CSU Question?

[Big Kev]

Hi All,

Just a quick question as I'm studying for my CSU endorsement. I have read in the books that the pitch must always be set to fully fine for take-off and just prior to landing to enable the engine to deliver maximum thrust but wouldn't the engine deliver max thrust at a lower RPM with the pitch blades in a more coarse position.

If they were in a more coarse position for takeoff sure, the engine wouldn't rotate as fast but the bit of air that the propeller takes would be greater than if it were at a finner pitch spinning faster. That bigger bite of air would result in a bigger production of thrust shouldn't it?

So why is it common practice to go pitch fully fine on takeoff when at a coarser setting more thrust is being developed?

Thanks all to those who reply.


:ok:

BK

[Chimbu chuckles]

Because at the coarser pitch you will not be developing full power....for one thing the engine driven fuel pump will be turning slower pumping less fuel. Additionally, because these engines are fixed timing the slower RPM will cause the spark to happen closer to TDC and the peak power pulse in the combustion chamber will not have the leverage to apply a maximum push against the piston. This will cause horrendous heat and pressures in the, effectively smaller, combustion chamber and you risk severe detonation which can destroy your engine within seconds.

[404 Titan]

It's also like trying to pull away from a set of traffic lights in your car with 5th gear. You have to start in first and work your way up as you accelerate otherwise you aren't going anywhere.

[Chimbu chuckles]

You may find this interesting.

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

It will lead you to other articles I highly recommend...'Mixture Magic' and 'Manifold Pressure Sucks' .

[M.25]

In addition to the fact that a piston engine creates max horsepower at a given RPM (usually max), the propeller is an aerofoil just like a wing, and therefore has a most efficient angle of attack. At low speeds (takeoff, climb, go-around) the propeller will be closest to this angle of attack with the blades in 'full fine' pitch. As you accelerate towards cruise speed, due to the changing direction of the relative airflow the AoA of the blades will decrease, requiring you to 'coarsen' the pitch to maintain the most efficient AoA.
Fixed pitch props use a compromise between climb and cruise performance. Variable pitch props allow us to get the most efficiency out of our prop in both climb and cruise configurations, giving the aircraft better takeoff/climb performance, and longer 'legs'.

[Milt]

You cannot select a prop pitch angle. The CSU only allows you to select the RPM you want the engine to reach. As you increase the power of the engine for takeoff with the CSU set for max RPM the prop stays at full fine pitch UNTIL the engine reaches max RPM. Then the CSU keeps adjusting the prop pitch as required to absorb the max power.

Only special instrumentation will determine what the blade angle happens to be when it comes off the fine pitch stops and as air speed increases the prop will increase pitch appropriately to maintain the RPM you have set.

On a landing approach whenever the RPM is less than your CSU setting the prop will be at full fine trying to recover the RPM.

Confusion will continue until everyone starts to call the CSU control an RPM selector.

FADEC will, under most circimstances, automatically adjust all significant engine/prop parameters to optimise performance.

[M.25]

Agreed. You can not select the pitch of the prop, you can only select combinations of MAP/RPM, which fortunately for us have been calculated by the manufacturers to provide us with an efficient range of AoA’s for a given combination. This is why you can’t just fit any old prop to any old aircraft. With reference to the original question, even if the engine could produce the same power at a reduced RPM, (resulting in a courser pitch once the engine has reached the selected rpm) the prop blades would be further from their best lift/drag ratio and therefore would not be as efficient. (Resulting in an increased ground roll distance)
In turboprop aircraft you can produce max torque through a range of propeller RPM, but we use a high RPM (resulting in a finer pitch) for takeoff since this is the AoA range that the prop is most efficient for our current TAS.

Feel free to correct me if I am wrong.:\

[UnderneathTheRadar]

Feel free to correct me if I am wrong.:\
I don't think you're wrong but can I borrow your glasses please? :8

[M.25]

I don't think you're wrong but can I borrow your glasses please? :8

I was simply elaborating on my previous post (off the top of my head) in case of any confusion and in reply to Milt... Hence my invitation for anyone to correct me if I am wrong.

[Like This - Do That]

I was simply elaborating on my previous post (off the top of my head) in case of any confusion and in reply to Milt... Hence my invitation for anyone to correct me if I am wrong.

M.25 I think UnderneathTheRadar was referring to your choice of ultra fine font in your post. My eyes are fine, but even I squinted a bit and leaned forward!

[*Lancer*]

I think UnderneathTheRadar was just implying you write very small on the chalkboard... :ok:

[M.25]

Cheers guys.:ok: Probably because I wrote the post in word and pasted it across after having to re-write a whole post earlier when the server was too busy to accept it.

[Big Kev]

Thanks to all that replied.

I understand that if you start of with say 25" and 2500RPM and then reduce to 25" and 2000RPM then the amount of power being developed is less.

But why should the thrust being produced by the propeller be decreased as well?

If we were to stand behind the propeller at 25"/2500RPM and measure the thrust and then do the same at 25"/2000RPM, shouldn't we feel the same?

I ask this because even though at 25"/2000RPM less power is being produced, the blade will slice a larger chunk of air because it is in a coarser position so even though the rate of air acceleration being produced by the prop is lower at the 25"/2000RPM combination, the larger chunks of air that the prop takes should mean that thrust pretty much stays the same - right??


Thanks to all that reply.....

:ok:

[Chimbu chuckles]

Kev the MP is not an indication of power being transmitted to the prop.

MP indicates how much air is available to be mixed with the fuel and the ratio of fuel to air is what gives you the power to do work....like spin the prop.

