Boeing Stearman Model 75

[smilo]

...and so, off we go, again,
into the wild blue yonder.
so many choices,
with so much conflicting
or insufficient data.
what to do? what to do?
i'm sure it will all work out in the end.

mean while, back at the ranch,
there's, still, so much work to do.

~~~~~~~~~~~~~~~~~~~~~~~

thanks for reminding me
about computers being on their last legs.
i need to back up the data
on my xp development machine.
there are so many years worth
of irreplaceable stuff.
one never knows when
the steadfast old beast
might want to give up the ghost.
it's not like there haven't been warning signs.

[Ivan]

...and so, off we go, again,
into the wild blue yonder.
so many choices,
with so much conflicting
or insufficient data.

Hello Smilo,

The Boeing Stearman Model 75 is hardly a case of insufficient data.
In fact there is so much data that there isn't much you CAN'T find out about the machine.
You have one in your local area.
I have one in my local area as well.

Around 8,000 total aircraft were produced. Enough spare parts to assemble another 2,000 aircraft were also produced.
Of the 8,000-something total airframes, about half still exist today.
Several THOUSAND are still airworthy.
The FAA has a LOT of data in their Type Certificate which is more than you can typically find about WW2 types.
I have come across 3 or 4 sets of pretty good drawings and I wasn't even looking hard.
There is a pretty easy to find flight instruction guide available and at least two or three pretty good flight evaluations to give a good impression of flying characteristics.
Basically it was pretty well behaved except for a tendency to ground loop rather easily.

Regarding conflicting data, a very small amount of reading will give a pretty good idea of what the original aeroplane really was.
One has to have the sense to distinguish between period sources and current sources. Each is good in its own way and for different details.

The first of these aeroplanes flew in 1934, so one has to consider how much equipment would be original in an example that is still flying today.
The fabric probably would have been replaced at least once.
The engine probably would have seen several overhauls or replacements and here is where life gets interesting:
If you owned one and needed an overhauled or replacement engine, would you keep it at the original 220 HP for historical reasons or would you go for a rebuilt zero-time engine with some modern accessories and 50% more power that weighed the same and looked the same?

This is one of those cases where there is so much data and it is so good that I have to actively resist the urge to start gathering information and build my own version of this beast.
It would not be hard, but is yet another diversion from things that deserve to be completed.


Hello Aleatorylamp,

The distinction between PT-13, PT-17, and PT-18 is basically the engine that was installed. All engines were of pretty similar performance and weight.
I believe there was only about 20 pounds difference between the lightest and heaviest of the engines originally installed.
The PT-27 as stated earlier was a Lend-Lease designation for the PT-17. I don't know for certain, but I suspect the designation change was probably because of some different equipment installed for the Canadians.

Here is some data I found a few days ago that I believe is reliable. I just copied it directly from a site to a MS Word document but neglected to save the URL.

PT-13 with a Lycoming R-680 engine (9 Cylinder) – 2,141 built.
PT-13 Initial production, R-680-B4B engine, about 215 HP
PT-13A R-680-7 engine, about 220 HP.
PT-13B R-680-11 engine, about 225 HP
PT-13C The same as the PT-13B, but modified for instrument flying
PT-13D R-680-17 engine, around 225 HP

PT-17 with a Continental R-670-5 engine, about 220 HP – with 3,519 delivered.
PT-17A Modified for instrument flying
PT-17B Modified with agricultural spraying equipment

PT-18 with a Jacobs R-755 engine, about 225 HP- 150 built.
PT-18A Modified for instrument flying

This information may also be found on Wikipedia in almost exactly the same format.

- Ivan.

[aleatorylamp]

Hello Ivan, hello Smilo.
Thanks, Ivan I´ve also seen the information you are giving. That´s why I said it was a bit difficult to confirm the scarce date I found about the 300 Hp and/or 330 Hp P-13D versions.

Of course there is no doubt about a P-17 with 220 Hp, so that one´s solved.

