Constant Speed Prop Help

[Bullethead]

Gruppen-

Never had a sim with constant speed props before so I'm a bit in the dark about how to use them.  Help would be appreciated here.

I understand the basic concepts, I _think_.  In AH, you have an RPM control that sets the constant speed the prop maintains.  You also have throttle, which changes the manifold pressure, aka how much macho is going to the prop.  So as you wiggle the throttle, the prop automatically changes pitch to maintain the pre-set RPM.  Is this right?

Now, seems to me that the various situations of takeoff/climb, cruise, combat, and landing would each require some optimum RPM setting.  IOW, in each type of situation, you set the RPM and leave it alone while you wiggle the throttle.  Is this true?  If so, can somebody give me some pointers on what amount of RPM is best for each type of situation?

Or is RPM like a trim tab for the motor, something we'll have to constantly tweak in flight?

Thanks in advance.

-Bullethead <CAF>

[Mt-Dew]

WOW, this game has constant speed props.  Thats gonna blow some people away.  Bullethead, your basic concept is correct.  Each plane should have its own sets of numbers best suited to it.  The supercharged engines really packed some horse-power in some planes.  One thing to watch for is over boosting at a given or set RPM.  You can't just cobb on the power if you have a lower RPM setting without hurting something.  

Could be lots of black smoke up here for a bit.  hehehee

Cool feature


Mt-Dew

[-blk--]

  OK, first off, AH isn't exactly modelling a CS prop totally correctly...but it's better than Brand W.

  And, you fly AH the same way as a real one.
It seems to have been missed in the FAQ for some reason, so I'll repost my MAP explanation here...

blk

PS-this is the same explanation I posted int he Alpha forum a few weeks back, so this may sound familiar if it did indeed end up in the FAQ...

------------------------------------------
How come the engine's rpm doesn't change when I move the throttle?


Basically, with a fixed pitch propeller, we can measure engine power with the tachometer (rpms). But, with a constant speed (CS) prop, the tachometer should remain constant. Therefor, we've got to measure engine power some other way.

So, we have this thing called Manifold Absolute Pressure (MAP). What this is, is we measure the air pressure inside the intake manifold (the pipes that carry air into the engine). First off, standard atmosphere is 29.92 InHg (you'll see where this is going).

First, I'll explain about normally aspirated (not supercharged) engines.  So, with the engine off, the MAP gage will read the ambient air pressure (29.92 InHg). We now decide to start the engine. But at idle power, the throttle plate is closed, and is restricting the flow of air (and sometimes fuel) to the cylinders. So, at idle, the engine will be at a low MAP setting (around 12 InHg MAP or so). It would try to be zero, but if there were NO air/fuel making it to the engine, it wouldn't run, so it'll be around 12 InHg MAP or so. So then, we push the throttle forward to the stop. This moves the throttle plate so that it's restricting the intake air as little as possible (if you don't know how a throttle plate works, most encyclopedias have a decent diagram). So, the engine is going to be as close to 29.92 InHg MAP as it possibly can be (since there's not supposed to be any restrictions). But the air can't be COMPLETELY unrestricted (there are turns in the induction system, the throttle plate still adds a little drag to the intake airstream and so on), so we can usually get around 29 InHg or so. So, you with me so far? As throttle setting increases, MAP increases up to 29 InHg. So, that's how we measure the engine's power with a constant rpm (high MAP equates to high power settings).

Now, remember how I said standard atmosphere was 29.92 InHg? Well, that's at Sea Level (and a "standard" day). As we climb, we lose a lot of that air density (approximately 1 InHg per 1000'). So, the maximum MAP the engine can possibly attain at altitude will decrease (because the atmospheric pressure decreases), and we get a corresponding loss of engine power at high altitude.

