Boeing Stearman Model 75

Hello Aleatorylamp,

Here are some general observations thus far:
You seem to want to build a nice high powered (and fast?) biplane.
You want to build a Boeing Stearman 75 Primary Trainer because of the article from Smilo's magazine.
You want it to be equipped as it would have come from the factory.

I do not believe that all of those features are possible.
A lot of Stearmans are still flying today, but none of them are absolutely original.
Those flying today have a different purpose. They are no longer military Primary Trainers.
They are no longer Military and are now subject to civilian (FAA) regulations.
Technology has advanced.
Parts have worn out and are no longer being produced, so to stay flying, they cannot be 100% original.
Engines have been rebuilt or replaced and often the replacements are better than the originals.
Folks don't use unshielded ignitions any more because it interferes with electronics many of which did not exist at the time these aeroplanes originally left the factory.

The Stearman 75 was a pretty good airframe, but had its problems as it came from the factory.
It was quite low powered for the size of the aeroplane and it only had two ailerons.
Folks who acquired them as military surplus have chosen to correct some of those issues because the airframe accepted those changes pretty easily.

The point I am getting at is this:
The Boeing Stearman 75 in its military configuration is quite a respectable project, but it is certainly no hotrod of a biplane.

- Ivan.
 
Hello Ivan,
I am putting together a document with the different versions, their power and their speed indications, to have an overview of what´s happening. As you pointed out, there´s only one PT-17 version with 135 mph, and the engine is a normal one. The empty weight, has a typo stating 1036 lb, and should be 1936 lb - but typos are more easily identified than hidden things.

I had been hoping to find the higher speed related to a -13B (or even a -13C) version...

The more I think about it, the more it seems that the -13B with 280 Hp was officially throttled or limited to the standard 124 mph, for bureaucratic certification reasons on the grounds of the fixed pitch prop, to keep everything within limits, but thatextraofficially, the -13B was capable of a much higher speed thanks to its power.

Otherwise there would be no reason to name this batch -13B. The naming always seems to have depended on the motorization. So, the R-680-13 engine on these units may well have been the higher compression type with a 7:1 compression ratio, and the top speed then may have been higher even than the 135 mph I´d thought would be applicable here.

Incidentally, the blower indication on the plaque of the atmospheric 1941 300 Hp R-680-13 on the "false treasure" 1996 Stearman is stated as being a direct drive blower. If I am not mistaken, this would then just be a fan on the crankshaft, blowing a bit more air into the carburettor to get a bit more oxygen, which would then not be a supercharger with any critical altitude.

Anyway, before deciding on something definite for the 2 planned military versions, we´ll have to study the matter a bit more.

I have just seen your second post, thanks, and shall we say, I am trying to find the fastest and most powerful factory-original military version possible.

Because of the 2 or 3 different and colourful attractive military colour schemes, there are 2 military versions planned. To make these more attractive, the first one would be the normal 220 or 225 Hp version with 124 or 125 mph. For the second one, I´d like a faster and/or more powerful one, IF this were to have existed, and there seem to be certain indications that this MAY be possible.

However, should this only be a chimera, I will settle for a difference of only 5 Hp and a couple of mph, because anyhow, there will be a third model, a 450 Hp P&W powered civil aerobatic super hot-rod, so the apetite for more speed and power will be satified anyway.

OK, then,
Cheers,
Aleatorylamp
 
Hello Aleatorylamp,

If you are really going for the hotrod version, there was at least one with a Pratt & Whitney R-1340 installed.
I don't think it would be all that fast, but it should climb REALLY well with such a high power to weight ratio.
My impression is that if you really look at the operating limits, the 450 HP Pratt & Whitney R-985 wasn't noticeably better than the current 360 HP Lycoming engines.
The 450 HP was just a Take-Off / 1 Minute Rating and normal maximum RPM was only 1950 which is really low.
Not only that, but the R-985 is a substantially heavier engine.

- Ivan.
 
Hello Ivan,
Hmmmmm... As I´m still undecided as to the power/performance for the more powerful military version, I havent´gone into studying the details of the planned hotrod yet.

However, I did notice that the 450 Hp 985 P&W is said to be rather heavy, and now that you mention it, perhaps the 360 Hp Lycoming series (is that the 4-cylinder Boxer aero engine?), or maybe the 415 Hp P&W R-1340 Wasp, would indeed be more appealing alternative. Because of its looks, the Wasp would probably be more fitting...

Trials on the simulator could give a general feeling as to the different behaviours resulting from the added power and weight of the three engines, so I might experiment a bit with that at the weekend and see what comes out!