However, there is a video of an authentic 1943 300 Hp Lycoming R-680 running on a trolley, comments in more than one site that the engine these were also used on our friend the AT-9 Jeep (R-680-9), and also a post in a forum of a guy that wants to use a vintage 300 Hp R-680-R on some build he´s making.

So, if they weren´t ex-factory, many of these birds must have been retrofitted WITHIN the production period, i.e. upto 1943, before the end of WWII, which is good enough for Smilo. So, even the 330 Hp version I found referred to could have been existed and may not be a typo, from the site I saw, but at least, with the video, and the comment on the contemporary AT-9 Jeep as having the same engine, we know that 300 Hp versions during the war did exist.

So, I´d propose one military version with 220 Hp and 7 cylinders, and another military version in different colour scheme with at least 300 Hp, but that´s just my suggestion.

Cheers,
Aleatorylamp

[smilo]

i say, do it your way, Stephan.
it may seem like i'm passing the buck,
because....i am.
as i see it, as long as you're the builder,
the choice is yours to make.
i'm sure, you'll do what's best.

as we are all well aware,
one can make one's self crazy
trying to make things gnat's ass perfect.
just remember, this project
is suppose to be a fun change for you.

that said,
may i direct you to your signature?
seems appropriate.

sorry, i am spent.
today's short trip,
giving my son a ride to work,
turned into a four hour production
of dealing with an ahole and my vehicle.
each unrelated, but a drag all the same.
i'm home safe and am in dire need of a nap.

[Ivan]

Hello Aleatorylamp,

You can of course build anything you like. That is the advantage of being the designer.

As we discussed earlier with the Baltimore, the engine variant (dash number) may make quite a difference and higher doesn't always mean better.
Consider that a B-26 Marauder, P-47N, F4U-1 Corsair, and F4U-4 Corsair all used a Pratt & Whitney R-2800 engine, but they did not all make the same amount of power.
Consider that the Curtiss-Wright CW-21 used the R-1820 with around 1000 HP while the General Motors FM-2 was getting 1350 HP also with a R-1820.
Look at the P-39 and P-63. Both used a Allison V-1710, but the power levels were quite different.

I have no doubt that a 330 HP Lycoming R-680 existed during this period. I do doubt that the military would have converted a significant number of PT-13s (if any at all) to use such an engine though. There was no point; this was a Primary Trainer. High performance was not necessary nor wanted in such an aeroplane.
After the war, many of these aeroplanes became surplus, but that would not have happened before the war ended.

The implausibility of a 330 HP PT-13 is my own opinion which is no more valuable than anyone else's.
I have not done any conclusive research to confirm my opinion.

Build what you want to build. You don't need anyone's agreement to do it.
Who knows, I may eventually try to build a "Stearman Special" myself at some point.

- Ivan.

[aleatorylamp]

Hello Ivan,
Thank you for abiding so patiently with my insistence on the possible use of 300 Hp engine during wartime use.
By the way, I can´t find the site where I saw the 330 Hp anymore, which was one of the first I found. It´s only 300 Hp now...

Most sites that go into any detail, coincide exactly with what you say, primarily that the 300 Hp engined PT-13D versions were post-war conversions (mainly acrobatics).

That might also explain the comment on other radial-engine related sites, that the PT-13D had the same R-680 (-9 or -13) as the contemporary 1941-1943 Curtiss AT-9 Jeep (300 Hp). I agree that the numbers after dashes on the engine model numbers are not a sequential increase indication for engine power. The dash numbers possibly add to the confusion, and a given dash number perhaps didn´t always mean the same engine power.

What throws me off, and has done so from the very beginning, it the figure of 186 mph top speed and 46 USG fuel tank stated in the AV History magazine scanned by Smilo, instead of the usual 124 or 135 mph and 40-43 USG tanks quoted for the 220-240 Hp engines used on this aeroplane.

Another comment I´ve read that also throws me off, is that some military bases were a bit afraid of using the higher powered Stearman 75 trainers on novices because of their difficulty and danger, and preferred to keep beginners on the lower powered, more forgiving ones. Also it was stated, that the higher powered Strearmans were more demanding on trainees´
skills, and were built because the lower powered ones were too easy.


