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Jetbuff
06-13-2005, 06:24 PM
I'd like to pose this question to those in the know:

Constant-speed props utilize a hydraulically linked (usually) mechanism to maintain RPMs within a desired range. i.e. you select RPM rather than prop-pitch. However, isn't it's ability to do so related to the mechanical limits of blade pitch? In the IL-2 series, CSP never over-speed in a dive. Is this correct behaviour? i.e. I would have thought that initially, the auto-pitch would auto-compensate by coarsening the pitch, but it must hit an upper limit of how coarse the blades can go right? Beyond that point, esp. with the throttle full open (i.e. no engine preload) shouldn't any extra speed translate to excess RPM's and over-revving?

Similarly, shouldn't a plane set to 100% pitch/RPM and closed throttle blow a cylinder or something? Or are both symptoms only common to variable-pitch aircraft where the pilot feeds in incorrect pitch for his speed/throttle?

I'm honestly interested in knowing - this is not a whine.

Jetbuff
06-13-2005, 06:24 PM
I'd like to pose this question to those in the know:

Constant-speed props utilize a hydraulically linked (usually) mechanism to maintain RPMs within a desired range. i.e. you select RPM rather than prop-pitch. However, isn't it's ability to do so related to the mechanical limits of blade pitch? In the IL-2 series, CSP never over-speed in a dive. Is this correct behaviour? i.e. I would have thought that initially, the auto-pitch would auto-compensate by coarsening the pitch, but it must hit an upper limit of how coarse the blades can go right? Beyond that point, esp. with the throttle full open (i.e. no engine preload) shouldn't any extra speed translate to excess RPM's and over-revving?

Similarly, shouldn't a plane set to 100% pitch/RPM and closed throttle blow a cylinder or something? Or are both symptoms only common to variable-pitch aircraft where the pilot feeds in incorrect pitch for his speed/throttle?

I'm honestly interested in knowing - this is not a whine.

diomedes33
06-14-2005, 12:35 AM
Take a look at this article (http://www.avweb.com/news/maint/185020-1.html) if you haven't seen it already.

The governor is probably something like this (http://en.wikipedia.org/wiki/Centrifugal_governor) and is connected to the driveshaft and the prop pitch hydraulic valve.

--DISCLAIMER--
The following is mostly dedecutions made from what I've read. The only sources I could find on CSP props were what I posted above. In my defense, I will have a degree in Aerospace Engineering in about 6 months. However, if anyone sees something that is wrong let me know and I'll change it.

Read on at your own risk. (gotta watch yourself around these parts, these days)
--------------


The way that I understand it, the CSP mechanism doesn't know or care how fast the plane is moving. It keeps adjusting the pitch to stay at the given engine setting. There is most likely a factor of safety built in to the system. For example, the governor maybe able to handle speeds up to 600 mph. Since the aircraft could never hope to go that fast, you'd never see it over-rev.

Along the same lines, the older planes like the hurricane do over rev. This happens when you go from really slow to really fast (diving). I believe these were mechanical systems rather than hydraulic ones. The time response for these was a known issue and this is why they went to hydraulic systems. The mechanical ones can handle the same range as the others, but it takes them longer to get there.

At 100% pitch and closed throttle. The throttle is in no way linked to the CSP system. So if the engine is in idle it stays in idle. The CSP will go as far is it can go then reach some sort of stop. That's why the rpm drops when you're sitting on the runway in the sim.

Kernow
06-14-2005, 07:12 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Jetbuff:
I'd like to pose this question to those in the know:

Constant-speed props utilize a hydraulically linked (usually) mechanism to maintain RPMs within a desired range. i.e. you select RPM rather than prop-pitch. However, isn't it's ability to do so related to the mechanical limits of blade pitch? In the IL-2 series, CSP never over-speed in a dive. Is this correct behaviour? i.e. I would have thought that initially, the auto-pitch would auto-compensate by coarsening the pitch, but it must hit an upper limit of how coarse the blades can go right? Beyond that point, esp. with the throttle full open (i.e. no engine preload) shouldn't any extra speed translate to excess RPM's and over-revving?

