[jhb] Re: Getting Too Real
- From: "Fossil" <fossil@xxxxxxx>
- To: <jhb@xxxxxxxxxxxxx>
- Date: Tue, 28 Jul 2009 03:42:21 +0100
This can be a tricky subject. The first major hurdle is that we still call
them throttle and pitch levers purely through historical usage when most
aircraft now have throttle and RPM levers.
The throttle lever is easy enough - it alters fuel flow as in any combustion
engine so that more fuel provides more power. In many engines (car, boat,
lawn mower) increased power means an increase in rpm because they are direct
drive. The same applies to most GA aircraft too where the prop is bolted
directly onto the crankshaft.
Direct drive is all very well for engines being subject to a constant load
because they will maintain fairly constant RPM. If you use a device where
the load varies - such as sticking the Flymo into thick grass - the extra
load will cause RPM to drop. Most devices have throttle control to overcome
this (except a Flymo) so if you encounter a situation where you know the
load is going to be sustained for a while you can increase power to
compensate. Go uphill in a car and you automatically hit the pedal to stop
speed dropping off. In a fixed prop aircraft you likewise know you have to
add power to climb and reduce it for descent.
The above works well for most engines so why the complication of pitch
control on aircraft? Again we have a complex answer because of the evolution
of the system - and these reveal the confusion between pitch and RPM.
Early aircraft had fixed props with a fixed pitch. The pitch of the prop
determines the thrust provided and although you can make different props
with different pitches the optimum variation was quite small indeed - only a
few degrees. A fine pitch on a prop will give good thrust for take off but
thrust decays with increasing airspeed and can limit maximum speed. A coarse
pitched prop will give added thrust and therefore a higher cruise but the
prop can be semi stalled on take off and this will result in longer take off
distances. For most fixed prop aircraft the pitch is chosen as most
appropriate to the operational speed range of the aircraft. Some of you may
recall the early S6B racing aircraft used in the Schneider Cup races which
had fixed pitch props but were expected to fly at extremely high speeds
(over 400mph). Here there was no compromise and the pitch was extremely
coarse - see
http://www.raf.mod.uk/downloads/wallpapers/1920s_1930s/s6b1024.jpg. The
pitch was so coarse that the prop produced hardly any thrust whilst on the
water and the take off runs were horrifically long - over a mile. I presume
that on these aircraft you never applied full power on take off because the
huge load would blow the engine.
By the 1930's they were experimenting with variable pitch props. With the
first of these you had a crude mechanical system which could give you a Fine
setting for take off and then you moved the lever into Coarse for the
cruise. Variations of this exist to this day and the early Cherokee Six
range had such a Pitch lever. Indeed this is rightly called a Pitch lever
because it does exactly that.
The invention of the CSU change things somewhat. This was a brilliant idea
designed to provide a better solution than the two position pitch lever by
allowing a constantly varying prop pitch relative to the loading on the
prop. The importance of the CSU is that it removes direct control of prop
pitch from the pilot and the lever in the cockpit now controls RPM. The
pilot sets the RPM required and it is the CSU that determines the prop pitch
to maintain that RPM. In the cockpit there is no indication of pitch at all
- just the RPM indicator.
When you fly an aircraft with a CSU you have to remember that the device is
doing some of your work. As the pilot you just set power and RPM to the
recommended settings and in nice stable flight these will not change - and
neither will the prop pitch. If you are in slight turbulence there will be
varying load on the props and here the CSU gets busy trying to adjust prop
pitch to maintain the RPM you have set. You can also set a gradual climb
without touching any levers because the increased load with initially slow
engine RPM but then the CSU will compensate by making the pitch finer,
reducing load and keeping RPM up.
As Peter rightly says you never apply more power when you have reduced the
RPM because you are going to bust the engine. If you are going uphill in a
car and it starts to slow down you can apply power if speed is relatively
high but you automatically know the point where it is more prudent to change
down a gear. You would never stay in top gear and let the speed decay to
30mph and floor the accelerator.
The crude rule is that the RPM lever stays ahead of the throttle at all
times. Once you bring back the throttle you can then move the RPM lever and
if you need to increase again it must be RPM first, then throttle. More
substantially the written rule for many years was that you never have more
Manifold Pressure than RPM and we used this in the Aztec. After take off
power was reduced to 25" and then RPM to 2500. In the cruise we used the
square rule - either 23"/2300 or 22"/2200. More MP than RPM (over square)
was always considered bad for detonation.
Nowadays there is increasing debate about the square rule but despite this
the actions should always be the same. When reducing power always work from
left to right - power, RPM, mixture - and when increasing power work right
to left.
bones
bones@xxxxxxx
-----Original Message-----
From: jhb-bounce@xxxxxxxxxxxxx [mailto:jhb-bounce@xxxxxxxxxxxxx] On Behalf
Of Peter Dodds
Sent: 27 July 2009 14:47
To: jhb@xxxxxxxxxxxxx
Cc: pdodds@xxxxxxxxxxxxx
Subject: [jhb] Re: Getting Too Real
Remember the mantra Gerry "Pitch up, throttle back"
When increasing power advance the pitch lever first
When reducing power throttle lever first.
The object of the excercise is always to avoid overspeeding the engine,
so adding power, you need to give the engine more work to do by
coarsening the pitch, before advanceing the throttle (adding energy)
Similarly to reduce power, (top of climb) you need to reduce the energy
input before giving the engine less work to do.
Some VP aircraft like the 182 I have flown on occasion you cannot
overspeed the engine, on others it is all too easy.
No doubt the Maestro will have a lot more to add about the whys and
wherefores.. :-))
Peter
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