So, although the majority of the thousands of Stearman 75 trainers used during the war had 220-240 Hp engines, I don´t think that a 300 Hp wartime version can be completely discarded... but maybe you´re right, maybe there wasn´t a 300 Hp Stearman during the war.

Anyway, it won´t affect the model itself - the 9 cylinders are there anyway, so there´s time to decide yet!

Cheers,
Aleatorylamp

[aleatorylamp]

Hello Ivan, Hello Smilo!
I found something else! This time it is probably 100% reliable:

It´s an E-book titled: "American Military Training Aircraft: Fixed and Rotary-Wing trainers Since 1916", by E.R, Johnson, Lloyd S. Jones. It contains a long chapter on the Boeing-Stearman PT Series that was built starting in 1934.

The interesting line says there was a delivery of
"...255 PT-13Bs (280 hp R-680-11 engines) in 1940, making it the Army Corps´ most important type of military trainer." - the "most important type" comment obviously includes the numerous lower powered versions ordered on several occasions before 1940.

Attached is a screenshot of the first part of the text on the Stearman PT´s. The chapter has lots of photos and further details, but the main thing is here, in context. If you save the .jpg image it´s easier to read.

So, Ivan was totally right about there not being a 300 or 330 Hp powered model until after the war, but we do get a nice and powerful 280 hp powered Army one for Smilo!! Then we can do the Navy one with 220 Hp, and the Aerobatics one with 450.

That´s good...

This new information would account for the pieces of information I mentioned in my last post that didn´t seem coherent with there only being 220 Hp or so versions.

Smilo, sorry to hear about the car and the ahole...
Things seldom come alone. They come in three´s , so be careful about a ppossible third problem. Life seems to send what one hates. I often get aholes on the road, and the result is that now I hate driving, which I used to love. Now I love taking the tram, but I get aholes on pedestrian crossings walking to the tram. So it´s on the road too, but not in the car. Less stressing, but perhaps more dangerous...
I mean... Why can´t life be perfect?!

Cheers,
Aleatorylamp

[smilo]

it's very hard to read, but,
doesn't this page, basically, confirm
what was said in post #3?
okay, the engine - designations weren't there,
but.....oh, never mind, i've got pixels to go count.

it's interesting that only 150 units
were produced with the jacobs r-755,
because the engine was needed
to power the sherman tank.
priorities

[aleatorylamp]

Hello Smilo,
OK, I understand - but Post #3 didn´t contain Hp information.
Anyway: 3 models, 220, 280 and 450 Hp.
It´s nice to be able to confirm historically correct specs though.

So the 2 military ones would be: 280 Hp Army in blue-and-yellow, and 220 Hp Navy "Yellow Peril".

Unfortunately, even though the image I put together, taking out the photos, for upload is clearer, downloading it, it is not as clear. As the images come from an E-book available, it´s not possible to get them any clearer.

I noticed the comment on the Sherman tanks´ need for the engines too! Radial engines probably came in handy because they were shorter than the usual in-line ones.

Here´s the second part of the article, where I also took out all the photos. It talks about the Navy versions, and contrary to what I was expecting, they didn´t have any of the powerful ones.

By the way, I could split the first part of the article into two and sent it in two different posts. It may be a bit clearer to read. The second part in this post is smaller, and looks clearer. So, if you want, I can split the first part and re-post.

Cheers, and have a nice cup of tea!
Aleatorylamp

[aleatorylamp]

Hello again,
Perhaps it is best I post the link to the E-Book in question. There are also lots of other interesting trainers described!

The content available is given with permission, so I expect there is no Copyright infringement.

https://books.google.es/books?id=kIy...engine&f=false

If this link works, I could delete the images in the attachments, so as to save megas on my posts...

Cheers,
Aleatorylamp

[smilo]

no need to split the first and re post.
i forgot about the Ctrl + keystroke
to enlarge the screen view. works fine.
also, the link works.
i might suggest leaving the images,
until you get a upload ceiling warning,
then, delete them.
anyway, thanks for your efforts.

