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anarchy52
08-17-2005, 02:51 AM
I found this on warclouds forum, I thought it could be interesting since there was a lot of debate about prop pitch (especially in german aircraft):
http://www.war-clouds.net/forums/viewtopic.php?t=427


A few notes on engine management and trim-

I listen to people conversing in missions and it makes my teeth grit sometimes when I hear someone give another pilot totally incorrect pitch and power advice.

I feel many people are getting this wrong because it's too simple the way it's set up in this sim. Both throttle level and propeller pitch are set up by percentages in the IL-2 series. So.....for example...on a manual prop system(constant speed shiftable), as in most allied planes, 85% of total throttle range corresponds with what percentage of available prop pitch? Of course the answer is 85%. Throttle and prop percentages should always match in the 5% range, but never throttle over pitch. For 80% pitch therefore, proper throttle position is 76 to 80% throttle. If the throttle is set above the pitch mark, no power or performance increase beyond pitch percentage is possible, and can only lead to overheating, because the excess power is shunted by the prop clutch, and a higher throttle percentage simply is over-fueling, with the catastrophic fouling effects on sparkplugs mercifully not present in FB-AEP-PF. Overheat is present, then controlled by an open radiator, thus affecting flight performance even more negatively. For a pitch control of this type[early, midwar, and late US planes, Japanese, Brit and Russian(with the exceptions of late-model spitfires)], throttle and pitch percentage are equal for cruise, and always 100% in combat. The only esoteric setting used is to make a Zero fly 1200 miles and back, and the pistons were case-hardened to prevent heat from low mix and low rpm from crystallizing the piston skirts.

A constant speed shiftable prop control works on both blade pitch and prop hub rpm. The blades are gear-driven to the most efficient pitch for a set RPM. If a pilot has his engine throttle too far below pitch percentage, the result is "luffing" the propellor, which means that air pressure striking the front side of the blade is a high percentage of powered thrust, causing incidental drag and inefficient flight. During cruise, when efficiency, economy, and engine cooling are important, a pilot also must regulate prop rpm on the positive side as well, to avoid what's called "beating the air". Think of that prop as being a screw(props used to be called air-screws) threading thru the air efficiently with a slight pressure on the trailing side of the "thread". As we all know, according to Bernoulli's theorem(week two groundschool), the forces of drag, forward motion and lift are in perfect balance in a properly trimmed aircraft cruising on the level. Most in the community don't run their planes this way, but once optimal design altitude has been reached and the aircraft trimmed nose level at max cruise speed, the aircraft will happily fly along at 45% throttle and pitch, and will amazingly sound like an old prop plane going pockata-pockata-pockata over the airfield. In the B-25, for example, 85% pitch puts the engine on the far end of the rpm and boost warning markers. In this aircraft, anything over 85% throttle and pitch should be considered War Emergency, as it will overheat rapidly. Max cruise indicated rpm is the beginning of the warning marker on both Tachometer and Boost Pressure guages.

German theory is quite different. Auto-prop on the Bf-109 and 110 as well as FW-190 series is pure constant speed. RPM is always between 1950 and 2500 on 50% throttle or more, the blades being much more automatically activated by a governance system that keeps the higher-torque, high compression German engines at rpms geared to take advantage of peak torque ft-lbs and the resulting bulk horsepower ratings within 550 rpm. Most allied engines have a variance of 500rpm at each 5% increment of pitch. In the 190, BMW designers went even further with intricate control, the KommandGerat system the had direct linkage from the throttle to the governance system. 190 engines will live if taken off auto, 109 engines will not, 3000rpm blows up inverted vees. In the 190 on "manual" the same rules apply as for allied, but the Kommandgerat in general can't be effectively matched by a human pilot, as it performs automatically and instantly in any speed situation, although a pilot might want to open to 100% pitch if too much speed is lost(below 275kph).