Similarly, shouldn't a plane set to 100% pitch/RPM and closed throttle blow a cylinder or something? Or are both symptoms only common to variable-pitch aircraft where the pilot feeds in incorrect pitch for his speed/throttle?

I'm honestly interested in knowing - this is not a whine. </div></BLOCKQUOTE>

You're right that there is a limit to blade angle travel - the high-pitch stop - and beyond that rpm will increase. I don't know, but maybe there is enough travel for the prop to cope with most speeds, even in a steep dive? Probably designed to cope with quite a dive.

100% pitch and closed throttle shouldn't be a problem; that's how you're supposed to land. All the manuals say prop speed 'fully forward' or 'fully fine' (which is 100%) for the pre-landing cx. That's to allow for best acceleration in the event of an overshoot. It also acts as a brake, because the only way the prop can rotate at max rpm if the engine is not producing power is to fine the angle off and act like a windmill - engine is rotated by energy from the airflow ie drag, not fuel burn. Once the speed decays you'll see that the blade has reached the low-pitch stop, because rpm will decrease indicating the CSU can't produce the desired max rpm.

I believe leaving MAP (manifold pressure / 'throttle') at a high value while reducing rpm is what can cause problems. You should reduce MAP first and then rpm, likewise when accelerating put rpm up first and then increase boost. Not modelled in game yet, but probably will be in BoB.

JG53Frankyboy
06-14-2005, 07:19 AM
CSP planes that overrev in 4.0m , out of mind:

all 3 Brewsters
G.50
Cr.42

Kernow
06-14-2005, 10:49 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by JG53Frankyboy:
CSP planes that overrev in 4.0m , out of mind:

all 3 Brewsters
G.50
Cr.42 </div></BLOCKQUOTE>

ie, the older ones, which ties in with what diomedes was saying about mech v hyd systems.

Link to prop theory - includes windmilling etc:

http://www.pilotfriend.com/flight_training/how%20aircraft%20fly/PROPELLERS.htm

and this is why low rpm and high MAP is bad (TDC = piston at top of stroke & PPP = point of peak pressure - in cylinder):

'What would happen if we got all the way up to this very high power, and then pulled the RPM back? (Don't try this at home, folks!) Well, with the spark occurring at 22? before TDC and PPP occurring at 16? after TDC at full RPM, there is a very precise time interval between spark and PPP. The crankshaft rotation is 38? (22? + 16? ). If we reduce the RPM by 20%, the crankshaft will turn only 30? (80% of 38? ) by the time PPP occurs, so now the peak pressure will occur at only 8? past TDC.

The preceding analysis is actually a bit of an oversimplification. In reality, there are other factors at work such as the rate of compression at the slower crank speed, so the relationship between RPM and PPP timing isn't really linear. In this case, the PPP would probably occur more like 12? after TDC.

In any case, at maximum MP and reduced RPM, the peak pressure will be much higher, because the combustion chamber will be much smaller when PPP occurs. Since the pressure will be much higher, the temperature will be much higher, and this increases the risk of detonation.'

From this article:

http://www.avweb.com/news/columns/182084-1.html

Art-J
06-14-2005, 11:19 AM
As far as I recall Zekes in 3.04 could overrev their props in steep dive too (all versions?). I don't know what's the situation in 4.0, since I don't have it http://forums.ubi.com/groupee_common/emoticons/icon_smile.gif

diomedes33
06-14-2005, 11:42 AM
So this would be 0% prop pitch and ~100% throttle? So it seems with the added back pressure you could loose a piston here. At the very least blow some piston rings, manifold seals and really piss off the crew chief.

I have a situation. Lets say you have a plane with a CPS prop with the governor set at 2500 rpm. You first reach this rpm at 50% power. It sounds like you get what Kernow described if you go to 100% power. Is flying at 50% power any different than flying at 100%?

Would putting the throttle right at this transition help reduce overheat and increase fuel economy, but still be able to maintain speed?