side note;
the jacobs being used in the sherman tank
is interesting, along with how it was done,
(how did they cool the thing, etc?)
but, it would take us way off topic,
so, i'll leave it at that.

stay cool

[aleatorylamp]

Hello Smilo,
Off topic? Naaaahh. Radial engines are on topic, so the Sherman is too!
I just found a diagram.
I was wondering how they´d done it too - horizontally? in the middle?...
Actually, they stood it up, and it was so short that it fit snuggly into the aft section. Cooling by water, it seems, because there´s a water pump and a radiator, so they must have put jackets or whatever you´d call something like that around the cylinders.

If we put some wings on it it will surely fly in CFS1 !!! ...and we´ll ask Ivan to fine tune the FD!
Cheers,
Aleatorylamp
P.S. Thanks for the Ctrl + keystroke. Depending on the size of the image, it works quite well!

[Ivan]

Hello Aleatorylamp,

I would not be so certain that ANY data is 100% reliable.
This book is certainly more reliable than my unresearched conjecture, but if it is important to you, do some additional research.

I am glad you found what you were looking for.

I actually have been debating on revising the flight model for my Kawasaki Ki-61-I-Tei because a source I had used for reference for fuel tank arrangements is probably not as accurate as I had first thought. The book has pretty good data otherwise.

- Ivan.

[aleatorylamp]

Hello Ivan,
Well, OK, then maybe 95%...

Another possibility would be to e-mail some flying clubs that participate in Stearman Fly-ins, that could possibly have members who would remember actually flying these aeroplanes, but I´m satisfied enough as it is.

The 186 mph top speed in the AV History magazine, as opposed to the usual 124 or 135 mph, is accounted for, and so are the other comments elsewhere, relative to the greater difficulty of higher powered Stearman trainers.

It´s rather fortunate that the data on this biplane is contained in the first 89 freeware pages of the 471-page E-book!

Cheers,
Aleatorylamp

[Ivan]

Hello Aleatorylamp,

Since I don't have the Aviation History article to reference, I will not comment on its contents.

Not sure what you mean about a 186 MPH top speed.
If this is maximum diving speed / indicate air speed, it makes sense.
If this is maximum level speed, it makes no sense whatsoever unless the airframe has been heavily modified.

I don't know what you mean about the greater difficulty of higher powered Stearman trainers.

- Ivan.

[aleatorylamp]

Hello Ivan,
Thanks for your comments! The AV Magazine I mentioned is the September Issue of the Aviation Magazine 2017 that Smilo scanned and posted in Post #6, that shows a top speed of 186 mph, whereas normally it is stated elsewhere at 124-135 mph.

Actually, I was just going to ask you if this would perhaps be IAS, because it seemed to me to be a bit much to get an increase of about 60 mph for a difference of 60 hp. Perhaps more plausibly it would be about 140 mph TAS?

Update: This makes even less sense, because IAS on the Beckwith gauge stack is lower than TAS.
Maybe it´s not 186 mph because it´s difficult to read on the AV Magazine article, and it´s really 136 mph, as opposed to the 124 mph of the 220 hp SPT-17. If this difference is more plausible, then the AV Magazine is referring to the 280 Hp Stearman PT-13B. It would also account for the fact that top speed is quoted at 135 mph by some sources instead of 124.

The greater difficulty of the more powerful Stearman 75 for novice pilots was that the more powerful engine increased the problems on the ground - ground loops were mentioned.

I hope this helps.
Cheers,
Aleatorylamp

[Ivan]

Hello Aleatorylamp,

Not everything in print is correct. I do not believe the information in that article is reliable.

I found the You-tube video about the 1943 era 300 HP Lycoming R-680 engine on a trailer.
It is a later series engine than the ones in the Stearman because it uses a Hamilton Constant Speed Propeller and the earlier series engines could not handle a propeller governor.
I found a copy of an operating manual for the R-680 and it show there were many models of the R-680 and it appears that most of the power gain was due to a higher compression (and higher octane fuel) used by the later versions. I just have not found what version corresponds to the R-680-11.

