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KraljMatjaz
08-09-2005, 05:49 AM
Hi

If we compare prop pitch to gears in a car, isn't it weird, that at max. speed on most airplanes prop pitch is in "lowest" (100%) gear? I would expect it to be somewhere in between, thus lower setting used for climbing (so the plane climbs with slow speed, but optimal RPM), and higher for diving (so the plane exceeds max. speed, but retains optimal rpm).

This happens only on Bf109.

IMO all planes should have pitch like it is on Bf109 series, but without the speed leap (if you use 100% prop pitch for a few seconds, you will instantly get a dozen or two kphs which sounds almost like a cheat).

Some pilots were even known to lower the pitch a bit to overrev the engine and gain some kphs more, but such engines had lower operational times... (now this is possible only on Bf109 and on Fw190 by a way smaller margin). It is like shifting down in a car, when you go overtaking, you reach very high RPM, but also additional speed.

Comments please, thanks

Art-J
08-09-2005, 10:10 AM
"This happens only in Bf109", because the 109 has the ability of direct pitch control by the pilot (along with a few of Fw190 versions, as You noticed), which is quite a good analogy to gears in the car, but nearly all other planes have Constant Speed Propellers. In these crates, by hitting the "prop pitch key", You actually set the desired engine RPM, while the governor is constantly setting and changing the prop pitch to hold this RPM, no matter what direction and speed the plane is flying at. That makes the "prop pitch ... %" writing on the screen completely off-topic, but it's just one of the well known unfortunate translations in our sim.
You cannot compare CSP props to gears in the car, because it would give You the car with wheels rotating at the same, previously set speed no matter how fast the car is going http://forums.ubi.com/groupee_common/emoticons/icon_biggrin.gif.

Cheers, Art

tigertalon
08-09-2005, 09:25 PM
Ah, thanks Art, this pretty much clears things up.

fabianfred
08-10-2005, 05:36 AM
so how does it work then...?

in 'Fighter Boys" they talk about Hurri and spit having to be in 'fine' for t/o and landing

in il2 i drop to 30% only if I want to slow down for landing... but i could leave it at 100% really all the time...

this has no comparison to the real thing then..?

once in the air did they go to coarse pitch to reduce revs and save fuel...? so did they go back to fine when in combat?

effte
08-10-2005, 09:35 AM
You want to be in high RPM for T/O and landing. Higher RPM means a higher maximum power output. During T/O, you obviously want all the power you can get. For landing, you want to have all that power available without waiting for the engine to increase RPM. You also often want drag to get a steep approach angle.

High RPM is what you set in aircraft with a propeller speed governor rather than a simple adjustable pitch propeller €" that means all but a few of the aircraft in Il-2. In the Il-2 series, high RPM has been misnomered as 100% pitch in the text messages on the screen. This has been a never-ending source of confusion.

For slowing down or getting down fast, stick it in high RPM (that means 100% in the on-screen message) to get the flattest pitch and the most drag from the propeller. Yes, there are people who (often rather loudly) advocate other procedures but I did the flight tests to prove that Il-2 handles this correctly a long time ago.

A few commercial aircraft are certified for flying approaches at a lower RPM than maximum. This is good for the noise footprint under the approach, but requires extensively documenting the spool-up times etc to the certifying authority.

Cheers,
Fred

Art-J
08-10-2005, 10:31 AM
Originally posted by fabianfred:
so how does it work then...?

in 'Fighter Boys" they talk about Hurri and spit having to be in 'fine' for t/o and landing

in il2 i drop to 30% only if I want to slow down for landing... but i could leave it at 100% really all the time...

this has no comparison to the real thing then..?

once in the air did they go to coarse pitch to reduce revs and save fuel...? so did they go back to fine when in combat?

What "they" talk is right. In CSP plane, setting high RPM (or "prop pitch 100%" as our sim calls it http://forums.ubi.com/images/smilies/35.gif) is good both during take off (for acceleration and effective power usage) and landing (for slowing down because of more "effective" windmilling). If You tend to drop to 30% on final approach, Your prop actually creates LESS drag. Try it Yourself: accelerate to a constant speed with stopwatch in Your hand, then shut the throttle and measure time needed for slowing down, let's say, 100kph. Do it once with high and once with low RPM setting.

Effte did a very nice brief description of CSP usage. Much better than I would do (as a non-native English speaker http://forums.ubi.com/groupee_common/emoticons/icon_wink.gif) For much longer, yet interesting article (with drawings, governor function etc. ) I strongly recommend Pelican's perch:

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

Cheers - Art

GH_Klingstroem
08-10-2005, 03:09 PM
You are all missing the point in my opinion. The reason for changing the prop pitch is to get the ultimate angle of attack for the propeller blade at diffrent forward speed. At low speeds you want a high pitch because that gives ultimate angle of attack for the prop blade=most possible thrust. the faster you go the more you will have to corsen the prop pitch to get the ulitmate angle of attack. Remember that the propblade is a wing that is rotating! So in the game 100% prop pitch means fine pitch and 0% means feathered, or should mean feather at least.
It is also correct that the engine has a RPM that is most suitable for it. Usually at around 2500rpm...

effte
08-11-2005, 03:14 AM
Klingstr¶m,
I beg to differ. The main purpose of the CSP is to keep the engine and propeller system operating at the optimal RPM. For performance, optimal RPM is maximum RPM as this will create the most power for a given amount of torque. For this reason, aero engines are designed for power output and torque output to peak at the same RPM, which is close to the design maximum. If you want to increase range and/or loiter time (and save a bit of wear and tear), optimum RPM will be lower than maximum RPM. The latter is the only time you should stray from 100% prop pitch in the game.

You then adapt the propeller in order for it to be as efficient as possible under these circumstances. This adaption involves making sure that most of the propeller will be at an angle of attack which isn€t too far from optimal during typical operating conditions.

In the game, €œ100% prop pitch€ on the CSP aircraft means maximum RPM. This means the finest pitch possible under the given conditions without overrevving. You are unlikely to be on the fine pitch stops, i e fully fine pitch, unless you are parked on the ground at low power. When you do hit the fine pitch stops, the RPM will fall below the selected RPM.

0% prop pitch in the game means the lowest selectable RPM. This most certainly does not mean feathered! Most single-engine aircraft can€t even feather the propeller, read the Pelican€s Perch articles linked above for a good explanation of why. It is very unlikely to mean bumping the propeller into the coarse pitch stops either, unless you happen to be in a steep full power dive. When you do hit the coarse pitch stops, the RPM will increase above the selected RPM.

Cheers,
Fred

GH_Klingstroem
08-11-2005, 04:34 AM
Ok you are right Effte, even though most of ur post just said what I said but in diffrent wording...i was just trying to point out that the reason the various prop pitch was invented in the first place was to get max efficient propeller at various speeds! You are talking about the the rpm and im talking about the reason u can select an optimum prop pitch..

0% prop pitch should however be a feathered prop. if you go below that u will end up in the reversed and beta mode. usually around 9-10 degrees negative pitch

effte
08-11-2005, 04:45 AM
No, 0% prop pitch is fully coarse. A feathered propeller is way beyond coarse. Coarse may be perhaps 50 degrees while feathered is around 90 degrees of prop pitch, depending on profile and reference section. A feathered propeller will not produce any thrust at all. Feathering is merely for the purpose of reducing windmilling drag with a dead engine.

To get into reverse, you go below <I>fine</I> pitch. Below coarse pitch, and also below feathered, lies the normal operating region.

Beta mode is merely a mode of controlling the reverse thrust output and not a mode of the propeller as such.

Cheers,
Fred