Now for trim, buy a book on it and do a little reading on trim theory. I could fill 200 pages with the info. But, remember not to "fly with the trim". Trim to reduce pressure in level flight. When u dive, trim down first, to anticipate gain of speed. When u gain or lose speed check the ball and attitude for adjustments. It is very important to learn a bit about it to get performance out of your ride. Rudder trim in German or other non-adjustables is a function of throttle. German theory was that besides the weight and space needed of the mechanisms necessary for trim adjustments, a fighter pilot should be ready to turn and burn at any time and only have to worry about elevator. Aerilons and rudder are pre-set for cruise at 18,000ft at 65% pitch and throttle.

Get it Right!!! ;o)

JV44Boelcke

anarchy52
08-17-2005, 02:51 AM
I found this on warclouds forum, I thought it could be interesting since there was a lot of debate about prop pitch (especially in german aircraft):
http://www.war-clouds.net/forums/viewtopic.php?t=427
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">
A few notes on engine management and trim-

I listen to people conversing in missions and it makes my teeth grit sometimes when I hear someone give another pilot totally incorrect pitch and power advice.

I feel many people are getting this wrong because it's too simple the way it's set up in this sim. Both throttle level and propeller pitch are set up by percentages in the IL-2 series. So.....for example...on a manual prop system(constant speed shiftable), as in most allied planes, 85% of total throttle range corresponds with what percentage of available prop pitch? Of course the answer is 85%. Throttle and prop percentages should always match in the 5% range, but never throttle over pitch. For 80% pitch therefore, proper throttle position is 76 to 80% throttle. If the throttle is set above the pitch mark, no power or performance increase beyond pitch percentage is possible, and can only lead to overheating, because the excess power is shunted by the prop clutch, and a higher throttle percentage simply is over-fueling, with the catastrophic fouling effects on sparkplugs mercifully not present in FB-AEP-PF. Overheat is present, then controlled by an open radiator, thus affecting flight performance even more negatively. For a pitch control of this type[early, midwar, and late US planes, Japanese, Brit and Russian(with the exceptions of late-model spitfires)], throttle and pitch percentage are equal for cruise, and always 100% in combat. The only esoteric setting used is to make a Zero fly 1200 miles and back, and the pistons were case-hardened to prevent heat from low mix and low rpm from crystallizing the piston skirts.

A constant speed shiftable prop control works on both blade pitch and prop hub rpm. The blades are gear-driven to the most efficient pitch for a set RPM. If a pilot has his engine throttle too far below pitch percentage, the result is "luffing" the propellor, which means that air pressure striking the front side of the blade is a high percentage of powered thrust, causing incidental drag and inefficient flight. During cruise, when efficiency, economy, and engine cooling are important, a pilot also must regulate prop rpm on the positive side as well, to avoid what's called "beating the air". Think of that prop as being a screw(props used to be called air-screws) threading thru the air efficiently with a slight pressure on the trailing side of the "thread". As we all know, according to Bernoulli's theorem(week two groundschool), the forces of drag, forward motion and lift are in perfect balance in a properly trimmed aircraft cruising on the level. Most in the community don't run their planes this way, but once optimal design altitude has been reached and the aircraft trimmed nose level at max cruise speed, the aircraft will happily fly along at 45% throttle and pitch, and will amazingly sound like an old prop plane going pockata-pockata-pockata over the airfield. In the B-25, for example, 85% pitch puts the engine on the far end of the rpm and boost warning markers. In this aircraft, anything over 85% throttle and pitch should be considered War Emergency, as it will overheat rapidly. Max cruise indicated rpm is the beginning of the warning marker on both Tachometer and Boost Pressure guages.

German theory is quite different. Auto-prop on the Bf-109 and 110 as well as FW-190 series is pure constant speed. RPM is always between 1950 and 2500 on 50% throttle or more, the blades being much more automatically activated by a governance system that keeps the higher-torque, high compression German engines at rpms geared to take advantage of peak torque ft-lbs and the resulting bulk horsepower ratings within 550 rpm. Most allied engines have a variance of 500rpm at each 5% increment of pitch. In the 190, BMW designers went even further with intricate control, the KommandGerat system the had direct linkage from the throttle to the governance system. 190 engines will live if taken off auto, 109 engines will not, 3000rpm blows up inverted vees. In the 190 on "manual" the same rules apply as for allied, but the Kommandgerat in general can't be effectively matched by a human pilot, as it performs automatically and instantly in any speed situation, although a pilot might want to open to 100% pitch if too much speed is lost(below 275kph).