Cheers,
Aleatorylamp
 
Hello Aleatorylamp,

Please note that the Pratt & Whitney R-985 "450 HP" Engine was only good for 450 HP for ONE MINUTE pf Take-Off (WEP) Power.
Normal maximum was only 310 HP @ 1950 RPM. (See Post #8)
That puts it way above a purely stock Stearman Model 75 engine but not above the other hotter Lycoming R-680 engines AND the engine weighs about 150 pounds more.

The 360 HP Lycoming would be a modern version of the same R-680 that we keep seeing on the PT-13 and the beautiful thing is that they give a lot more power but are in the same weight range as the stock engines.

The R-1340 is the same engine as found on the AT-6 Texan / SNJ Harvard and would be giving about 600 HP in stock configuration and probably a bit more if tuned a bit.
At that point, I suppose you would need to change the designation to AT-13, AT-17, AT-18, or AT-27????

Need to go out again.
- Ivan.
 
Hello Ivan,

Wow! - as I said, I hadn´t done much research on the P&W R-985, and then, upon seeing your last post I decided to discard the heavy P&W R-985 with its 450 Hp only for 1 minute. Then I had a quick glance at the info on the other two engines, but it seems that it wasn´t quite enough to obtain their correct power data to see what we really have afoot. Thanks for the clarification!

Hello Smilo:
Just out of curiosity, which of the two engines would you prefer? The modern 360 Hp Lycoming R-680, or the 600 Hp P&W 1340?

Cheers,
Aleatorylamp
 
to be completely honest,
it doesn't really matter.
my main interest
is in the WWII era trainer.
the post war crop duster
or modified civilian hot rod,
i'm sorry to say...not so much.
(i hope that wasn't rude)

i patiently await
the visual model.
 
Hello Smilo,
No, it wasn´t!
OK then, then, I´ll put the 360 Hp Lycoming R-680 into the hotrod.
I tried both out both ( with CV props), with correct displacement
and compression settings (albeit with standard propeller tables...)
and got exact power and RPM readings from the Lycoming, and quite
reasonable approximations for the other one.

The 600 Hp P&W 1340 with twice the cyl. diplacement seems a bit outlandish,
also a bit more complicated to programme, because it is supercharged, but it
was good enough to see what it feels like. Engine torque , accelleration and
climb is much greater than on the other one, and top speed is 170 mph.

However, I like the Lycoming one better, and got 150 mph top speed from that,
with the same airframe that I had for the standard military version where I got
exact 220 Hp/125 mph, and 280/136 mph for the mysterious, elusive, yet possible
other one.

The probabilities of this military, stock 280 Hp version having existed, with an
official speed of 125 mph, limited to that by possible caution because of the
fixed pitch prop, but extraofficially capable of achieving 136 mph, could be:

1) It can´t have existed.
2) It might have, but probably not.
3) It may have, but the again it may not have.
4) It may probably have existed, but it might not have.
5) It may well have existed.
6) It must have existed.

If Ivan and yourself were to choose Options 5) or 6), it is reasonable enough
to make the second military model a 280 Hp powered one.

Opinions, please?

Then, I don´t mind if we don´t have a cropduster - I´m non-plussed about that.

I´ll very soon start on the visual model. I only have to scale print out the plans.
Cheers,
Aleatorylamp
 
please, correct me if i'm wrong.
you like the "mysterious"
280/136 lycoming, right?
if so...then, use it.
make a note in the readme file
explaining (as best you can)
why you're using it and call it good.

heck, you can even note it
in a 280/135 mph air file description,
so it shows when selecting aircraft.
if others don't like it,
they can choose another model
with a different air file.

when you're ready (and if you want),
i can help you put together a package
where texture, engine configuration,
and three model options are available,
all, in one model 75 folder.
basically, allowing pilots to choose
which ever variant they want to fly.
...but, that's way down the road.
 
Hello Smilo,
Great! A 3-aircraft package sounds like a good idea: One standard Navy version, one Army model with a choice of two motorizations, and a third model, the civil aerobatic hotrod, all with 9-cyl Lycoming R-680 engines of different Horsepower.

Your help, when the time comes, will be necessary to put the package together. I´ve done it very often, but very rarely because I found it rather tricky.

Cheers,
Aleatorylamp
 
it's been a while, but,
when the time comes,
it's very doable.

i was thinking, maybe,
two navy versions,
two air corps versions,
and the civilian hotrod.
heck, while we're at it,
we could even add,
high powered,
fantasy military versions,
but, that might be pushing it.

so, no seven cylinder choice?
don't get me wrong,
i'm not complaining,
just askin'
 
Hello Smilo,

Once I saw that the standard 220 Hp powered PT-13/17 could also have 9-cyl Lycomings, and I was more concentrated on power than on cyl-number, I automatically discarded the 7-cyl Continentals, but I´ll gladly supply one if you like. No Prob!

