CSU vs VPP

[ravenx]

Can someone please exlpain the difference between a constant speed unit and a variale pitch propellor and what the benefits are

[IanSeager]

A CSU will vary the pitch of the blades in flight to ensure that the prop turns at a constant speed. It is fairly common for aeroplanes to be described as having a VPP when really what is meant is that there's a CSU changing the pitch of the blades.

99% of the time CSU and VPP are one and the same.

Some kit aircraft have VPP that can be adjusted on the ground only - an example of a VPP without a CSU.

Ian

[Field In Sight]

As Ian said,

A VPP is a propeller that can change it's angle of incidence and thus angle of attack so that it can vary to an optimum coefficient of lift for a particular operation i.e. cruise or take-off.

This is a benefit over a fixed pitch prop that has it's angle of incidence set at an optimum angle for one operation only. This is usually cruise as you spend more time cruising around than taking off, although the take-off performance will be slightly degraded.

The CSU automatically controls the VPP so that it maintains a certain RPM by varying the pitch depending on the engine torque and load applied to the prop. This is to maintain the engine RPM at a certain value that gives the best performance/economy.

I think the Diamond Katana has an electrically operated VPP that has 2 settings only i.e. take-off (fine pitch or 1st gear) and cruise (course pitch or top gear).

[FlyingForFun]

Hi ravenx,

I fly an aircraft which has a VPP, but not a CSU (Europa with a Rotax 914 engine). I have also flown Piper Arrows which have a CSU.

Here's how I understand it - I know someone will correct me if I'm wrong ;) I suspect my understanding is at least over-simplified, even if it's not wrong, but it works for me.

First of all, propellor blades are aerofoils, just like a wing. They're shaped a little differently, and have a different purpose in the grand scheme of things, but they work exactly the same way.

We know that angle of attack is a crucial factor in finding the performance of an aerofoil. Angle of attack is defined as the angle between the aerofoil chord-line and the relative airflow.

Obvious point coming up here: if you twist your propellor blades round, you change the angle of attack. Increased angle of attack is known as coarse, decreased is known as fine.

Less obvious point: if you change your airspeed, you'll change the angle of attack of the blades. Imagine a prop blade striking a stationary parcel of air. The angle at which it will hit it will be the angle between the aircraft's lateral axis and the blade's chord line (I'm sure there's a name for that, but I don't know what it is!) Now, imagine that the air isn't stationary, that it's moving towards the blade (or that the blade is moving towards it). The angle of attack decreases. This is very hard to visualise - I had to use fingers and hands in some very strange poses before I could visualise it! (It also follows that increasing the speed of the prop increases the angle of attack.) If the angle of attack decreases, the lift which the blade produces (and therefore the thrust which the propellor produces overall) decreases. Therefore, a prop with too fine a pitch won't work well at high airspeeds.

One solution is to use coarser blades. However, if you make the blade too coarse, it will stall. It will also slow the engine down because it will meet more resistance from the air - piston engines produce more power at high speeds, and in fact running a piston engine at low speed and high power can damage the engine (try driving off in 5th gear going up a steep hill, and see what nasty noises your car engine makes!) Plus, the propellor turns slower, and lift is a factor of speed, so the prop will produce less horizontal lift (i.e. thrust). I'm led to believe that the effect on the engine power is small compared to the effect on the angle of attack and speed of the blade, but I don't know any numbers. So a coarse prop, although it works well at low speeds, won't work so well at high speeds. These props are known as "climb props" because they help you climb faster, versus "cruise props" which are finer and help you cruise faster at the expense of climb performance.

The next step is a variable pitch prop, such as the one I have. Some VPPs have a "climb" setting and a "cruise" setting, which are roughly analagous to the two different types of fixed-pitch prop, except that you can switch between them in flight. I have a continuously-variable prop, i.e. I can set any pitch between two limits. There is one optimal pitch for each engine speed/airspeed combination. But, in practice, I can't continuously change the pitch. And the effect of the engine speed is minimal compared to the effect of airspeed. So I tend to use ultra-fine for taxiing, fine for take-off and in preparation for go-around, slightly coarser for cruise-climb, and almost full-coarse for cruising.

Imagine I'm flying along at cruise power, in the cruise, with a cruise setting on my prop. Then I pull up. Immediately, my airspeed drops, and I need to reset my prop. A CSU attempts to get around this by fining the prop automatically for you. By attempting to maintain a constant engine speed, the CSU is forces to fine the propellor in response to the decreased airspeed. This results in the angle of attack remaining almost, but not quite, constant, and avoids slowing the prop down - close enough that the pilot of a CSU-equipped aircraft won't necessarilly need to reset the propellor controls in response to a moderate change in pitch. (It also avoids loosing engine power due to reduced engine speed, but this is negligable anyway.)

