Aces High Bulletin Board
General Forums => Aces High General Discussion => Topic started by: EDO43 on January 08, 2001, 02:39:00 PM
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I was talking with HT and a few others about this a few nites ago and though they had originally convinced me that the +/- control (keypad) for engine power worked correctly, I have since rethought the idea, changed my mind to my original standing and here's my reasoning.
Given the fact that most of us (if not all) have separate throttle controls on our joysticks (or stand alone) which controls engine power (i.e. manifold pressure) and given the fact that these propellors are modeled to be controllable pitch (constant speed) props (per HT),the joystick throttle should (and does) control engine power. If the +/- key is for increasing or decreasing engine power,why is it there? I for one don't need it for that purpose, I have a joystick throttle. When landing at a field, do you keep your throttle at idle on the descent? I do. Pressing the +/- key to - reduces the engine rpm even more. An additional throttle? what for? IMHO, this is a propellor control. In actuality, throttle primarily controls engine power, and engine RPM is primarily controlled by the propellor.
Take this set of conditions for example: F4U-1D 50% internal fuel, 2 150 gal drop tanks, 1 notch flaps departing from a land base. With the throttle at full power, you get off the field with little trouble. Sound ok? Now try the same thing with the +/- key all the way to the - side. You may get off the ground but it'll take a much longer TO roll and you won't develop the engine power/thrust to get you in the air as fast.
Now if you agree with that, lets talk props. Specifically, constant speed hyrdomatic props which are the ones modeled here (again per HT). As a propellor increases blade angle, (goes to higher pitch by moving the prop control +)engine RPM will decrease. While this is happening the maniflold pressure increases because of the additional work the engine has to do to drive the prop. Consequently, airspeed goes up because each blade of the prop takes a bigger "bite" out of the air. Now,if the propellor control is moved to a lower setting, the exact opposite will happen; Manifold pressure will decrease, blade angle will decrease and airspeed will go down. However, since there is less load on the engine, the RPM will increase. The exact opposite of what happens when you press the +/- key in the game. I have tried this numerous times at all different altitudes. Pressing on the - key drops airspeed, manifold pressure AND engine rpm. This is incorrect, engine rpm should increase. Now, we're not talking 800-1000 rpm but it should increase. Likewise the opposite is also incorrect. That is why I say that the +/- key is a prop control and as such operates backwards.
Since there is no constant speed range modeled, we won't discuss what happens when you change settings while in that range suffice to say that engine RPM will stay relatively constant no matter where the throttle is positioned. Hence the term "constant speed".
I love this game even though I don't seem to land as much as I'd like to after a sortie. (http://bbs.hitechcreations.com/smf/Smileys/default/smile.gif)
Rebuttals and replys always welcome.. (http://bbs.hitechcreations.com/smf/Smileys/default/smile.gif)
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EDO the + - do not control pitch. They set the desired RPM preset of the prop. Hence the term constant speed prop. The prop pitch is changing constantly as you change manifold pressure or just speed up or slow down.
HiTech
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so the prop is always in a constant speed range? Ok, now how about adding a mixture control? (http://bbs.hitechcreations.com/smf/Smileys/default/smile.gif) auto rich, auto lean, full rich and idle cutoff? Too complicated for the user? I wouldn't think so... (http://bbs.hitechcreations.com/smf/Smileys/default/smile.gif)
HT are you a pilot by chance? Aeronautical Engineer? I'm curious.
[This message has been edited by EDO43 (edited 01-09-2001).]
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prop control responds to slow.
it should go to high rpms as nearly as fast as the manifold pressure can be adjusted.
currently you will be 1000 feet down the runway by the time the rpms have gone from idle to full power.
it's to slow
some fluctuations in rpm when a change is made would be a nice touch as well.
while were on the subject it would be nice to have full feather props not automatically feather unless the oil lines and oil pressure drops off.
to be able to use the rpm +/- controls to bring a windmilling engine to full feather shutdown would sereously kick ass.
i dont like the autofeather. its not any fun.
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Well one thing I never understood was why if you went to idle, why does the engine stay at full RPM? Is it the propeller shaft RPM? Is it simply a result of the windmill effect in AH?
