Constant Speed Prop Help

[funked]

Thanks for the scoop about the prop pitch!  I had no idea!

[indian]

-blk-- Was trying to state that a turbo supercgarger are two different type of boost agents on the same engine. a turbo is exhaust drven where a supercharger is crank driven either directly or geared. Thought you were talking about constant speed props and how they would really work not in AH.

And to the guy who says a constant speed prop does not need a gear box prop pitch will not maintain the speed you can still run it up with engine rpms. Prop pitch helps keep from over torqueing the prop. In an airplane with a power lever and a throttle lever the rpms would be set to max 100% and the prop (or power lever) would be used to run up the torque to get take off speed. Those plane without a power lever the prop pitch does not change and the rpms ( and this is where I will agree with -blk--)and the map guage to control torque.

My apologies to -blk-- for not reading his explanation as a simple one for AH only. I have 20 years in the aviation field nd my first reaction was this is all wrong. I see now what he was aiming at.

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Tommy (INDIAN) Toon

http://www.geocities.com/~tltoon

[-blk--]

  Hehe, isn't that wierd indian...
 
  Just because I fly 20 hours a week and work on 'em the rest of the time doesn't mean this tuff isn't difficult for others to understand...  I forget that a LOT.

blk

PS-Indian, what do you do?  Repair Station, A&P?  Pilot?

[indian]

-blk-- I wokr at a very large repair station for a major airline. I hold an A&P. I also just realised that I was think turbo prop so my mistake, I read that late at night and thought it sounded wrong.

See what I mean about the Trubo charger and supercharger not being the same though. it is possible to have both and did see I didnt make it very clear. So sorry again.

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Tommy (INDIAN) Toon

http://www.geocities.com/~tltoon

[shdo]

the other thing indian is that your refering to turbine engines not recipricating engines.  so indeed your explination is a little off.


shdo

[bod]

Indian and blk, remember WWII planes had piston engines, not turbines. At least not those with props.

Bod

[Lugnut]

Unless I missed something, there is no pitch control associated with the throttle position, correct?

Couple of observations, after I first startup, with throttle pulled all the back to idle RPM indicator says 2500, throttle position and engine sound would have me believe 600 rpm. Work in progress or bug?

Second,if I'm at 10,000 ft AGL at 100% throttle at max cruise (say 300+), if I go into a dive, should not the engine pitch sound/rpm increase?

Third if Im at 3/4 throttle *position* and RPM indicator is at 2500, when I push the throttle forward, assumming the RPMs read the same and a constant speed prop, why does the sound pitch increase with MAP? Aren't you just increasing the load on the engine and not the RPM?

To me it looks like your not modeling CSPs, just monitoring MAP and keeping the RPM indicator at MAX rpm regardless of the situation.

ignorant and slightly confused...

Lugnut

[This message has been edited by Lugnut (edited 09-29-1999).]

[Stiglr]

yow! It's not the propeller that's spinning, it's my decidedly insufficiently scientific head!!!!

Math! Math! Run away! Run away!

 

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[-blk--]

  vv indian, them turbines can get a mite confusing, especially since any type of turbocharger has an exhaust driven turbine  .

  Now to answer the throttle/pitch relation question:

  Throttle basically has no direct impact upon RPM with a constant speed prop.  However, when we increase the throttle setting (MAP), the engine produces more HP, which makes the engine want to speed up.  In order to keep the RPM the same, a propellor governor regulates oil pressure to the propellor to INCREASE the blade angle.  This in turn, loads the blades and the engine does not perceptibly gain speed.  The opposite is also true.  If I reduce the throttle setting, the load on the prop will be the same.  This will cause the RPMs to decrease (we humans shouldn't be able to percieve it), but the governor senses the decrase in RPM, and lowers the blade pitch.  The lowered blade pitch unloads the engine, thus the RPM remains relatively constant.  See, this is why they are called Constant Speed Props.

  To answer your question about diving at max throttle:  with a CS prop, basically, the airspeed would uload the prop and thus it would want to speed up.  However, the governor would increase the blade pitch and the RPM would remain constant.  Although at some point (very extreme point usually--never happened to me in RL) you would hit the limit of the blade travel and they could go no further.  The RPM would at that point increase, but I don't think you can hit it.  Especially military aircraft tend to have props with a very large pitch range, so it's possible, but not probable.

  As to the sound question:  an engine model and a sound model are completely different.  That's a nearly direct quote from HT a few nights ago BTW.

blk

PS-I really simplified that, but the prop class I took was a full semester and some people still didn't get all that...

