious but could be found with some specific testing.

- Ivan.

[aleatorylamp]

Hello Ivan,
I have also commented on this on the other Stearman thread,
but in less detail.

The problem appears to lie in the specified RPM for 105% power:
An increase of 12 Hp for a 100 RPM increase won´t tally - at least in the sim.

Specs: 100% = 220 Hp at 2100 RPM, and 105% = 237 Hp at 2200 RPM.
The best difference I obtained was 45 RPM, and it was making me ill, so I gave up.

The Continental engine didn´t have an RPM specification for 105% power,
and with 100% at a close 2080 (instead of 2075), and 105% at 2020 RPM,
it all seemed more correct. Even more so when I eliminated the extra 5% altogether.

Later, comparing the normal 100% performance for both engines, the
7 x 95.4 cubic inches with 5.4:1 compression ratio of the Continental, and the
9 x 75.55 cubic inches with 5.5:1 compression ratio of the Lycoming, there was
virtually no difference at all in the 100% power performances of both engines!

So, you are right in that the engines are virtually the same,
even in the simulator, of course!

Cheers,
Aleatorylamp