Just weird stuff, like setting the lowest RPM setting with very little throttle (IE at a very coarse pitch) and then clobbing it...wouldn't that result in the greatest torque reaction (or an engine stall out) because the governer doesn't react instantly and lots of power being applied?
Same with setting for max RPM and low MP, wouldn't clobbing the throttle result in some over revving before the governer could increase the pitch to RPMs back to the set level?
Oh yeah, and can reversable pitch props be made a perk? (http://bbs.hitechcreations.com/smf/Smileys/default/wink.gif)
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EDO the + - do not control pitch. They set the desired RPM preset of the prop. Hence the term constant speed prop. The prop pitch is changing constantly as you change manifold pressure or just speed up or slow down.
HiTech
If you're setting the rpm preset of the prop, then it is a pitch control. Prop pitch controls are usually labeled "increase rpm" on the end nearest the instrument panel, and "decreases rpm" aft of it. The term "constant speed" refers to engine speed, not prop rpm; albeit that the engine/propeller reduction gearing is fixed and for a given engine rpm, the prop will maintain a given rpm. I'm sending copies of some pages out of a couple A&P training manuals so if you have time skim over them and you'll see that what I'm saying is correct.
This is gettin way to nit picky (no pun intended) so this ends it here. (http://bbs.hitechcreations.com/smf/Smileys/default/smile.gif) I'm not sure that people even mess with the control other than myself an a few others. I'm going to pick up my 1 Gig Athlon powered computer and play some AH this evening. (http://bbs.hitechcreations.com/smf/Smileys/default/cool.gif)
[This message has been edited by EDO43 (edited 01-10-2001).]
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EDO, if you are setting the RPM then it is only a pitch control *if* the load on the prop is constant and the power output of the engine is constant - neither of which are true. They will be approximately constant in cruise, but are not necessarily so.
Constant speed refers to *both* prop and engine as they are *GEARED* and always operate at a fixed ratio (the gearing ratio). If the prop is holding a constant RPM then so is the engine though it will be different by the gear ratio.
Hitech is 100% correct in his explaination. I have manuals too - manuals that show how the governor works by changing prop pitch to maintain a constant speed (RPM). Detailed explanations. These are manuals by the engine manufacturers.
Constant speed governors work by changing propellor pitch to hold a constant propellor and engine speed that is set with the "Increase RPM" and "Decrease RPM" speed control.
As you increase *throttle* you open up the butterfly and the engine develops more power. It tries to increase RPM but the governor senses this and increases blade pitch to compensate. The end result is still more power transferred to the air and the plane climbs, goes faster, etc.
As you decrease throttle, the opposite occurs. Where the RPM would decrease because there just isn't the power to match the prop load, the blade angle is reduced to where the engine can turn it at the desired RPM setting.
Decrease throttle/power and the blade angle is reduced. Increase power and the blade pitch angle is increased. As long as the governor isn't at limit, the RPM will be constant (within governor tolerance).
If you set the prop pitch directly, it would be logical to also label it as increase RPM and decrease RPM as that is the effect it would have - but that is not what these planes were equipped with. They were equipped with constant speed props as they were the most efficient and gave the best performance at all operating points.
If you want I can send you copies of the pages from Pratt & Whitney, Allison, Wright or Hamilton Standard manuals that explain this concept very well. They go into great gory detail on how electric and hydraulic constant speed prop mechanisms function and the end result is exactly as Hitech explained.
He really is correct.
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Rape, pillage, then burn...
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Jigster - couple of points...
As long as the plane is moving through the air, it doesn't matter if the engine is driving the prop or the prop is driving the engine - the governor will try to hold RPM and it can. It is a simple mechanism that controls prop pitch in response to RPM and the hydraulic ones would only care if they had pressure with which to operate.
(Not all engines can handle being driven though and in real life some always have to be pulling or they can be damaged.)
As to really hitting the throttle and getting overrev - it all depends on how fast the governor responds compared to how fast the engine revs. Remember these things aren't like car engines. Going from idle to full throttle was a good way to stall these engines - no accelerator pump. It probably would also wreak havoc on the gears and other internal parts. A lot of horsepower trying real hard to accelerate a lot of mass is probably really hard on the internals. I think pilots would generally make big power changes slowly and smoothly in which case the governor would follow it nicely. Small power changes wouldn't have that big a pitch change associated with them and the governor would respond pretty fast all in all. Any overrevs or underrevs would be minor.