An easy way to prove what models of the R-680 would have been installed in the Stearman would be to find out what the compression ratio and octane requirement are for the R-680-11 and what standard of fuel was used for training in the United States. (The B series engines used 73 octane minimum and I believe the E series used 87 octane.)

The Wikipedia page lists Jane's in its bibliography and I actually own the book, but although the book entry has an excellent technical description, it does not have what I am looking for either.

- Ivan.

[aleatorylamp]

Hello Ivan,
Your expertise is definitely bearing fruits towards clarifying the enigma of some possibly more powerfully motorized military version. Thank you for your comments on the video with the 300 Hp version and the CV propeller, which further confirm that I can safely discard this one, as I did yesterday and as you had already pointed out before.

The most commonly quoted top speed for the Stearman 75 is 124 mph. I seem to remember you once saying that performance increases were about 5 Hp per Mph, so the speed difference between 220 Hp and 280 Hp (60 hp) would be about 12 mph, making it plausible that the more powerful units could have a top speed of 136 mph. If a biplane´s airframe were to further reduce this difference a bit, we would arrive at 134 mph or 135 mph, which is a figure quoted on several other sites.

OK, I agree that just because it´s printed, it doesn´t mean it´s correct... our literature teacher at high school often said that, but here there are quite a few discrepancies that have piqued my curiosity. Granted, initially, in my ignorance, I was misguided by more exaggerated mistakes in some sources, but now it seems to be narrowing down to more plausible results.

Update:
The most commonly quoted top speed for the Stearman 75 is 124 mph, like the Wikepedia in Spanish amongst others.
Other sites quote 135 or 136 mph, like the Wikipedia in English, amongst others.
Both refer to the 220 Hp Engine!
Obviously something is quite wrong: 124 mph and 135 mph for the same engine power?!
Then, the R-680-11 is quoted elsewhere as giving 225 Hp, but 5 Hp don´t account for 11 or 12 mph speed difference.
Wouldn´t it be more likely that 280 Hp would account for that?

It is interesting you mention Fuel Octane. I saw a rather unusual one, I thought: 78 Octane. If usually 73 was normally used for the 220 Hp engine, maybe the 78 Octane would coincide for the 280 Hp one.

I´ll get there in the end!
Thanks again, and cheers,
Aleatorylamp

[aleatorylamp]

P.S.
There are even a couple of South American sites in Spanish, that go even further, claiming that the 220 Hp engines conferred 186 mph, which sounds even more mistaken.

The error may come from the fact that several Stearman PT-13 batches were exported to South America with 300 Hp R-680 radials, and one site actually cites 186 mph top speed for this model. Even this seems a bit optimistic, to say the least, and
at most I suppose it could have been 145 mph for 300 Hp, but we don´t need that anyway.

In conclusion, most probably the 186 mph come from a misinterpretation of the 186 mph Vne which is quoted for the Stearman 75, and Vne doesn´t care about Hp. Then, I said before, the Aviation History Magazine article´s top speed quote that I mistakenly took for being 186 mph, must be a blurry 136 mph, so that´s my own fault for being stupid.

Incidentally, I just discovered that the cowled red model I found on the simviation page does not really look like a PT-17 at all, but rather like Stearman Model 6 "Cloudboy".

This was the predecessor, or base model that led to the Stearman 75, of which several units were built, with a list of different YBT- designations and motorizations that anyone can get lost in. It is even difficult to see how many were built - 7? 15? Who knows? - with all those re-engined ones, it´s impossible to tell.

Two had 300 Hp P&W Wasp Junior and 300 Hp Wright R-975-1 engines, and some others had different 165 and 170 Hp engines of different makes (Lycoming, Continental, Wright, Kinner). From what I can deduce, it appears that the 300 Hp "Cloudboys" were the ones that were too difficult for novices to handle safely on the tarmac, not the PT-13´s. Then, the military YPT- designations of these models also had one with 200 Hp engine which was quoted to have had 200 mph top speed, which is incomprehensible as the Model 6 Series was basically the same as the Model 75.

All this just adds to the confusion. It seems to have been normal in that time for manufacturers to do short production runs as experimentation led to further innovation.

