Constant speed props
 

I have never flown an aircraft with a variable pitch prop but am going to learn at Easter. However, I am having problems understanding the operation of the prop lever and throtle.

I think I have a clear picture of engine managment in flight, ie rev up and throttle back. But as I understand it, on the ground, do you taxy around with the blue prop lever full forward at 2400 rpm ('ish) and the throttle right back or does having the throttle right back mean it reaches the fine stop and thus the rpm decreases?

On a Piper Arrow, just before engine start up, is the prop lever full forward and the throttle open a bit. You crank the engine and move the mixture to rich when it starts. then in the checklist, it says adjust power for 1000rpm and then warm up at 1400 rpm. forstly this is unclear because there is no 'power' lever and secondly how do you adjust the rpm up to 1400 rpm if the prop leveris slrady full forward.

I hope this isn't too full of waffle, and would be very grateful if anyone could shed some light on this subject and make it a bit clearer for me. Thanks.

skips

Also, if anyone flies an arrow II, do you have the cruise speeds at 55, 65 and 75% powers handy? Ta

[This message has been edited by SkippyX (edited 19 March 2001).]

It's the equivalent of an infinitely variable gearbox. On the ground and during take off, the pitch lever is fully fine. Use throttle just like any fixed pitch a/c. On climb out at 300/500', throttle back to 25", pitch lever back to 2500rpm. In the cruise 23"/2300 rpm is a typical setting. To climb, increase pitch first, power second. To descend, reduce power first, if required increase pitch second. Usually, leave pitch alone, adjust power with throttle. Downwind, or on final when you get smart, pitch first - to fully fine, then adjust power second with throttle as before. You must have pitch fully fine for the go-around.

Clear as mud? Ask me if not happy.
Numbers for A2 not to hand, sorry but they're all in the POH.

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Yabbadabaadoo. It's OK Boo-Boo.

[This message has been edited by Yogi-Bear (edited 19 March 2001).]

Yogi-bear,

yes, still clear as mud. I have read most of what you said but am still not sure what happens on the ground when you have full fine and little throttle, will you be taxying around with 2400rpm or what?!

Skippy - No, the engine can't acheive that RPM at a low power setting. With the lever forward, the prop will stay in fine pitch and the RPM will depend on the power setting until it reaches 2400, whereupon the prop will automatically go more course to absorb the extra power and maintain selected RPM.

Once in the air, with the lever set somewhere between full fine and full course, it will 'govern' engine RPM to the level selected by the prop lever. If the RPM starts to rise above the selected level, it cousens the blades, if it starts to fall below the selected level, it fines-off the blades.

Hope this helps.

In the Yak, RPM is shown as percent RPM, rather than actual RPM - like a jet. Thus, for aeros, we set 82 percent RPM and full power and haul it around the sky. The prop will maintain engine speed regardless of airspeed. Very useful!

SSD

Skippy, I endorse all said before.I fly an Arrow 4 so similar to a "2". Once you fly it for an hour or so it will all fall into place - it is really not a major problem. It will all seem a little busy on and just after take off for a while until you get used to it, then 2nd nature.

re the power settings :- there are many combinations and on mine there is a easy to interpret table attached to the pilots sun visor - you want 65% power at 4000'? Look at the visor and it will give you a choice of RPM / MP settings - eg 2300rpm / 23" or 2400 rpm / 22.5" etc etc (these are not actual figures - just illustrative).

You will also see that the fuel flow meter is marked with white lines for the range of flows to be set (with the mixture lever) for each of 55%, 65% and 75% power. It is all there in front of you so you do not have to keep looking at or memorising the handbook.

Hope this helps,

A2

On the ground, at low rpm the prop is not in the constant speed 'range' and it stays fully fine. Rpm is controlled by the throttle and it functions as a fixed pitch prop. As you increase the rpm the prop enters the constant speed range whereupon the blue rpm lever will start to control prop pitch and therefore rpm. When you do the runup checks on the ground, you check the operation of the prop pitch mechanism - on the Arrow at 2000rpm I think. If you try it at say 1200 rpm the prop lever will have no effect, likewise in the descent at low power settings. I remember being very confused by it all at the start but it does fall into place, don't worry. Think of it as the gear lever.

Skippy, it is extremely simple.
You have 3 control levers, Throttle (black), Mixture (red) and the Prop control (blue).
The Throttle and the Mixture levers are connected to the carburettor or the injector and the Prop control is connected to the CSU (Constant Speed Unit).
In order for you to understand the way that you control the prop I have to give you a quick explanation on the operation of the CSU.
The CSU is made of three components (there are more but they are not relevant for this discussion)
1. Weights
2. Spring
3. Valve Selector

There are two weights by the shape of ‘L’ facing each other (one looks like an ‘L’ and the other will be a mirror image creating two vertical lines and one horizontal line).
In the centre there is a valve selector that seats on the horizontal lines of the ‘L’ shaped weights. This selector is fitted in 90º to the weights (in our case vertically). The spring is fitted in the centre and pushes against the horizontal line of the ‘L’ shape i.e., pushing the weights and the selector valve down around a shaft that is fitted in the corner between the vertical and the horizontal lines of the ‘L’.
When the engine runs the weights of the CSU turn around the selector valve, by doing so, a centrifugal force is created and the weights want to be thrown out, as a result, the vertical part of the ‘L will move outwards, by doing so it lifts the valve selector. When the engine slows down, the centrifugal force will be reduced and the weights move inwards (by the force of the spring), letting the valve selector down.

When you operate the Prop Control you do one thing, you change the spring’s force against the horizontal portion of the weights.
When the Prop Control is fully forward, you mechanically lock the CSU weights and the valve selector at a position that does not allow the engine oil to enter or exit the prop causing it to stay in fine pitch at all times. Any other position of the control will cause to a different spring pressure against the weights. Both forces (spring’s and centrifugal) wants to be in an equilibrium.
On take off, the control is fully forward, hence, the CSU is locked and the prop is in a fine pitch. You throttle back to 25” and than pull the prop control to 2500rpm. What you actually do, you reduce the pressure of the spring on the weighs, allowing them to open, that in turn causes the valve selector to go up which allows some oil to escape from the prop, causing the prop’s pitch to become coarser (and vice versa).
On the ground, the engine power and RPM are controlled by the throttle only, however, in the air, power is controlled by the throttle (hg&#8221 ;) and the RPM by the Prop Control. Yet, you must be careful not to overboost the engine above its limits (usually max 3” over square). You will find that every engine has a power setting table that will provide you with the correct throttle/RPM combination for 55%, 65% and 75% power settings, it also will tell you what is the fuel flow for each settings. As a rule, if you don’t know what is the fuel flow for 75% power use full rich or you will damage the engine.
Prop Control forwards = fine pitch
Prop Control backwards = coarse pitch
If you are still confused, find a diagram of a CSU and ask your mechanic to explain you how it works.

Thanks very much for all your comments, it is a lot clearer, especially about ground ops. I'll look forward to trying it out for real soon!

Skippy,
you might look at the columns at www.avweb.com: John Deakin has a column (from a year or two ago) on the CS prop. Explains operation and construction very well

-me

(the column's address is
http://www.avweb.com/articles/pelperch/pelp0016.html )



[This message has been edited by Low_and_Slow (edited 20 March 2001).]