[Lugnut]

Gotcha on the sound issue, sounds like they aren't synched up yet.

I still maintain that you aren't going to  spool up a Merlin at 2500 rpm right at start up...

Lug

[Uedel]

So if i read all these stuff and my head is swirling because i have to translate all these technical stuff in german in my head lol.

But now the question. Did all WWII Prop Planes have constant spinning Props ? And what was then the advantage of the engine managment of the FW 190 i read so many times about.




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Uedel not the best, but better then rest

[Bullethead]

Uedel said:
>>>>>>>> But now the question. Did all WWII Prop Planes have constant spinning Props ? And what was then the advantage of the engine managment of the FW 190 i read so many times about. <<<<<<<<<<<

As I understand it, the FW had 1 handle to control everything related to the motor:  throttle, prop RPM, and maybe mixture.  Other planes had 3 handles, 1 for each control.  Somehow the FW's was all integrated and balanced by 1940 vintage automatic control systems.  I don't know if this gave any advantages other than reducing pilot workload and the amount of training needed.

-Bullethead <CAF>

[-blk--]

  Lug, lemme ask around.  I think I can have a definite answer by Monday or Tuesday (as to whether a Merlin could turn 2,500 static)

blk

PS-most of the WW2 fighters has CS props.  Some early Hurricanes and I think spits has fixed pitch or two pitch props, but those were VERY early war (as in outdated by august '39), or before the war.  Other than that, yes they did, with the exception of the Fw thing.  Which, as a pilot, I don't like the idea.  But then, I'm a control freak and don't like the idea that there's a set ratio of engine output to MAP/RPM combinations (which the FW lever essentially did assume)

[tshred]

blk said:

"Which, as a pilot, I don't like the idea. But then, I'm a control freak and don't like the idea that there's a set ratio of engine output to MAP/RPM combinations (which the FW lever essentially did assume)
 "

Unless I read your post wrong, here goes..

Ah, but there is, and it's published by the engine manufacturer, to get the best performance and long life out of your engine. And as a pilot, you should be following their guidelines! Them FW engineers were kind to their pilots by removing their workload! Just as an example, how many of you run your cruise setting 'square'? How many of you have actually looked at the cruise charts in your Engine Manual or POH? I own, fly and maintain a 172 with a 220hp Franklin engine. It's manuall(engine manual) recommends MP as high as 26hg with RPM's as low as 22 for optimal cruise at lowest recommended power percentage. It has handy charts for each power setting, ie 65%, 75% etc...and yet people out their say never run over square! LOL, just goes to show you how much misinformation is really out there!

All you pilots and mechanics out there should read the articles "Go Ahead, Abuse Your Engine!" found at http://www.avweb.com/articles/pelperch/pelp0008.html  , and "The Ten Biggest Lies
About Piston Aircraft Engines" found at http://www.avweb.com/articles/englies.html  in addition to the two mentioned in my post above. Again, you have to be a member, but it's free and painless!  . If you want to learn about all the "Old wives tales" still being taught and passed around, read these articles. There has been a lot of stuff posted in this thread, but no one has explained it correctly or better than the 4 articles I have referenced, IMHO.

ts

[Lugnut]

blk

To me it isn't a question of could it, what I've seen and read, whether its a Merlin, P&W, Allison, BMW, DB, or whatever, when you start the plane, your going to run it at 600-800 through warm up and mag checks and get the oil temp up to normal before you do a run up it otherwise you risk wrecking the engine. 2500 is max engine RPM you don't start an engine at that setting.

Imagine starting your car up cold on a winter morning and keeping your foot in it after it catches and holding it at 5000-6000 rpm while the engine is trying to pump cold oil through it, then shortly thereafter, do a full throttle acceleration for a couple of minutes. Not likely.

Lug

PS (Stop me if you heard this) After Beechcraft made the Starship they got a call from a guy building a 3/4 scale FW replica who inquired about the Starshhips single lever engine management system that contolled throttle, pitch and mixture that he was trying to duplicate for his 190 replica (as the original had a similiar system). Beech engineers originally thought they had been innovators of a revolutionary concept in engine management, and were quite surprised to find out Focke Wulf had beaten them to the punch 40+ years prior.

PPS From what I have heard the system in the original 190s was a bit quirky in that in cruise it continually "hunted around" and made small adjustments to either the pitch, throttle or mixture, thus making it difficult for pilots to fly a constant speed, and thus fly tight formations.

[This message has been edited by Lugnut (edited 09-30-1999).]