Citabria - The pitch controls I've seen only have normal operating limits for blade angle. To feather there is a switch or valve of some kind that defeats regulation and drives the blades to limit. This is probably also a safety feature so some numbnut doesn't to easily put an engine into feather accidentally.
Also, at least the Curtiss electric propeller had a safety switch design where if oil pressure were lost, the blades maintained their last commanded pitch - they didn't autofeather.
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Rape, pillage, then burn...
[This message has been edited by PakRat (edited 01-11-2001).]
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Edo - another point in response to your point regarding taking off with the RPM set to full low (high pitch).
You get a certain amount of power out of each pop that happens in the cylinder. If you cut RPMs, you are by definition getting fewer pops whcih means less power can be developed by the engine.
Remember, prop pitch is very similar to gearing in a car or on a bike. How easy is it to start out in high gear? Not very.
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Rape, pillage, then burn...
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Actually these engines might be a little more adaptive to instant throttle changes then you might think...
Now in a car where you have an RPM range of say, 600 RPM boggle idle and a max @ 6000 RPM, changing fuel flow from these two is possible for the light mass of the engine's moving parts.
Now in planes, max revolution is normally half that of a car...say an inline like the Merlin -- 3000 RPM. They don't have accelerator pumps but they really don't need them. Fuel pressure is high because of the transfer distance, and the real restriction on radials or inlines of this size is air flow, because engine RPM must go up before the supercharger kicks in, and it doesn't start working till RPM increases! Hence the black smoke that occurs during rapid throttle changes -- way too rich, and the carbon build up can cause some nasty problems later in that flight. Very small changes in RPM are going to account for massive changes in power...that's the principle behind such big engines, more bang per turn.
Uh anyway, enough of my ramblings...what I was getting at is the governers are generally pretty slow. Usually at least a 2-3 second delay and thats enough to make things hairy...but I was kinda refering specifically to AH. (yanno, set pitch fine, rapid increase from idle to 75% or so, assuming it doesn't stall, is going to cause some over rev until the governer compensates)
The only thing I found odd is the engine maintaining full RPM (say the Allison or Merlin at 3000 RPM) with idle throttle, or about MP 12. At that power level, the capable RPM of the engine is less then the prop can adjust for...it's beyond the fine pitch (or even a runaway prop (http://bbs.hitechcreations.com/smf/Smileys/default/smile.gif) ) I though maybe the windmill effect was causing this, as the RPM's only drop for the engines when they are on the ground and not moving what so ever.
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Some of you people ought to actually get your hands dirty actually work on some of these things rather than just reading books about them. Almost anyone can read and understand but it's a different story when you read, and then go work on the thing that you've just read about. Watch it work and then really find out how and why it works.
PakRat, ANY control which adjusts a blade angle or setting on a propeller is a prop pitch control period.
"Constant speed refers to *both* prop and engine as they are *GEARED* and always operate at a fixed ratio (the gearing ratio). If the prop is holding a constant RPM then so is the engine though it will be different by the gear ratio."
I believe I said that in my earlier post.
"Hitech is 100% correct in his explaination. I have manuals too - manuals that show how the governor works by changing prop pitch to maintain a constant speed (RPM). Detailed explanations. These are manuals by the engine manufacturers."
No he's not, and no the governor doesn't work by changing the prop pitch, it works by being geared to the accessory drive pad on the engine. It's function is to provide boosted oil pressure to operate the pitch changing mechanism of the propeller.
'Constant speed governors work by changing propellor pitch to hold a constant propellor and engine speed that is set with the "Increase RPM" and "Decrease RPM" speed control."
Well what are you changing when you do that? Mixture? Butterfly valve (throttle) opening?...NO YOU'RE CHANGING THE PROPELLER PITCH. SPECIFICALLY, THE TENSION ON THE SPEEDER SPRING INSIDE THE GOVERNOR.