So, someone got smart, and decided that if we could somehow get more than ambient air pressure at altitude, the engine (and thus the aircraft) would perform better. So, basically, they compress air going into the intake manifold back to that sea level pressure (29.92 InHg). So, the engine can develop its sea level Horsepower at altitude. This is called "Normalizing." When we compress the air with a compressor driven by an exhaust driven turbine, we've got a "Turbo-Normalizer," or what's generally referred to as a "Turbocharger."  but then, someone else got even smarter, and decided that if we could boost MAP back up to sea level pressure at altitude, we could increase it even more. Thus, at sea level we could get MAP in excess of 29.92 InHg. So, we're actually increasing the Horsepower of the engine to an artificially high level. This is called "Supercharging."  When an exhaust driven turbine drives the Supercharger, it's called a "Turbo-Supercharger." So, how do we operate such an engine?

Well, on the P-51D in AH, the normal full throttle, on the runway MAP is 60 InHg (which corresponds pretty well to the 61 InHg redline in real life). So, 60 InHg is full throttle. And, that idle MAP setting is about 20-30 InHg because the turbo-supercharger is compressing the air just a bit there too (and that engine will draw at least 20 InHg MAP in order to keep all 12 of those high compression cylinders working...).

                         WEP is just giving you a boost to 65 InHg. In RL, you've got to be careful not to overboost the engine (create more HP than the engine can absorb, and the heat associated with such high power settings does a lot of damage as well).  But since this is AH, you can push the engine hard, just watch how long you fly with WEP engaged.

[funked]

Bullethead:  There seems to be no prop control in HA - RPM is constant.

Blk:  P-51D has two-stage two-speed mechanical supercharger system - no turbo.

[This message has been edited by funked (edited 09-28-1999).]

[Bullethead]

-blk-- said:
>>>>>>>>> PS-this is the same explanation I posted int he Alpha forum a few weeks back, so this may sound familiar if it did indeed end up in the FAQ... <<<<<<<<<<<

Thanks, but your post got cut off while still in theory before it got to practical application  .

So, back to the basic question.  How do I know what RPM setting to use in a given situation?  I assume there are curves somewhere of HP vs. RPM, fuel consumption vs. RPM, max level speed vs. RPM, etc, each for various alts.  Where can I find this info?  I figure it'll make quite a difference in combat, especially vs. folks who don't know how to get the most HP out of their motor  .

-Bullethead <CAF>

[Bullethead]

funked said:
>>>>>>>>>> Bullethead: There seems to be no prop control in HA - RPM is constant. <<<<<<<<<<

I beg to differ.  KP- and KP+ are the default RPM change keys and they do indeed work in the beta.  You can use these keys to set the RPM over a wide range of values, which it then holds constant while you wiggle the throttle and change manifold pressure.  This is why I want to know what the optimum RPM settings are  .

-Bullethead <CAF>

[Pyro]

For now, you're best to keep your RPM maxed out.  There's more to be implemented in this area, but it's pretty much roughed in now.  Once we get the fuel consumptions curves all done, this will be more of a factor.



------------------
Doug "Pyro" Balmos
HiTech Creations

[Bullethead]

Pyro said:
>>>>>>>>>>>>>>> For now, you're best to keep your RPM maxed out. There's more to be implemented in this area, but it's pretty much roughed in now. Once we get the fuel consumptions curves all done, this will be more of a factor. <<<<<<<<<<<<<<<<<

Thanks for the info.  I look forward to seeing more added to this feature.  I foresee it being a great separator of the quick from the dead.  "Why does his 51 go faster than mine?" <EG>

BTW, ain't had a chance to say so yet, so thanks for the work here and good luck with this venture.

-Bullethead <CAF>
 AW:  3841-
 WB:  blt-hd
 AH:  BH3841

[Pyro]

Don't look for performance boosts out of it.  The real benefit in AH will come in the form of fuel conservation at lower throttle settings.  We do want to stress the importance of fuel more, and that's why we're running faster fuel consumption rates and have drop tanks available.



------------------
Doug "Pyro" Balmos
HiTech Creations

[chisel]

Just for fun set the prop to full coarse and try to takeoff.

So,we gonna get mixture controls or are you gonna stop here?