Anyway, Cheers,
Aleatorylamp

[aleatorylamp]

Hello Guys:
Here is further proof of the existence of the 280 Hp powered versions.
On the following internet page:
http://all-aero.com/index.php/54-pla...-1--n2s--pt-18

...it says, just before the end of the first section between the first two photos:

"Then came 220 PT-13Bs with the 209-kW (280-hp) R-680-11s of which six became blind-flying PT-l3Cs, and 895 PT-13Ds with R-680-1 engines."

The other text had mentioned 255 of these, and here there are 220, but that wouldn´t be all that important.
So, in conclusion, there was a 280 Hp poered PT-13B, and my calculation for a top speed for this aeroplane would be 134-136 mph. Is this plausible?

Cheers,
Aleatorylamp

[Ivan]

The most commonly quoted top speed for the Stearman 75 is 124 mph. I seem to remember you once saying that performance increases were about 5 Hp per Mph, so the speed difference between 220 Hp and 280 Hp (60 hp) would be about 12 mph, making it plausible that the more powerful units could have a top speed of 136 mph. If a biplane´s airframe were to further reduce this difference a bit, we would arrive at 134 mph or 135 mph, which is a figure quoted on several other sites.

Hello Aleatorylamp,

I don't know where you got that idea from, but it wasn't me because it simply makes no sense.
A Late model Spitfire Mk.IX had about 1000 HP more than a Spitfire Mk.I but it certainly didn't go 200 MPH faster!

The 124 MPH maximum speed is probably correct for a stock Stearman 75.
The 186 MPH is a maximum diving speed.

I haven't done any calculations to check what the 136 MPH maximum speed might be but I am guessing it might be a racing version of the Stearman 75 with more streamlining, more horsepower and probably a Constant Speed Propeller.

The actual minimum requirement is 65 octane for Lycoming R-680 B series engines and Continental W-670 engines.
Jacobs R-755 engines require minimum of 73 octane.
Lycoming R-680 E series engines require minimum of 87 octane.

Pratt & Whitney R-985 also requires minimum of 87 octane.

There is a LOT of data out there on the Internet and it is very easy to find with a search engine.

- Ivan.

[aleatorylamp]

Hello Ivan,
Thank you again for your counsel and your comments.
Yes, there IS a lot of data.... and sometimes in some particular aspect, it´s difficult to pinpoint.

I suppose that the 5 Hp giving 1 mph increase in speed would apply only to slower speeds, not to fighter plane speeds like the Spitfire, where the power difference you mention would probably only mean about 100 mph - i.e.10 Hp for 1 mph.

I remember it only as a general rule of thumb, for more "normal" speeds, perhaps for speeds of the old bomber biplanes, well below 100 mph. I must have got it from somewhere else. And of course it would depend on the plane.

OK, I agree that 186 mph is Vne, so it is confirmed that we can discard that speed reference.

The airframe of the 280 Hp versions was identical to that of the 220 or 225 Hp powered ones. This means we can also discard streamlining, and as it was a basic trainer, not intended to complicate trainee´s lives, it had no flaps and props were of the fixed type. Thus, we can discard CV props too. There was only an intentionally created difficulty on the Model 75 to make it a less easy but still basic trainer, namely placing the tank in the wing - hence the ground loop risk.

The sim appears to give me roughly 20 mph difference between 220 and 280 Hp readings, depending on some of the drag settings, and I´m getting about 138 mph for 280 hp, and 219 for 220, but that still seems too fast. I´m trying not to meddle with the propeller tables, only with the different Drag types, and Torque + Friction.

Anyway, things seems to be falling into place slowly.

Thanks a lot! Cheers,
Aleatorylamp

[Ivan]

Hello Aleatorylamp,

Regarding Maximum diving speed, if I were you, I would keep it as a reference for tuning the AIR file instead of discarding.

Regarding a stock 280 HP version of the PT-13, the more I read, the less inclined I am to believe that such a thing actually existed.