NonWonderDog
06-14-2005, 12:43 PM
Well, when you increase power the governor sets the prop blades to a coarser pitch to maintain RPM. So, while the prop's still spinning at the same speed, it's pulling more air with every turn.

It's a bit more complicated than that, really. The prop blades also have to be set more coarse at higher speeds. This is necessary for the prop just to maintain the same AoA, because the direction of the relative wind to the prop depends on both speed and RPM.

As far as I know, the props on WWII fighters could move through a relatively huge range of motion. Overspeeding was possible, but not much of a worry. Just watch the prop clocks on the German birds -- what do they read, 60 degrees of motion?!

Jetbuff
06-14-2005, 12:46 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by diomedes33:
The way that I understand it, the CSP mechanism doesn't know or care how fast the plane is moving. It keeps adjusting the pitch to stay at the given engine setting. There is most likely a factor of safety built in to the system. For example, the governor maybe able to handle speeds up to 600 mph. Since the aircraft could never hope to go that fast, you'd never see it over-rev.

Along the same lines, the older planes like the hurricane do over rev. This happens when you go from really slow to really fast (diving). I believe these were mechanical systems rather than hydraulic ones. The time response for these was a known issue and this is why they went to hydraulic systems. The mechanical ones can handle the same range as the others, but it takes them longer to get there. </div></BLOCKQUOTE>
Interesting, I had no idea how the older CSP systems were built and assumed they were all hydraulically linked. More interesting would be to find out if the FB planes that do over-rev were equipped with this mechanical governor.
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">At 100% pitch and closed throttle. The throttle is in no way linked to the CSP system. So if the engine is in idle it stays in idle. The CSP will go as far is it can go then reach some sort of stop. That's why the rpm drops when you're sitting on the runway in the sim. </div></BLOCKQUOTE>
The throttle is indirectly linked to the CSP through the engine isn't it? i.e. if the throttle was physically closed (i.e. 0 fuel getting in to the cylinders) wouldn't the engine come under great stress as it tries to resist the high rpms due to the windmilling prop?
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Kernow:
100% pitch and closed throttle shouldn't be a problem; that's how you're supposed to land. All the manuals say prop speed 'fully forward' or 'fully fine' (which is 100%) for the pre-landing cx. That's to allow for best acceleration in the event of an overshoot. It also acts as a brake, because the only way the prop can rotate at max rpm if the engine is not producing power is to fine the angle off and act like a windmill - engine is rotated by energy from the airflow ie drag, not fuel burn. Once the speed decays you'll see that the blade has reached the low-pitch stop, because rpm will decrease indicating the CSU can't produce the desired max rpm. </div></BLOCKQUOTE>
Rgr about it not being a problem at low speed such as when landing. (since the rpm is probably low in-spite of the governor in such conditions) I was wondering about say being in a dive and as your speed increases, rpms in the red, you forget and cut throttle while leaving the RPM selector as is. Now the prop wants to run away but your engine, in the absence of combustion from fuel in-flow doesn't want to move the pistons at all.
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">I believe leaving MAP (manifold pressure / 'throttle') at a high value while reducing rpm is what can cause problems. You should reduce MAP first and then rpm, likewise when accelerating put rpm up first and then increase boost. Not modelled in game yet, but probably will be in BoB. </div></BLOCKQUOTE>
Yes that's probably where I'm getting confused. I'm pretty sure I came across that very same advice somewhere else while reading up on CEM pre FB.

Then again MAP is not about what your throttle is set to and has more to do with the suction (i.e. negative pressure) building up inside the engine's fuel in-flow valve, right? High negative pressure = high fuel flow. BUT, if the throttle is closed, MAP will be high as the pistons try to suck fuel against the closed valve right? Oh cr@p, I'm getting confused again! http://forums.ubi.com/groupee_common/emoticons/icon_biggrin.gif

diomedes33
06-14-2005, 02:41 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Jetbuff:
The throttle is indirectly linked to the CSP through the engine isn't it? i.e. if the throttle was physically closed (i.e. 0 fuel getting in to the cylinders) wouldn't the engine come under great stress as it tries to resist the high rpms due to the windmilling prop?
</div></BLOCKQUOTE>

