Fixed Pitch vs. Constant Speed

In an airplane with a constant speed prop, the throttle controls the manifold pressure and the prop control (usually just to the right of the throttle on the panel) controls the RPM. The prop control attaches to the engine driven governor which regulates the oil pressure sent to the prop resulting in various pitch setting of the blades. Adequate power must be present to attain a certain RPM, but this RPM stays constant, until the prop control is moved in or out.

On take-off with the prop control full forward, the RPM will reach red line on the tach as the throttle is advanced but it will not over rev even with increasing airspeed. A constant speed prop makes flying the airplane much simpler as the RPM stays where you set it. Manifold pressure of course decreases with altitude and increases when descending.

As a rule of thumb, any power increase from cruise flight to climb is accomplished by increasing the RPM first and then advancing the throttle. Power reduction back to cruise is accomplished by pulling the throttle back first and then reducing the RPM with the prop control.

Some older airplanes had controllable props that did not have a governor and pitch change was controlled by a valve directing engine oil pressure to a rubber bladder that operated a jack plate controlling the prop pitch. Any increase of power with the throttle or any increase in airspeed also increased the RPM. The workload in these planes was greater than with a constant speed but the performance increase was the same as it allowed the prop to be set to low pitch for maximum RPM at take-off and the pitch increased for an efficient setting at cruise. The early Beech Bonanzas had electric props whereas the pitch was changed with a small electric motor and they also required more attention than a prop operated with a governor.

Hope this has not been confusing.

Ron Judy

 

Tim,
Some use the analogy of a transmission. Flat pitch + high RPM = lotsa fast little bites of air (low gear). More pitch + lower RPM = less but slower and bigger bites of air (high gear). Pretty simplistic and doesn't tell the whole tale but it helps a bit in grasping the concept.

Takeoff: Max power - props full fwd, throttle full fwd
Climb: METO power (Max except takeoff) - power back a bit then props back a bit, in that order.
Cruise: Power back, props back (and synched )
One of the last items on the pre-landing checklist is props full fwd - in case of a go-round so you can advance throttles to full if necessary.

Anyone tell you the old pre-landing check acronym "GUMP?"
Gas
Undercarriage
Mixture
Props
I believe this is originally a Brit thing, hence the terminology.

Now I'd go back and reread rjnavions more eloquent and tecnical post. Hey, maybe rj will even tell you why if you have the throttles fwd and pull the props back first (or vicey versey) something might go bang.

When you advance to "Complex Airplane", i.e., retractable gear and constant speed prop, the training manual should explain it all. Same with multi. No law against you picking up one of those manuals now.

Geez, this thread is alot more fun than trying to convince a bunch of wooden-headed liberals that the FairTax is a good deal for everybody.

Cheers,
Tom

 

When you pull the throttle(s) back to the point where the RPM begins to drop (sound begins to change) with throttle movement, then you have rached the point where the props(s) are/is as flat as possible. From that point forward, there is no reason not to put the RPM control(s) forward, because you will not hear the obnoxious dissonant whine of engines suffering from an oblivious ramming forward of the props because of a GUMPS checklist, and you will be able to keep the engines perfectly in tune by adjusting the throttles once the props are flat. It's all part of the understanding of what airmanship is about, so don't worry.

 

I was thinking about this the other day: why does the manifold pressure increase when you decrease the rpm? This is most noticable to me during runup when I exercise the prop. Once the prop lever is pulled back enough to get a drop in rpm, the manifold pressure increases at the same time.
I've reasoned through it, and the best I came up with was that at a lower rpm, there is less suction coming from the cylinders due to the lower frequency of the piston strokes, thus the manifold pressure would increase. Is this accurate, or am I missing something?

 

You've got it exactly.

The Manifold pressure though is, (I think) a confusing term.
It is exactly the same thing that advances your timing in an automobile...
There, it's more fittingly, called Vacuum...( Lower than ambient pressure)

But Auto's don't have vacuum operated instruments, driven by additional pumps, and the two could become confused.

I guess it's a matter of Po-tay-toes....Po-tat-oes, To-may-toes....To-mat-oes.
It's the same no matter what it's called.

"Manifold pressure", is simply trying to return to ambient pressure, when running at a lower RPM,
as opposed to a state of higher vacuum.

Watch the MP gauge before the engine runs... it'll usually read (within design tolerances)
the same as your altimeter setting ( 2X.xx in. Hg.) if Alt. is set for actual elevation.





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