Now for trim, buy a book on it and do a little reading on trim theory. I could fill 200 pages with the info. But, remember not to "fly with the trim". Trim to reduce pressure in level flight. When u dive, trim down first, to anticipate gain of speed. When u gain or lose speed check the ball and attitude for adjustments. It is very important to learn a bit about it to get performance out of your ride. Rudder trim in German or other non-adjustables is a function of throttle. German theory was that besides the weight and space needed of the mechanisms necessary for trim adjustments, a fighter pilot should be ready to turn and burn at any time and only have to worry about elevator. Aerilons and rudder are pre-set for cruise at 18,000ft at 65% pitch and throttle.

Get it Right!!! ;o)

JV44Boelcke
</div></BLOCKQUOTE>

anarchy52
08-17-2005, 08:53 AM
Nothing to comment?
Leadspitter?

Taylortony
08-17-2005, 12:32 PM
Nice post http://forums.ubi.com/groupee_common/emoticons/icon_smile.gif Loffing is I take it another term for prop slip, this is the theoretical distance traveled by a prop in one rotation against the actual distance travelled allowing for air pressure, poor aerodynamics, temperature, inefficency and anything else you could think off.

effte
08-17-2005, 02:31 PM
In short: Go tomichapmas CEM pages ( http://people.ee.ethz.ch/~chapman/il2guide/cemguide/intro.htm) instead. This will only cause more confusion. Lots of information, but the percentage of home-brewed theory is on the high side of acceptable in my opinion.

As for why, here are a few reasons:

First off, he has bought into the square/oversquare theory of RPM and power relation.

You never want to open the throttle without bringing the RPM up first €" that is true. Power as transferred by the prop is a function of RPM and torque. Maximum power is reached when the RPM is the design maximum and the torque is the design maximum. The system will be designed for this power output to correspond with the engine maximum power output. If the RPM is less than maximum but the power is less than maximum, the torque will have to be above maximum... and things will break.

One convenient way of making sure you were not overtorquing, on some crates, was the €œsquare€ method. 19€ of manifold pressure meant 1900 (or more) RPM, 21€ meant 2100 RPM etc. To just assume this to translate into 85% €œprop pitch€/RPM for 85% power, well... I wouldn€t and I wouldn€t bother with reducing RPM in most cases. Just stick the RPM all the way up there if you want to use any kind of power, and only bring it down if you are trying to stretch the legs of the aircraft for long flights (and we all see those all the time in PF, right?) or if you are trying to nurse a damaged engine back home. The only downside is a bit of wear (which isn€t modelled) and fuel use.

The suggested relationship is a convenient thing to have in the back of one€s head though!

As for the propeller clutch, in these crates there is no such thing. You bend things or strip PGB gears. That€s about the only €œclutch€ there is on aircraft. Propeller clutches are found on boats (and other vehicles). I think the guy may be coming in from the maritime side of things?

Higher throttle will not foul the plugs, either. That€s what happens from poor mixture management. Plugs may, at times, foul if an engine is run at low power for too long. During that long glide in idle, bring the engine up to power every now and then. You did remember to bring the prop RPM up first, didn€t you? http://forums.ubi.com/groupee_common/emoticons/icon_wink.gif

Most variable pitch propellers are hydraulically powered, not gear driven.

With €œluffing€, I think he is using a maritime term to describe windmilling. For this to happem, you have to reduce power a lot. You will not have inefficient flight, you will have help from the propeller windmilling and causing drag when you are trying to descend.

As for week two of ground school, Bernoulli vs. Newton has been beaten to death in this forum and others. The Bernoulli only school of thought has lost the battle a decade or two ago. The force generating pressure difference is more due to a low pressure in front of the blade (or on top of the wing) than a high pressure behind (even though an elegant propeller model is to have a pressure rise across the disc which translates to a velocity increase a short distance behind it). The four fources of flight (not three) are thrust, drag, lift and weight.

Cheers,
Fred