MP is actually an indication of suction. Sitting on the ground with the engine shut down you will see the MP gauge reading 29.5 inches at sea level in ISA conditions. Engine running at idle might be 10 inches which is lots less air pressure because the throttle butterfly is almost blocking the air intake up stream of the venturi which is where MP is measured creating a partial vacuum in the intake manifold. Mixture in this case would be full rich so fuel being delivered to the combustion chamber is dependant only on the engine driven pump which is a function of prop RPM. Not much fuel and not much air means not much power.

Another example is climbing up to say 5000' where you will be indicating, with throttle wide open, about 25 inches and setting 2000 rpm. Then pull the mixture to idle cutoff. The MP won't change and if you lower the nose to maintain speed the RPM won't change but is the engine producing any power?

In your example the MP is unchanged but the RPM is reduced. Reducing the RPM means the engine driven fuel pump turns slower delivering less fuel...less fuel/same air means less power developed by the engine.

The only control you have in the cockpit that is capable of adjusting power from 0-100% is the mixture control.

[Delta_7]

This may not be the simplest way to explain this :ugh: but here goes.

As we are talking low forward speeds (T/O and LDG), imagine an aircraft doing a ground run in the run-up bay.

Thrust generated is the mass of air you are moving multiplied by its velocity.

During a ground run, as you change from 25"/2500 RPM to 25"/2000 RPM, four important things happen

1. The prop takes larger chunks of air (results in more thrust)
2. Because the prop blades are moving more slowly they move less mass of air (results in less thrust)
4. More induced drag is generated by the props at their greater AoA (results in less thrust)
5. Less parasite drag is generated by the props at their lower speed (results in more thrust).

These four combine to produce an OVERALL reduction in thrust.

However, speed up to cruise speed and some things change. Induced and parasite drag on the blades change due to the forward speed of the aircraft. Simply put, at higher airspeeds reducing the RPM means you reduce the TOTAL drag produced by the propeller. This means less energy from the engine is wasted making drag. I'm not saying you get more thrust because you don't, but you get similar thrust (slightly less) for much less fuel (due to reduced RPM).

Power is simply the rate at which thrust is happening, and so also reduces as the RPM reduces as the 25" of fuel/air charge is entering the cylinders less often. This means the rate at which this thrust is being produced is lower - hence, less power too.

[APMR]

Big Kev,

My two bob answers to your questions:


So why is it common practice to go pitch fully fine on takeoff when
at a coarser setting more thrust is being developed?


It is normal practice to select max RPM for takeoff and landing because only then can you get maximum power from the engine. Instead of thinking of the selection as being for "fine pitch", think of it as being for maximum engine RPM.

To get maximum thrust from the propeller the engine must first be giving it maximum power. The engine is at maximum power when the product of the torque and RPM is greatest (i.e torque x RPM). It has been engineered that max torque occurs at max RPM.

Interestingly, the petrol burning piston engine is capable of greater torque (and therefore, power) than what it gives on an aircraft but cannot provide it because it would need to rev at somewhere around 3,500 RPM (roughly the max torque RPM for petrol burning piston engines - including car engines).

A geared prop allows the engine to rev at/near it's most efficient (at/near the value for max torque) whilst allowing the prop to rev at its most efficient (1,500 - 1,800 RPM). The downside of gearing is the extra complexity and weight due to having a gearbox and the increased wear and tear on the engine (due to it having to do millions more revolutions than a non-geared). A better solution again is the diesel engine, which gives max torque at much lower RPM than petrol engines.


I understand that if you start of with say 25" and 2500RPM and then
reduce to 25" and 2000RPM then the amount of power being developed is less.

But why should the thrust being produced by the propeller be decreased as well?


If you reduce RPM from 2500 to 2000 the MAP will increase slightly, to say 25.5". That little MAP increase is no anomaly or coincidence - it is an indication that the power (and thrust) are staying the same!

The effect of reducing RPM (without touching the power levers) is much more clear on the turboprop engine (due to its superior instrumentation). Reduce the RPM as per this example and you will observe the torque increase by 25%. The end result is that the power is still the same (because the power is the product of torque and RPM). Some example numbers for a turboprop:

Before: 2000 RPM, 1500 ft-lbs torque
After: 1500 RPM, 2000 ft-lbs torque

These are some typical numbers that any turboprop pilot could verify. They show that by changing the RPM (only) the power output stays the same. In practice, the power output does change slightly, due to slight differences in efficiencies between the two regimes.

The slight MAP pressure rise that you observe on the piston engine when you reduce RPM is akin to the torque rise on the turboprop but because MAP variations are not true reflections of torque variations (see below), the MAP rise is misleadingly small.

The manifold pressure gauge for the piston engine is "the poor man's torquemeter". Although it still allows accurate setting of power, manifold pressure is not proportional to torque. Of course you could put a proper torquemeter on a piston engine but why bother when MAP does the job satisfactorily and is far more simple and reliable?

Big Kev, you are correct in your assertion that the thrust should stay about the same when you reduce RPM whilst leaving MAP unchanged. In theory, the thrust should stay exactly the same (because the power given to the propeller should be remaining the same), but due again to slight efficiency differences between the two regimes, the thrust ends up being slightly different - possibly greater, possibly less.

Best of luck with your CSU conversion. You can forget all the above and just remember this:

Aside from the mixture control (refer Chimbu's excellent description), the only levers that change the power (and thrust) are the throttles! Also true on the turboprop, of course - but those levers are aptly named "the power levers". You may now be wondering why there are pitch levers in the cockpit but hopefully now you can answer that little question yourself!

[Big Kev]

Thanks to APMR and Delta_7 for there excellent descriptions.

The theory books do not really go into the CSU operation in proper depth and leaves wondering minds like myself unaswered to a certain extent. So thanks again to those who replied.


BK.

:ok: