dasintex

My Feedback: (10)

I have read so much info on this subject which just leaves me confused as to which is which; Lets' assume that I am using the correct Diameter Prop for a given Engine; what are the differences in performances one can expect from Lower Piched Props versus Higher Pitched Props, which one gives better out of the hole performance, speed, etc; which one slows the plane down quicker, 3D performance; etc.

Thanks.

Lnewqban

If you are familiar with how wings work, you can see that all that applies to propellers.

Lets assume same engine and same rpms:
A low pitch prop is equivalent to a small wing flying at a constant speed and a fixed low AOA.
A high pitch prop is the same thing but flying with a fixed high AOA.

As both rotate at the same, both have the same velocity of air flowing over and under them.

The forward speed of the airplane makes a huge difference in the AOA that each "sees".
As that speed increases, that AOA gets smaller and the prop's blades "lift" less, demanding less torque from the engine.




The basic problem with a high pitch prop, is that for zero or low forward speed of the airplane, the AOA may be excessive (above the critical AOA) and the blade is stalling, just like a regular wing does when we force the elevator up too much at low speeds.
When the blades are rotating in a stalled condition, they lose much of their "lift" and the propeller thrust is very small.

http://en.wikipedia.org/wiki/Stall_%28flight%29

If that little thrust or a catapult or a hand toss increases the forward speed of the plane, then the AOA that the blades "see" gets reduced and the propeller "bites" the air and pulls harder than the equivalent low pitch prop, forcing the airplane to move faster.

The additional "lift" and speed supplied by the high pitch prop (when is not stalled) costs additional torque (HP) that the engine has to provide.

For slowing down the airplane, the same principles apply, only that the AOA that the blades "see" becomes negative sooner for the low pitch prop, which is better for that task.

Propellers behave a little different than wings due to the speed to size ratio, which sometimes makes the blades "flight" close to supersonic speeds for high rpms'.

That is a simplified summary, ............ or so I believe.

http://www.youtube.com/watch?v=6UlsArvbTeo

Top_Gunn

My Feedback: (6)

Non-technically, you can think of lower pitch as being like lower gear on a car. Better for slow speeds and climbs, not so fast at high speeds.

dasintex

My Feedback: (10)

Yes, I get the lower gear part, which makes sense; would it also be true that a higher pitch prop, until the plane gets up to speed, would cavitate or just churn the air up without actually pulling until air speed cathes up? but which prop would slow the plane down when the throttle is cut?

Lnewqban

Cavitation only applies to liquids:

http://en.wikipedia.org/wiki/Cavitation

Please see edited post above for braking effect.

jetmech05

Props don't bite, pull, or push air it's all lift

Top_Gunn

My Feedback: (6)

Well, lift is partly pushing. But this isn't the aerodynamics forum (where there was an extensive though pointless discussion of whether wings get pushed up or sucked up).

A lower pitch prop will give you lower speeds than a higher pitch prop when you cut the throttle. A high pitch prop will move the plane forward even at low speeds, but the engine will have to work harder to get it going, so a lower pitch prop will give you better acceleration when starting out.

I don't do any kind of flying that requires a lot of speed, so the only time I use high pitch props is when I can't use a lower pitch one because it would have to be too high in diameter for the plane and engine (ground clearance, mostly). Or when the LHS is sold out of low pitch props.

invertmast

My Feedback: (23)

Low pitch props give better "out of the hole" and climb performance but at a sacrifice of lower top speeds. High pitch props are the opposite. Some High pitch props will also stall at lower speeds, depending on the prop pitch and the rpm it spins.

Now, since a lower pitch prop puts less load on the engine/motor than a high pitch propeller of the same diameter, the engine has to work less to get to RPM, since it is now possible for the rpm to go above redline (more a full scale issue than model) you need to increase the low pitch prop's diameter to increase the load on the engine to bring the rpms down. This larger diameter prop now has more surface area to help slow down the model.

then there are over square props, which mean the pitch is greater than the diameter (7x10 for example). These props have a smaller diameter so as to allow greater prop RPM without making the tips go above roughly .8mach (that really loud scream you hear some airplanes make, is actually the prop tips near or over the speed of sound, this is VERY inefficient and LOUD, not to mention annoying IMO). So since the propeller Can spin higher RPMs due to thr reduced diameter, you must increase thebladr pitch in order to get the RPM back to an acceptable range. These props provide high top speeds, but due to there high pitch angles, they suffer from a lack of out of the hole performance

landeck

My Feedback: (1)

Invertmast, very well said.

Bruce

da Rock

It would be nice if we could consider pitch as if there were no other details, but we can't.

With our models, just as with full scale airplanes, there are usually a few fixed or almost fixed parameters that drive our choices and we often don't have much choice.

We don't usually have much ability to alter the power available. When we do, we most often choose the most power. We almost never have the ability to increase our ground clearance. When we do, we seldom consider doing so. So when it comes to our propeller choices, we seldom have much choice of diameter.

When you're stuck with a set diameter, and we're working with a fixed pitch, the power of the motor often affects how our choice of pitch far more than happens in full scale. Full scale can adjust pitch with the twist of a knob or pull of a lever. The pitch can be adjusted as needed throughout the flight envelope.

What happens when the motor is stuck with one pitch depends on the power band of that motor. It's fun to consider pitch as one simple decision, but with models it's not.

If your prop's diameter and pitch don't fit into the power band, your model suffers somewhere in the envelope. If you're stuck with one diameter, your choices of pitch are narrowed considerably. Even choosing to cut your diameter to match higher pitch fixed props isn't really much of a choice. That "speedier" prop might not fit the power of the motor, or suit it as well, and you're almost certainly getting less efficiency out of the lessened diameter. Think that throwing more power at the problem solves it? Overpowered models often lose throttle response and give you a throttle that works like a switch. Lots of overpowered models out there give you fits landing. When you've got a plane you're flying around at half throttle, the stick is twice as sensitive in that lower half where you're trying to control your "normal" airspeeds. A lot of overpower motors have no problem flying too fast at idle and demand you put a lower pitch prop on just to get the idle speed down so you can land the sucker. That pitch then lessens the stick sensitivity even more. The model winds up with even less throttle movement in the normal airspeeds. Lots of people fly those things. Fun or not, they prove that pitch is much more than a one trick pony.

Nothing in aerodynamics is sound byte simple.

Lnewqban

This is a very accurate post !

I would add one more variable to the game: the Cd of the whole airplane.

A high overall coefficient of drag means that the higher pitch propeller may be unable to overcome the huge growing of the antagonist force of drag (that opposes the thrust force) as speed of the model tries to increase.

The high pitch propeller will pull all it can, but once the model reaches that critical speed, all the poor prop can do is "spinning the wheels in the mud", eating much HP with little result (efficiency goes down the tubes) and possibly going to a stalled condition.

A less pitched prop could better match the draggy airframe and work efficiently at that critical speed.

Aerodynamically clean airframes can better use the speed advantage of high pitch props; however, they may struggle taking off from grass and slowing down for landing.

Hence, when discussing ideal propeller for an specific model airplane and engine (or motor), trial en error may be a better approach than simple and hard rules about performance.

Any engineering design is a series of compromises. Any successful airplane is a series of compromises flying in formation.