The point of constant speed props?

Champcar

Champcar

Well-Known Member
Having my first multi flight today it was also my first flight in a complex aircraft. I understand how a constant prop works but what i cant figure out is why? Is there really that great of an advantage. I didnt really see my self moving the props that much or that greatly in range. To me they seemed to be more of a pain but help and ignorant aviation out.

By the way who ever designed the panel on the 81 Turbo Seminoles what were you thinking!?!?! I can barely see the engine instruments...the yoke and my leg is in the way.
 
Yes, there really is that great of an advantage. If you have a fixed pitch prop, it is really only efficient in a narrow band of airspeed. You may have noticed that when you were flying a single engine plane, that you could not get full rated rpm in the climb, and that once you were leveled out in cruise, it was necessary to throttle back to keep from exceeding the redline on the tach.

The reason for this is that most airplanes with a fixed pitch prop use a compromise between a propeller pitch that will give best climb performance or best cruise performance. If the fixed pitch prop had a fine pitch it would increase climb performance by allowing the engine to reach full rated rpm while flying at the relatively low airspeeds that give best climb. The penalty is that once in cruise, you have to throttle back quite a bit to keep from overspeeding the engine. A climb prop could easily cost you 10 knots in cruise airspeed. If you opt for best cruise performance you will have more of a coarse pitch, and will be able to cruise at higher power settings, but the engine will not be able to climb at full power. A cruise prop may cost you several hundred fpm in climb.

To mitigate some of the undesirable effects of having either a pure climb or cruise prop, most airplanes are fitted with a compromise. Since the majority of an aircrafts time is spent in cruise flight, the manufacturer will try to fit a prop that allows as high a cruise speed as possible while giving acceptable climb performance.

Or you can spend more money and get an aircraft with a constant speed prop. This allows full rated power in the climb for best climb performance and allows the airplane to cruise at high power settings.

One other advantage that a constant speed prop has over a fixed pitch prop for multi-engine flying is that you can usually feather them. If you loose an engine and cannot feather the prop, you are not going anywhere but down. A windmilling prop creates a huge amount of drag and will murder your climb performance on one engine, in most cases you will not even be able to maintain altitude.
 
The way I thought about it when I first started flying planes with a constant speed prop is that its essentially like shifting gears in a car with a manual transmission. You change to a different prop setting (or gear) for different phases of flight to get increased performance.
 
Think of it like the gears on a car. As you get faster, it makes more sense to shift into a higher gear to optimize your power, right? Its kinda the same idea. They allow you to fly at a faster (or the same) speed but at lower RPM, which saves fuel. Think of a fixed-pitch as a car with only one gear that can go descently fast, but accelerates kinda slowly. Its all a trade-off. A contant speed prop is like a car with gears. It allows you to always run the props at the most efficient angle of attack for the given airspeed.

If you want to get real technical you can start thinking of the prop's angle of attack, which will be a combination of it rotational velocity and its forward velocity through the air.
 
you don't fly at riddle anymore? did your instructor go over what changing prop settings did?

the advantage to having variable pitch prop systems is instead of having one type of a prop (cruise or climb prop) like on a fixed pitch cessna, you can adjust the blade angle with movement of the prop levers for whatever conditions you're in.

the PHAK says "the main advantage of a constant-speed propeller is that it converts a high percentage of brake horsepower into thrust horspepower over a wide range of RPM and airspeed combinations."

say, during cruise, you set your power and props, the blade angle will change and allow the engine to run at a lower throttle and RPM for that given airspeed.
 
you could equate a constant speed prop to that of a transmission in your car. to accelerate, you want full power and a high RPM setting. In cruise, you do not need that high rpm setting. also, as you speed up, the relative wind on your propeller changes, reducing the angle of attack, reducing drag, speeding up the propeller. a constant speed prop will increase the blade angle, keeping the prop at the desired speed. this also reduces noise.
 
Well now its all clicking in the head. For some reson i was really having trouble understanding it but now i do. Thanks

I never flew riddle, just goto school there.
 
Champcar said:
Well now its all clicking in the head. For some reson i was really having trouble understanding it but now i do. Thanks

I never flew riddle, just goto school there.
Click to expand...

so you're at north aire i'm assuming using the turbo seminole and all, right? do you know dan jackson?
 
You must log in or register to reply here.
Back
Top