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  #1  
Old 06-22-2012, 06:15 AM
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LAMPSguy LAMPSguy is offline
 
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Location: Pensacola, FL
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Question CS props are confusing me...

Nearly all of my time in the last ten years has been turbine powered helicopter time. When I fly fixed wing it is more focused on flight than paying attention to engine...I put levers to the number the instructor says.

Maybe the way my helicopters fly is making it hard for me. Turbine engines with 2 gas coupled parts, gas generator governed in a narrow range for best efficiency of making power. Power turbine governed to give me what I demand from said power, no more. I pull up on collective, rotor pitch increases, I get more thrust. Very simple.

How is piston engine/CS prop so different...is it the gas coupling vs the direct drive (but the prop spins through the air, like gas coupling) the difference?

It almost seems like the piston CS is exactly the same, I push RPM lever forward to go faster...instead of having a governor control the fuel/mixture, I have to do it myself...do I have it?

So, anyone have piston AND turbine CS time able to fix me?
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  #2  
Old 06-22-2012, 06:32 AM
WingsOnWheels WingsOnWheels is offline
 
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Default

The two system are quite a bit different.

On your helicopter the governor (a completely different kind) controls the power output (throttle) for you to maintain RPM while you manage the rotor pitch (prop control) manually.

In the case of a CS prop it is the opposite, the prop control sets the RPM and the governor manages pitch to maintain that RPM. Then you manage power with the Throttle.

To develop full power you need full RPM, however in cruise you can pull the RPM back with the prop control while leaving the throttle wide open (depending on altitude) since you won't get full maniford pressure at cruise altitude and overstress the engine.
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Last edited by WingsOnWheels : 06-22-2012 at 06:39 AM.
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  #3  
Old 06-22-2012, 06:39 AM
rgmwa rgmwa is offline
 
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This post may be helpful:

http://www.vansairforce.com/communit...ad.php?t=48713
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  #4  
Old 06-22-2012, 06:43 AM
WAM120RV WAM120RV is offline
 
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Default Helicopters?

Forget helicopters, they dont actually fly anyway, they are just so ugly the earth repels them

Constant speeds are simple....... its all in the name. You want max power on take off so you set the pitch to fully fine. This will give you max rated engine RPM and max power (Manifold pressure MAp) for take off and and climb.

In the cruise you decide what speed you want the engine to run at, say 2400. Pull back on the throttle to reduce the mainifold pressure so the engine is making less power, then wind in the prop untill you get the RPM you want to run at.

It will then run at the speed RPM you set it at, if you climb the pitch will flatten to maintain the rpm you set, if you descend the pitch will coarsen to maintain that constant speed.

Now if you push in the throttle the engine will produce more MAP (power) so the pitch coarsens maintaining that RPM you set, pull back on the throttle and it goes fine to maintain the RPM. Constant speed.......

However, pull back far enough on the gas and it might hit the backstops so RPM may reduce.

So, when you have set the RPM where you want it you can pretty much fly it like a fixed pitch, this only changes when you get it back to the pattern when you want to slow down CS Props help with this when you pull them back to fine pitch, but of course you have max available power available for a go round.

Changing the pitch of a rotor blade on a helicopter is more akin to changing the pitch angle on the wing, than what you are doing with a constant speed.
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  #5  
Old 06-22-2012, 06:52 AM
Sig600 Sig600 is offline
 
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In easy helo to plane speak, a CS prop acts the same way. Set a given RPM, and as you increase or decrease throttle... the prop pitch will increase or decrease to maintain RPM. Helo being a little more complicated, with a collective you're commanding an increase in lift, and a bunch of magic stuff happens to adjust fuel flow/torque output/blade angle/etc. C/S prop is simpilier.

i.e. if I'm at 20" of manifold pressure with 2400rpm set, the prop blades will be at some AOA generating X amount of thrust (lift).

If I increase power to say 23", then in order for the prop to maintain 2400rpm the prop blades need to increase pitch, which will thus increase the thrust (lift) and increase the drag on the engine. So the net result is more thrust, while maintaining rpm.

The inverse is true if I pull the power back from say 23" back to 20". The engine is not putting out as much power, the prop blades will decrease pitch/aoa/drag on the engine, maintaining RPM but decreasing the amount of thrust.


