LAMPSguy

LAMPSguy

Well Known Member
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?
 
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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Helicopters?

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

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.
 
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.
 
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.
 
Phlyan Pan said:
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.
Click to expand...

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.
 
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.
 
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,
 
Obviously we don't all use them the same way

I fly with wide open throttle all the time except when I want to land.

I record the engine start time and tank as the first item.

I take off with the prop control forward to allow max rpm (I have the governor set so I can get 2720 rpm) but after airborne I pull back the prop control for noise abatement in the airport area - still wide open throttle.

As I climb I lean out the mixture to get good performance out of the engine. If I am tardy in my attention to the mixture the engine lets me know by sound and roughness that it is being choked to death with fuel. As soon as I pull back the mixture it smoothes out.

When I reach altitude on a trip I pull back on the prop control to 2450 RPM for the long cruise and I set the mixture with the Cylinder #4 EGT at 1300 F. I maintain those numbers to my destination with fine tuning if necessary and never touch the throttle.

I fly 1/2 hour on the first tank, then switch to the other tank recording the elapsed time used out of the first tank and the clock time that I switched to the new tank along with its identity. Then I run 1 hour out of the second tank so the maximum imbalance is 1/2 hour of fuel and the fuel valve has only been switched once in 1.5 hr fuel burn. Then I go back to the first tank with the appropriate fuel management entries on my flight plan and burn 1 hr out of it then I switch tanks again. At this point I have .3 hr fuel left in the first tank and .8 hr in the second which leaves me at .6 hr fuel remaining plus the 1/2 hour VFR reserve. My next switch is a little arbitrary at the end of 1/2 hour (not really - I stay on the same tank because they are equal) and I have .3 hr left in each tank and If I haven't already I will soon see the yellow warning lights for both tanks. After that I fly with my hand on the
fuel valve and closely monitor the fuel pressure gauge. When it starts to drop that tank is dry and I switch to my last .3 hours of fuel. My travel fuel burn is 10 gallons per hour. For racing such as the AirVenture Cup Race which can be the better part of 500 miles I have to plan and monitor ground speed carefully because the burn rate jumps to just under 14 GPH.

When I approach the destination airport I pull back on the throttle for the first time until the manifold pressure bottoms out and I can push the prop forward without overspeeding the engine thereafter using throttle only for thrust control like a fixed pitch prop.

Bob Axsom
 
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