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Variable Pitch Propellers

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Hello, I am new to these forum, but I have been reading it for a few weeks, my question to all fo you is Why no one uses a VAriable Pitch Propeller in Canards?

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Hello, I am new to these forum, but I have been reading it for a few weeks, my question to all fo you is Why no one uses a VAriable Pitch Propeller in Canards?

There are numerous canards flying with variable and constant speed propellers, both on aircraft and auto conversion engines.

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thanks Mark I have not seen any discussion about them and was just asking. The other day I was reading about the landing approach in which you have to redice RPM and that there was always a risk of carb ice or engine stalling, and another question is :

CAn you keep your RPM to a save level and work with the propellor pitch to land and avoid the risk of engine stalling?

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There are three main factors to consider in the "fixed" vs "constant speed" debate;

 

1) I would say the main reason is cost;

 

a top quality fixed prop will cost about $2,000, an MT variable pitch/constant speed will cost about $10,000.

 

2) Performance. If you are interested in only one area of the flight envelope, i.e. speed, and are willing to sacrifice performance in all other corners of the envelope, then a fixed pitch will normally outperform a constant speed.

 

On the other hand, a constant speed offers very good performance in all corners of the envelope.

 

3) Weight,

 

A constant speed prop and its pumps, motors, control system, etc, will be much heavier than a fixed pitch. A good quality fixed prop will weigh approx 20 lbs. My MT electric weights in at 45 lbs.

 

 

 

I've been flying with an MT prop on my LongEZ for over 1600 hours, I like the takeoff and landing performance, and I love the ability to cruise at 165KIAS while the prop is turning 2300 rpm. I used to run 175KIAS with a Great American turning 2900. I sacrificed a little top end speed (about 5 kts) for this performance.

 

Lower rpms means less fuel burn, less engine wear, and considerably less noise.

 

Waiter


F16 performance on a Piper Cub budget

LongEZ, 160hp, MT CS Prop, Downdraft cooling, Full retract

visit: www.iflyez.com

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Thanks Waiter, I am glad to know it can be done, it is my believe that a variable pitch propeller is much safer in take off and landings, which is a great concern to me when flying family members. The cost is a good factor but safety is number one.

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Thanks Waiter, I am glad to know it can be done, it is my believe that a variable pitch propeller is much safer in take off and landings.....

Do you have any data to back up your belief? Any info suggesting that fixed pitch props have a higher accident rate than CS prop aircraft of the same type?

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Not really prop safety, but carb ice factor when lowering RPM, both props do their job accordingly, but carb ice can occur at any time given the conditions, not only in cold weather( vaporization of fluids).

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Not really prop safety, but carb ice factor when lowering RPM, both props do their job accordingly, but carb ice can occur at any time given the conditions, not only in cold weather( vaporization of fluids).

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I'm not sure "safer" would be the correct term. A constant speed prop can provide a much wider range of runway conditions that would be acceptable for takeoffs and landings; i.e. shorter runways, greater crosswind angles and speeds, heavier gross weights, etc. However, If you operate your aircraft within its design envelope, then there really is no "safer" prop.

 

Actually, from a strictly MTBF analysis, would say that a CS is less safe, as it has more parts that could fail during a critical takeoff or landing.

 

I'm not sure if a CS prop vs a fixed prop would have any impact on carb ice or engine stalling.

 

Carb ice is a factor of OAT, Relative humidity, and throttle position (manifold pressure) I don't see how a CS prop would reduce these factors.

 

As for engine stalling, the CS prop would probably delay this, but I don't think this is a significant factor. If the idle speed is set to low, the engine will stall regardless of fixed of constant speed. Fix the problem by adjusting idle speed.

 

Waiter


F16 performance on a Piper Cub budget

LongEZ, 160hp, MT CS Prop, Downdraft cooling, Full retract

visit: www.iflyez.com

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What you describe is generally how TurboProps operate. Basically, you change the prop pitch (power), and the fuel controller changes the fuel flow (throttle) in an attempt to maintain a certain engine rpm.

 

This type of control scheme would certainly work with piston engines, but I don't think I ever heard of this type of control scheme implemented in a piston engine.

 

As far as Piston engines;

 

To make sure we are not confusing terms;

 

CS - Constant speed - The propeller controller automatically changes prop pitch in an attempt to maintain a preset "RPM", Regardless of throttle settings, climb, dive, altitude, or airspeed. This is the mode that is found on the vast majority of aircraft. (This is like an Automatic transmission in a car)

 

Variable Pitch - The operator can alter the propeller pitch at any time, This is strictly a manual operation, Any change to throttle settings, climb, descent, or speed, will alter the corresponding engine RPM. The operator Must ensure the engine rpm stays withing its operational and design limits. (This is like a manual transmission in a car; yes Bunky, you can put it in first gear while going down the road at 80mph)

 

Although my MT can do either (variable pitch or constant speed) I almost always leave it in the CS mode.

 

Normally, I select 2700 rpm for takeoffs and landings. I select 2500 rpm for climb, 2300 for cruise, and occasionally, 1900 rpm for economy cruise. The throttle can be moved freely and the rpm stays the same.

 

The only time that use the Variable Pitch mode is when I am on a Long cross country. I generally climb to my cruising altitude, i.e. 17,500 ft. When the speed is stabilized, I select Variable pitch. I do this so that any minor changes in speed or inadvertent climb or descent, I immediately fell/hear the propellor speed changing.

 

 

 

Waiter


F16 performance on a Piper Cub budget

LongEZ, 160hp, MT CS Prop, Downdraft cooling, Full retract

visit: www.iflyez.com

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