"As you increase *throttle* you open up the butterfly and the engine develops more power. It tries to increase RPM but the governor senses this and increases blade pitch to compensate. The end result is still more power transferred to the air and the plane climbs, goes faster, etc. "
Correct, but what happens if you change the propeller pitch "preset" setting for any given throttle setting i.e. the +/- key?
"As you decrease throttle, the opposite occurs. Where the RPM would decrease because there just isn't the power to match the prop load, the blade angle is reduced to where the engine can turn it at the desired RPM setting."
"Decrease throttle/power and the blade angle is reduced. Increase power and the blade pitch angle is increased. As long as the governor isn't at limit, the RPM will be constant (within governor tolerance)."
RIGHT, NOW CHANGE THE PROP SETTING WHEN THE THROTTLE IS UNCHANGED...WHAT HAPPENS THEN?
"If you set the prop pitch directly, it would be logical to also label it as increase RPM and decrease RPM as that is the effect it would have - but that is not what these planes were equipped with. They were equipped with constant speed props as they were the most efficient and gave the best performance at all operating points."
WRONG AGAIN, the F6F and the P-47 were labeled EXACTLY that way, increase and decrease RPM. I'll post a picture here if you'd like.
If you want I can send you copies of the pages from Pratt & Whitney, Allison, Wright or Hamilton Standard manuals that explain this concept very well. They go into great gory detail on how electric and hydraulic constant speed prop mechanisms function and the end result is exactly as Hitech explained.
He really is correct.
No he's not and you've beat around the issue. We're not talking throttle adjustment here, were talking prop pitch control. I have dribbled oil on more manufacturers maintenance manuals than you can probably imagine. I do this for a living and can categorically state that this is the way it is. Now unless you've got more than 15 years experience changing everything from pal nuts to cylinder jugs to exhaust collectors on these big ole round engines I suggest you defer to those who are actually in the know.
Tell me I don't know what I'm talking about....I thought ended this issue but I guess not.
[This message has been edited by EDO43 (edited 01-15-2001).]
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"Originally posted by PakRat:
Edo - another point in response to your point regarding taking off with the RPM set to full low (high pitch).
You get a certain amount of power out of each pop that happens in the cylinder. If you cut RPMs, you are by definition getting fewer pops whcih means less power can be developed by the engine.
Remember, prop pitch is very similar to gearing in a car or on a bike. How easy is it to start out in high gear? Not very."
I never said you take off in high pitch. (low rpm) RTFS
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I realy don't have a clue what this discusion is about.
HiTech
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It might be a bit easier to follow if I could figure out how to use the bold in "reply with quote"..since I cannot, I use quotation marks... (http://bbs.hitechcreations.com/smf/Smileys/default/smile.gif)
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Bold text is made by embracing the text with a and .
I am having a hard time following this thread as well, but from what I gather couldn't you just get the effect you want (keys should be reversed) by going into setup, key mapping, flight and then switch the two of them around?
[This message has been edited by LLv34_Snefens (edited 01-15-2001).]
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Thanks for the heads up re: bold. As for key mapping it in reverse, I'll have to get back to you on that. It should work.
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Heh, fix the flaps in AH first then you can toy with your engine settings... (http://bbs.hitechcreations.com/smf/Smileys/default/wink.gif) (http://bbs.hitechcreations.com/smf/Smileys/default/biggrin.gif)
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"Decrease throttle/power and the blade angle is reduced. Increase power and the blade pitch angle is increased. As long as the governor isn't at limit, the RPM will be constant (within governor tolerance)."
RIGHT, NOW CHANGE THE PROP SETTING WHEN THE THROTTLE IS UNCHANGED...WHAT HAPPENS THEN?
It depends (http://bbs.hitechcreations.com/smf/Smileys/default/biggrin.gif)
The reason for a constant speed propeller is to maximize the efficiency of the propeller over a wide range of speeds.
The faster you fly, the coarser you're propeller blades pitch will need to be to maintain it's most efficient AoA.
You will need to analyze each engine / propeller combination to determine which setting is best for each engine / propeller combination.
Changing the RPM with the CSU control may increase, or it may not.
HTH a little bit and doesn't cloud things further.
Cheers,
Rod