Low pitch, high rpm = max amount of thrust at low/no airspeed. This great for accelerating from a stop, and climbing out.

Higher pitch, lower rpm as you accelerate makes it more efficient and effective at higher airspeeds for cruise.
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  #6  
Old 06-22-2012, 07:28 AM
Phlyan Pan Phlyan Pan is offline
 
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Default

I tend to use a car as a metaphor rather than a helo. The constant speed prop is a manual transmission, in low gear (fine pitch) it has better pull and acceleration at low speed. In high gear (coarse pitch) it has better cruise efficiency. You're still controlling your power with the throttle though.

Now if you want to understand the mechanics of how it accomplishes these tasks then by all means go for it, but as far as the effects this is usually a pretty good metaphor.
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  #7  
Old 06-22-2012, 07:44 AM
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L.Adamson L.Adamson is offline
 
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Quote:
Originally Posted by Phlyan Pan View Post
I tend to use a car as a metaphor rather than a helo. The constant speed prop is a manual transmission, in low gear (fine pitch) it has better pull and acceleration at low speed. In high gear (coarse pitch) it has better cruise efficiency. You're still controlling your power with the throttle though.

Now if you want to understand the mechanics of how it accomplishes these tasks then by all means go for it, but as far as the effects this is usually a pretty good metaphor.
But..............unlike a car, for fastest speeds, the prop lever will again be pushed forward to the fine pitch end (or near so). However, since it's not a variable pitch prop being operated solely by the prop knob...........it's not actually at the finest pitch. That's where the car comparison falls apart, since it's not freeway speeds at high throttle and 5th gear.

BTW--- For those that don't know. For getting into the pattern, you only push the knob forward on final, with throttle pulled back. Even if you're at 2350-2500 for instance, the prop will still act as an airbrake. If you don't push the knob full forward, you'll notice a dramatic performance penalty on a touch and go, or go around. Push the knob forward to quickly before the aircraft has slowed down, with no load on the engine, and it's like shifting a vehicle into low gear at the wrong time.

Most of the time, I'd pull back slightly after takeoff ( noise), and then run around 2350 in cruise. My prop has the 2000-2250 rpm. restriction.
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  #8  
Old 06-22-2012, 08:05 AM
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Wesael Wesael is offline
 
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Quote:
Originally Posted by WAM120RV View Post
Forget helicopters, they dont actually fly
You are correct they don't fly. They just beat the air into submission.
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  #9  
Old 06-22-2012, 08:39 AM
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rv8guy rv8guy is offline
 
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well, I have time in both, and the truth is, I think they really do act similar.

In the turbine/helo, you read torque for power, in the cs/airplane you read manifold pressure. While flying, both the rotor and the cs prop maintain a constant speed, with one real exception.

In the helo, RPM stays up, even with the collective/thrust reduced to basically zero. There is a throttle or idle, depending on the setup, but that's for ground ops only (well maybe an emergency or two).

With the cs, you obviously override the speed when you pull to idle, and the speed goes way down. (not good in a helo)

Both have some system to change blade pitch to keep that speed/rpm constant. The reality of the two are quite different, but the speed/rpm part seems very similar to me. Of course there is that pull to add power in the helo, and push to add power with the cs, but that's just a detail.

In cruise for example, we think of adding power or manifold pressure with the cs, but the reality is we are increasing the blade pitch just like adding blade pitch with the collective/thrust. In the helo we generally talk "pulling pitch", while in the airplane we "add power". Outcome in both places is more power higher blade pitch etc.

Just a detail, but even with a lycoming powered helicopter, it does act just like the turbine, except in the TH-55 I learned in, it was the pilot that was the governor with the twist grip throttle.
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  #10  
Old 06-22-2012, 09:18 AM
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pierre smith pierre smith is offline
 
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Default Hi Nick.

I fly both my PT-6 powered Air Tractor and my -10 and the prop usage is the same. Both have the prop control full forward for takeoff and then later. pulled back to cruise settings.

The Garrett engines are direct drive and geared and run at a high, constant RPM and as the prop control is moved, the thrust and pitch change, still pretty similar but they're much noisier than my free turbined PT-6.

Incidentally, the last mechanic held my prop while I started the engine, holding it dead still! When he let it go, it spooled up in the feathered position.

Best,
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