Oops forgot about the prop turning the engine. Guess that's why they put in that warning light in the P-39. http://forums.ubi.com/images/smilies/16x16_smiley-very-happy.gif

Hmmm just did some more testing with different CSP prop equiped airplanes. Sitting on the runway with 0% pp and full throttle, the manifold pressure drops. This makes sense as long as the prop can create enough drag to slow the propellor down to the target rpm. For a 1500+ hp engine sitting on the ground this seems a bit tough. So ... maybe the throttle is linked in here somewhere. Maybe the governor gets to a point where it realizes that prop pitch can't do it alone, then starts choking back on the throttle too.

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Orignially posted by NonWonderDog:
Well, when you increase power the governor sets the prop blades to a coarser pitch to maintain RPM. So, while the prop's still spinning at the same speed, it's pulling more air with every turn.
</div></BLOCKQUOTE>
http://forums.ubi.com/images/smilies/35.gif I'll chalk that up to being groggy and no coffee. I should have read my own first post. Answered my own question before I asked it. So you get more acceleration out of the higher engine settings, but you risk blowing it up if you push it too far. Are there any fail safes that are at work here for WWII era? I'm curious how easy it is to break the engine by ignoring the manifold pressure gauge and running it balls out all the time.

TX-EcoDragon
06-14-2005, 04:40 PM
Jet buff said: "Then again MAP is not about what your throttle is set to and has more to do with the suction. . ."

Well, Manifold pressure is the pilot's direct indication of power as set using the throttle, so actually it has a lot to do with the throttle position. So any increased throttle will lead to a increased MP value etc. (though the MP will change with altitude changes, mixture adjustments, intake efficiency, supercharger/turbo operation, so it's not a fixed value, but neither is the engines power output). The Tachometer on the other hand will usually show *very* slight and brief increase as power as added before returning to the previously set value provided that the power is set sufficiently high (generally a few inches higher than idle) to drive the prop governor.


The prop full forward (100% in game) is also the usual takeoff and initial climb setting as well. Usually near pattern altitude the power will be brought down to the climb setting and the prop will be rolled back as well to a lower RPM, but initially max power will usually be demanded and this requires fine pitch and high MP.

About that power at idle quesiton, keep in mind also that when power is brought below the "governing range" the oil pressure is low enough in the aircraft that the prop will move to the fully fine setting despite the RPM set by the pilot(in most designs anyway).

As to the original question, most Constant speed designs are such that at maximal speeds the prop is usually well able to drive the RPM to lower values even once you are at Vne (never exceed speeds) so the chance of "out-running" your prop is pretty low in general. What is a lot more common in maneuvering flight is overspeeds that happen when a rapid power change is made from below governing range to high power, in this instance the powerplant response is faster than that of the prop governor response and the RPM will aggressively exceed the set value, and often the redline value on the tach as well, but this is brief since the RPM are brought back in check in a few seconds by the governor, though that doesn€t mean it doesn't cause problems. There are some detailed articles written by John Deacon at AvWeb that are probably the best ones to read.

props Driving Engines: http://www.avweb.com/news/columns/186778-1.html

Those Marvelous Props:
http://www.avweb.com/news/columns/182082-1.html

Manifold Pressure Sucks!
http://www.avweb.com/news/columns/182081-1.html

Jetbuff
06-14-2005, 06:08 PM
Those Pelican Perch articles are exactly the ones that I read, thanks for linking them. (I lost the bookmarks)

I guess my question regarding throttle/MAP is about how a closed fuel valve would influence intra-cylinder pressure. The pistons would essentially be straining against immense pressures when rising. (little if any fuel is entering and expanding through burning) I guess this can be averted if the minimal throttle setting leaves the fuel valve slightly open - is that the case?

As for over-revving, makes sense that so long as the max coarse angle is enough to maintain normal RPM's at max dive and the response of the governor is fast enough there should be little risk of it under most circumstances. Thanks for clearing that up.