I already have bleedwise half-way acceptable 5, 7 and 9 engines, with box-shaped cylinders, engine-block cones and pushrods on the front. Exhausts are either individual parts for each cylinder, or an exhaust ring with two pipes below the nose. These were the Siemens-Halske and BMW-X for the German Klemm and the Flamingo, so a Continental and a Lycoming reconversion won´t be much ado.

The 7-cyl. version works better that the 9-cyl one, so there´s no problem in having two different builds depending on the engine. The more bleedless one will be the 7-cyl one.

What I´ll try is make hexagonal shaped cylinders, if it doesn´t increase the bleeds, but the basically "v" shaped cylinderheads will probably be impossible and only cause extra bleeds.

I´ll start with the 9-cyl engine, and once I´ve got it done, I´ll make a description with screenshots and see if Ivan can suggest ways of improvement, which I´ll try out. I don´t want to make Ivan fire up his development computer and reduce its longevity!!

Interesting also is the tailwheel, steerable for the Army Corps and swivelling for the Navy one, I believe.

If there are 5 or 6 versions, I don´t mind. We can have as many as we want! No prob either!

Update: I have never been a fan of "what if" versions, either for download or for upload, and a fantasy high powered military version I find, would fit into this category. For the moment, the 280 Hp engine with such low performance is a contradiction and is totally ridiculous. Why would anyone build a -13B series with 280Hp engines to get the performance of more normal 220 Hp ones? The only way it could have existed is its the performance was officially disguised, and I need a bit more information to make it less of a fantasy, so I don´t think I´ll do one or worry about it any longer!
Then, I´m trying to figure out if it is reliable enough to say there was a 300 lb lighter edition of the P-17 to justify the 135 mph performance mentioned on some sources but not on others for the same model.

Cheers,
Aleatorylamp
 
Last edited:
i'm not much for fantasy aircraft, either.
i was just saying,
putting one together could be easily done
with a few lines added to the the aircraft.cfg.

my mention of no seven cylinder version
was just a query, not a request.
i'm good without it, but,
if you want to do it,
i'm okay with that, too.

visual engine details would be very cool,
but, for the most part,
i understand the limitations of af99.
all i can really do, is sit back,
watch and say, good luck.
sorry, i can't be of more assistance.
 
Hello Smilo, Hello Ivan,
I hope you´ve had a nice weekend. Here it was sweltering again, and the brain gets mushy with
temperatures that don´t fall below 26.5 C at night... but we get by.

There´s an excellent Stearman PT-17 Pilot´s report here:
http://www.airbum.com/pireps/PirepStearmanPT17.html
It describes a lot of the sensations, and makes a very good read.
Talking of sensations... I´ve seen a couple of videos with these planes flying SIDEWAYS, at 1 yard above the ground.
Quite spectacular, to say the least. These planes must really be absolutely fantastic to fly - even the normal 220 Hp ones!

Anyway, I got the link to the article from an ad where a 280 Hp Stearman is being sold in the Czech
Republic. Curious is, that it has a wooden propeller. Unfortunately there are no performance details.

This is the link to the ad:
http://www.planecheck.com/index.asp?...id=34151&cor=y
The ad says:
Boeing Stearman N2S Kaydet For Sale in Czech Rep.
Year of construction: 1943 - Country Czech Rep. - City/Aerodrome: LKOT
Description:
TTSN 5293 hrs
NORMAL Category plane
Radio + transponder
Lycoming R-680-11 Engine 280 HP
SOH 0 hrs
Wood Sensenich Propeller

I have sent them an e-mail asking about the engine installation date and aircraft top-speed. Let´s see if they answer.

Now, to make this whole 280 Hp tease even worse, there is yet a third place where it is mentioned:
Aerofiles.com also mention the stock 280 Hp Stearman, but don´t give any performance details.

http://www.aerofiles.com/_stear.html

They present a photo of a Boeing-Stearman PT-13B Final assembly (Boeing)
and also give unit numbers. Note the 255 factory -13B´s from 1940 and the 6 -13C refits from 1941.

PT-13B 1940 = 280hp R-680-11. POP: 255 [40-1562/1741, 41-787/861].
PT-13C 1941 = PT-13A repowered with 280hp R-680-11. POP: 6 modifications.

That´s why I think a stock 280 Hp Stearman can´t be a fictitious thing, but something that must have existed.

Cheers,
Aleatorylamp.
 
Hello Gentlemen,

We dropped off my Daughter down in Charlottesville on Saturday.
Yes, the same Charlottesville where all the disturbances are!
Just got back from another supply run today for some essentials.
Just finished watching the eclipse.
The peak was during a storm and hidden by clouds but we do have some good pictures.
It was clear about 3 minutes before the peak though.

