Hawk81A-2

[JtD]

If the AVG did use higher boosts systematically, then they'd have both higher power output and more engine failures, no matter if the engines were hand crafted or standard production line.

What 'certain, common failures on stock components' did you find they encountered?
How do you think they created the extra power output you mention?

Just to be clear - I mean no disrespect or anything, and do not intend to argue. I'm just curious. Personally, I see a connection between high boosts, high power and high failure rates, but if there's something on top of it and more specific to the AVG, I'm more than willing to learn about it.

[Former_Older]

I think I already mentioned I didn't come here to argue with you. I post here quite rarely and I'm sorry I decided to participate this time, too

[JtD]

It's of course up to you to decide not to share what you know, shame though. Does anyone else have an idea what 'specific things happened' and which 'certain, common failures on 'stock' components' occurred, and maybe also how this is related to a higher power output but not to the higher boosts used?

[ElAurens]

The AVG P40s had a very high failure rate of the prop reduction gear boxes.

Guys, I have worked with both of you on Curtiss related things. FormerOlder has done an exhaustive amount of research on the AVG over the years, and JtD is also well versed on things Hawk, I see no need for you guys to have a spat.

Even without the manual boost control issue, the hand fitted Allisons that were installed on the AVG Hawks did indeed produce more power than standard issue engines. This alone could be the cause of the high failure rates of reduction gears.

Just saying it's because of manual boost controls plays too much into the revisionist view that the AVG's pilots were inferior. Their combat record in the short time they flew stands in very stark contrast indeed to the modern, and wrong, thinking about them.

[JtD]

Thanks for the information. I still don't really get why the extra power is not supposed to be related to application of higher boost, in particular where Plane-Eater says he found that mentioned. If you built two alike engines just with different tolerances, the high quality version might be somewhat more powerful due to higher efficiencies (less losses), but that's not going to be excessive unless the standard production quality is abysmal. You'll need to increase boost or rpm in order to get more power out of it.

Regarding manual boost control means inferior pilots, imo that's not the case at all. I think looking at the record of the AVG and the conditions they operated under speaks for itself. Manual boost control means that it is simply a matter of clearance and application of boost levels in the field, suited to the conditions on site, with no technical changes to the engine or aircraft. With the P-40, this was done, too, with several squadrons considerably exceeding official limits. The point here is that higher quality engines should have less trouble with overboosting than standard versions might have, as they would most certainly be more reliable.

The application of 50" boost would yield an increase of about 340 hp (35%) at low altitudes over the standard emergency power settings. You'll get nowhere near this with just manufacturing (reasonably) higher quality.

Afaik, flight model wise there's no difference in game between a P-40B and the AVG Hawk 81. If I look at this topic, I think it would be better if there was a difference. But there's not much hard evidence to allow modeling a deviation from official specs.

[ElAurens]

Former_Older and JtD,

Check your PMs.

[Plane-Eater]

JtD:

Sorry to dig out an old thread. From a quick search in a few places online I vaguely remembered, the AVG had repeated issues early on with thrust bearing failures in their engines. I think it was Molesworth's Sharks Over China as well as Clemmon's book on the AVG that mentioned a field-modified oil recovery and sprayer system was devised to sump extra oil and deliver it back onto the bearing.

That kind of failure in the reduction gear case, from my understanding, is a symptom of higher power loads over a significant period of time than the reduction gears are rated for. I'm not an engine expert, though, so somebody who knows the old war engines better than me might be able to elaborate.

The AVG-vintage pilot manual on Dan Ford's site mentions 41" Hg as max boost at sea level. I've seen a multitude of sources, including my brief contact with Mr. Shilling before he passed, that indicated 41" was... we'll call it highly conservative. 41" was routinely exceeded by a notable margin.

Dan Ford has updated some of his research since I was last involved heavily, and his work is extensive and exhaustive. I'll need to brush up before I try to speak authoritatively on too many more specifics. : )

EDIT: Like I mentioned above, I'm not an engine expert, but wouldn't closer tolerances also allow (at least to some extent) higher loads than the design rating without the same level of wear or failure? IE, you could run the engine at - for example - 115% rated output without as much / as many of the corresponding wear and problems that would cause on a stock engine? So along those lines, by the time you did finally start getting high-load related failures, you'd be running at WAY more than just 5 or 10% above max rated output?

[MaxGunz]

Originally Posted by JtD

Thanks for the information. I still don't really get why the extra power is not supposed to be related to application of higher boost, in particular where Plane-Eater says he found that mentioned. If you built two alike engines just with different tolerances, the high quality version might be somewhat more powerful due to higher efficiencies (less losses), but that's not going to be excessive unless the standard production quality is abysmal. You'll need to increase boost or rpm in order to get more power out of it.

You would have to check the standards of the engine makers in those times. I don't think that P&W held Rolls Royce tolerances, FWIW. Maybe see how they compared to Detroit automotive. A whole lot depends on how many and which parts they hand-matched.

The difference can be a stock engine vs a "blueprinted" engine.

[JtD]

Originally Posted by Plane-Eater

EDIT: Like I mentioned above, I'm not an engine expert, but wouldn't closer tolerances also allow (at least to some extent) higher loads than the design rating without the same level of wear or failure? IE, you could run the engine at - for example - 115% rated output without as much / as many of the corresponding wear and problems that would cause on a stock engine? So along those lines, by the time you did finally start getting high-load related failures, you'd be running at WAY more than just 5 or 10% above max rated output?

Not necessarily so, closer tolerances can contribute to less power loss and less variance in parts failure. For instance, if you achieve the same compression in all cylinders with zero tolerance, tolerance will see some cylinders with higher than normal and some with lower than normal compression. Operationally, you'll have to run the engine at a boost level which is save for the cylinder with the highest compression, so it is save to increase boost level when you have no tolerance at all. However, you can increase the boost level so that with zero tolerance all cylinders fail, while you'd still have some cylinders running on the engine with tolerances.
For prop reduction gear I don't know the effects of tolerances, but frankly there's little I can think of to turn a rejected part into a superior one. That might be a reason they were the weakest link.