Of course, there are engine management and fuel consumption issues to worry about, apart from pure performance, so it's actually more complex than this once you start looking into it (and that's where I give up!)

I hope that's pretty close to accurate (and if it's not, well, we'll both have learnt something when someone corrects me!), and that it helps answer the question!

FFF
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[ravenx]

FFF - I can tell you've been paying attention in your ATPL reading :)

Thanks for the comprehensive response I think it's cleared that up.

How does the pitch affect the noise generated.

[FlyingForFun]

:o Erm, actually, I haven't got to that bit in the ATPL notes yet ravenx! If I had, I'd probably be more certain than I am. But this stuff mostly comes from background reading. I always thought that v.p. props were all to do with maintaining engine power, the same as the gearbox in a car. Then someone told me that's not the case, although it's a common mis-conception. So I did some reading to find out the truth.

As for noise, lower engine speed (i.e. coarser pitch) will always generate less noise. This is due to a combination of both the engine and the prop being quieter at lower speeds.

(The extra engine noise due to the increase in throttle that you'd need to get the same thrust would be minimal. And that's if you increased the throttle at all - you might not want to because of the damage it would do to the engine using high throttle at lower speeds, you might just not bother because you've got enough power anyway, or you might be at full throttle and not have any extra to give!)

FFF
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[Tinstaafl]

FFF

So a coarse prop, although it works well at low speeds, won't work so well at high speeds. These props are known as "climb props" because they help you climb faster, versus "cruise props" which are finer and help you cruise faster at the expense of climb performance.

I think you need to re-write this statement of yours. You have it backwards.

At low speeds eg take-off & climb a relatively fine pitch prop is needed. This is a climb prop. At cruise & high speeds a relatively coarse pitch is more efficient. This is a cruise prop.

As you quite correctly mentioned elsewhere, as speed increases an increasingly coarser pitch blade angle is needed to maintain the most efficient AofA.

If the blade angle of a fixed pitch prop is set so that it is most efficient at take off (a fine pitch prop aka. a climb prop) then it will become increasingly less efficient as the a/c accelerates in the cruise, resulting in reduced cruise performance.

If the blade angle of a fixed pitch prop is set so that it is most efficient at cruise (a coarse pitch prop aka. a cruise prop) then it will start at a low efficiency during takeoff becomeing increasingly more efficient as the a/c accelerates in the cruise. This results increased cruise performance at the expense of take-off performance.

Have a look at the Schneider Trophy racers of the '20s & '30s. Extremely coarse blade angles optimised for their eventual cruise speed (ie fast) but with the penalty of abysmal take-off performance.

In the imaginary ideal world, after take-off you would unbolt the very fine pitch take off prop after getting airborne, put on a slightly coarser climb prop for the climb, exchange that one for a coarser pitch prop for the cruise then back to the very fine pitch prop for the landing (in case maximum power & efficiency is suddenly needed during the flare).

Obviously this is impractical, hence the arrival of variable pitch props, commonly in the form of two speed props. Flick a switch & the blade angle changes from fine to coarse or vica versa. The blade is mostly efficient somewhere in the take-off & climb range of speeds, and also mostly efficient somewhere in the cruise range of speeds.

The next development was for a continously variable pitch prop. This replaced the two position type with a blade mechanism that allowed a blade setting anywhere between its finest & coarsest positions.

Great, but every speed or RPM change meant that the efficiency changed a bit unless the blade angle was reset.

Add a Constant Speed Unit to this VPP & it will automatically adjust the blade angle to maintain a designated RPM, relieving the crew of this task.

[FlyingForFun]

Tinstaafl,

You are quite right, I got that sentence the wrong way round - oops! I think I got it the right way round in the rest of my post, though - just that sentence I got wrong.

You said:

In the imaginary ideal world, after take-off you would unbolt the very fine pitch take off prop after getting airborne, put on a slightly coarser climb prop for the climb, exchange that one for a coarser pitch prop for the cruise then back to the very fine pitch prop for the landing
Ah, a practical use for wing-walkers then! Did you see that guy who broke the world record for the oldest wing-walker just a couple of weeks ago? I'm sure he wouldn't mind changing the prop while he's out there... :D :D :D

FFF
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