Now back to the Stearman.
Assuming that there WAS a 280 HP R-680-11 installed in a few PT-13B's and PT-13C's (and I am still not entirely convinced that there were), there is the possibility that the level speed performance did not change at all.

How can this be? More power and it should go faster, right?

What happens if the extra power is gained by increased engine torque without an increase in maximum RPM limits?
In that case, with a fixed pitch wooden propeller, you pretty much would not see anything unless you changed the propeller pitch which you cannot do.
The aeroplane would accelerate better and climb better, but would not necessarily go significantly faster because if engine RPM and propeller pitch were matched well for the standard power engine, you would overspeed the engine to go any faster.
That is the way it appears to me, but I have not thought about it a lot.

- Ivan.
 
Hello Ivan,
Here in the evening the eclipse was also visible as a partial one, but there was a lot of haze and only a small lower portion was cut away.

Well, about the Stearman:
Top speed being limited by the fixed propeller did not occur to me, and I don´t know enough to be able to tell at what RPM and Speed blade-stall would happen, but it is a possible explanation if the 124 mph was close to the limit of the propeller.

There also was a metal ground-adjustable propeller that was fitted to wartime trainers, and that could have been more useful for the more powerful engines.

I did notice a comment on one of the aerobatic models with a 300 Hp CV prop engine that did 157.6 mph, so obviously pitch adjustment made all the difference.

Anyway, let´s hope the seller of the plane ion the Czech Republic answers - I´m not too sure he will because I didn´t express interes in buying the plane.

Incidentally, I´m also having trouble with the vertical and horizontal dimension mismatch on some of the drawings, and on one where it doesn´t happen, left and right halves of the drawing are quite a few pixels difference, but I´m making the necessary adjustments.

Cheers,
Aleatorylamp
 
Hello Aleatorylamp,

The condition of "Blade Stall" would be at low speed, so this is not really relevant at the moment.
You easily have enough data to figure out how the Sensenich wooden propeller and others behave by carefully reading the FAA Type Certificate.
A useful thing to know is that in practical aircraft maintenance, one starts with a brand new (perfect) propeller which gets nicks and dings in service.
The nicks and ding are removed by sanding or filing them out and re-balancing.
Over time, the propeller gets progressively smaller and at some point becomes too small to use.

The main problem you will run into is that you don't have a good set of propeller tables to use that cover the low pitch angles that are used in the Stearman 75 and you will need those in order to figure out a few more details.

The process I would follow here is to first build a reasonable Propeller Efficiency Table covering the range of 5 degrees to about 20 or 25 degrees.
Make sure your engine power / torque curves are reasonable for the type of engine and RPM range you are dealing with.
Experimentally determine the power coefficient from the FAA data.
Extrapolate the rest of the Power Coefficient Table from looking at the Geometry of the propeller and where the efficiency would drop to zero.

The numbers for the Stearman 75 are so different from everything else I have worked with that I can't give you any specifics without actually following the same process myself.

- Ivan.
 
Hello Ivan,
Thank you, as always, for your clarifications, and also for your instructions to make
correct propeller tables for a possible testing platform to obtain some closer information.
One thing is that I understand what can be done, and quite another is to understand
enough to know how to do it...
OK, blade stall is only at low speed. To have blade stall at high speed you´d need a much higher
pitch, of course, and as in this case pitch would be too low, the effect is that the propeller would
act as a brake. Increased RPM would probably help, but maybe the possible 2300 instead of 2200
would probably not be enough.
Cheers,
Aleatorylamp
 
Hello Ivan,
Now I remember we made a wooden fixed pitch prop for the 1917
high-compression D.IVa Mercedes engine (max RPM 1400).
I´ll have a look at my notes on the angle entries upto 25 degrees.
Maybe I CAN do something!
Cheers,
Aleatorylamp
 
Hello again,
I´ve been downloading drawings and sketches with key measurements to double check, in order to discard inaccurate ones and fix the most usable one. A bit lengthy, but hopefully it will avoid later re-adjustments or corrections once building is underway.

The two places I´ve e-mailed about the possible 280Hp Stearmans haven´t answered, but I´ll discard the version completely - there´s little point. First of all, as Ivan says, performance would be limited by the propeller anyway. Secondly, I think I´ve discovered the reason for a possible incorporation of this engine, namely the severe engine shortage at the time, which forced the manufacturers to resort to the more powerful engine , of which there was more stock, but due to the wooden propeller, and perhaps even the gound-adjustable pitch one (which was also used), top performance had to be curtailed.

It is still strange, though, why so many sources state 135 mph instead of 124 mph for top speed for the same models. Even the 300 lb lighter version appears without weight reduction in other places.

Cheers,
Aleatorylamp
 
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