Nuther question will changing prop pitch reduce dive speed, decceleration acceleration in the future ,also overspeed? <-havent checked it out where's Wells?

Sound files for pitch change?

Really like the Gustav on takeoff. Even managed to keep a few on the runway  
Makes the 51 look like a popsiclecat

Looks pretty good after cusory inspection. no problems yet but I promise to break somthing.

<S> HTC!

PS nag.. nag.. nag  

[wells]

The rule of thumb is, match rpm to the speed (if flying 2/3 top speed, use 2/3 rpm) and then use MP to maintain that speed.  Fuel consumption will vary (almost directly) with both rpms and MP.  For example, best range in a P-51, the settings are 1800 RPM and 185 mph IAS.

[indian]

-blk-- you need to make a correction you stated that pressure and density are the same or atleast led to air density is about 14.75psi at sea level and drops at 5000ft intervals. Not going to get techno here. But you are refering to density in above post. And another unless we have a prop tach in these fighters the rpm gauge should and will change with engine speed, we would need a prop lever to run the prop from beta range to full feather where beta is flat as in wing tip to wing tip and full feather is flat front to rear MAP does not measure the power of the prop it only mesures the pressure inside the manifold. the only indication unless a prop tach or torgue meter is installed is the seat of the pants type of feeling. A constant speed prop is attached to a gear box (planetary type) and not a shaft straight from the engine, this type of prop spins at a constant speed of about 3200 rpm's depending on type and all, the rpms of the engine are used to increase torque to the prop. The rpm's speeding up and spinning the supercharger are what increases the MAP which again does not show prop output.

Turbo supercharging is a result of the exhaust driven supersgarger?

No turbo supercharging is a crank drivne supercharger with turbochargers driven by the exhaust of the engine.
Compund tubocharging is a turbocharger that drives a shaft that is connect to the crankshaft they give about 50 more hp to they engine and are used I believe on R3350 compond turbo engines.
 Almost everything you said has nothing to do with constant speed props. MAP is a good indication you are going to blow a head off or somehting not what the prop is doing.
------------------
Tommy (INDIAN) Toon
 http://www.geocities.com/~tltoon



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

[bod]

A constant speed prop does not need to be attached to a gear box (an normally isn't for modern airplanes), since the speed is changed by changing the pitch of the blades.

The reason for gearbox is to reduce the speed of the prop so that it wont go supersonic or to reduce noise level.

Bod

[-blk--]

  Disclaimer:  I'm not really going to debate the minute details of my explanation.  I wrote that because there were people in the Alpha test who wanted to understand it as it relates to AH.

  AH, is basically a CS prop with one setting (full forward).  As of now, if you decrease the rpm (yes, you can, just mine you have to map the keys yourself) you just decrease torque.  This is not a finished engine model.  For now, just leave it at full RPM (a tad over 3,000)  Then, use the MAP as a general indication of the horsepower being produced by the engine.  This is how it relates to AH.  Full throttle in the P-51 is 60 InHg.  Climb power I use is 55, even though there's not a real reason to in AH yet, other than fuel consumption.  Cruise around 45-50 InHg.  Descents around 30 InHg.  

blk

PS-indian, I fail to see the difference in your explanation of a turbo-supercharger.  You described an exhaust driven compressor which increases the air density to a point hich corresponds to an altitude below sea level...  Now I'm just curious, am I missing something?

[tshred]

If you really want to understand Props and Manifold pressure, and learn about all the myth's still being passed on, then I suggest checking out this website. Every week you get an email update about what's going on in Aviation, plus they have experts who write very good articles, ranging from maintence to flight ops. You won't be dissapointed. The following link will take you to the article, but first you must join them, free by the way, to be able to view it.
 http://www.avweb.com/articles/pelperch/pelp0016.html

The title of the article is: "Those Marvelous Props"

There is also another related article: "Manifold Pressure Sucks!"

Check out the rest of the website and the other articles, lot's of good reading.

tshred

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