You probably will have to tune propeller tables anyway because the pitch angles typically run from about 7 degrees to about 13degrees maximum thus the stock tables won't work all that well for you not to mention the power coefficients are unlikely to match up.
It is unlikely that your power / drag tests are meaningful currently because of other data mismatches.

I am a bit puzzled as to why you think a gravity feed fuel tank in the upper wing center section has anything to do with ground loops either as a risk or as a cause.

- Ivan.

[aleatorylamp]

Hello Ivan,
I meant 186 mph as a discard for a top speed reference, not to discard the Vne.

The Drag/Torque adjustments are actually working quite well, because the engine
is very similar to one we had some time ago. Possible data mismatches would not
necessarily have to be very large, as the difference in power is only 15 Hp.

I´ve moved the update from my previous post into this post, for context reasons:

The engine I´m using is a slightly down-toned version of your 295 Hp R-680-9 from
the AT-9 Fledgling. It has 75.55 cu. in. cyl. displacement, 7:1 compression,
(same as the AT-9), but with a fixed pitch propeller (same diameter, 8.5 ft),
at 22 degrees blade angle, but a little lower on the Torque - 64.1 instead of 65.0.
RPM are at 2200, whereas the At-9 Fledgling twins did 2300 RPM. < this line is corrected!

Adjusting Torque and Drag more conveniently, at the moment I´ve got S.L. top-speed
for 220 hp at 124.2 mph, and flooring it to 280 hp, gets me 134.5 mph. My uneducated
impression would tell me that the sim could be giving a reasonably good approximation.
A cook would say "If in doubt, throw it out", and I´d say, "When doubt sets in, try the sim."
However, I still suppose that you will say that a 10 mph increase for an increment of 60 Hp
is too high, even though we are talking about speeds of around ony 130 mph,
which is not terribly fast.

As regards the non-existence of the 280 Hp engine, I´m sorry, but I´m inclined to disagree
with you, as it´s already on two different sites that a couple of hundred wartime examples are
mentioned. Compared to the thousands of units produced, the small proportion in the number of
machines could be the reason they are not often mentioned.

According to one source, the central top wing main tank raised the centre of gravity and made the
plane intentionally top heavy. That´s what they said...

Anyway, sorry to be such a bother, but if there is a possibility of making the two military versions
have notably different performances, it would add to their appeal.
Let´s see what else happens...
Cheers,
Aleatorylamp

[aleatorylamp]

Hello Guys,
I think I´ve decyphered what´s going on with the different speeds and hp for stock Stearmen:

Summarizing:
- Some sites say top speed for 220 hp powered units was 124 mph, others say 135 or 136 mph. WHY?
- Several sites state top speed for 220 hp (e.g. R-680-5) powered units, at 124 mph. OK!!
- Two sites mention 220 or 255 PT-13B aircraft as having 280 hp R-680-11 engines. OK!!
- One of these 2 states top speed for the 280 hp R-680-11 powered units was 124 mph. WHAT??
- The other site quotes no speeds. WELL...
- Several sites ignore the existence of the 280 Hp engine on military Stearmans altogether. FAIL!!
- Several sites and Wikipedia mention the Stearman-75 engine as being the same on the AT-9 Jeep. OH?

But, wait a minute... the AT-9 Jeep (or CW-25 Fledgling) had 295 or 300 hp R-680-9 radials. So what´s going on?

Well...The improvement of the AT-9 Jeep was the AT-9A Jeep, with better hydraulics and avionics,
and the engines had improved ignition. They were two 295 or 300 hp pòwered R-680-11 radials!

So what´s the difference between the 280 hp R-680-11 engine on the PT-13B Stearmans and the
295 or 300 hp R-680-11 engines on the AT-9 Jeep or Fledgling?

On the engine? ABSOLUTELY NONE!! It is THE SAME ENGINE!
It was the AT-9A´s CV propeller and the fixed pitch one on the Stearman that made the difference.

15 or 20 Hp less on the Stearman because of the more inefficient, fixed pitch prop.

So... How fast can a 280 hp Stearman-75 go in level flight?
I bet it could do 136 mph!!

Cheers,
Aleatorylamp.