Propeller pitch

[Basil]

Can someone explain what is 'Pitch', in propeller terms, and  how is it measured.We have various props in our 'Club shop' so I would like to know what/how to look at them. I ask as its been suggested that I use a low pitch prop for a particular A/C.

Bas

[Gary Manuel]

The pitch is the angle that the blades are set at. Greater pitch equals greater angle. A higher pitch prop takes more power to turn at a given speed  than a smaller pitched one.

 

If you imagine that the prop is pulling the model through thick soup rather than air, so that it can't slip, the pitch is how far the model would move in one revolution of the prop.

Edited May 28, 2021 by Gary Manuel

[Don Fry]

Another way, pitch is like gears in a car. 
The plane that is a good candidate for a small pitch is a slow flyer. 1st gear, low ratio, low speed, lots of power. A racing aircraft, large pitch, lots of speed, won’t go uphill without badly loosing speed.

As a VERY rough rule of thumb, most ordinary models run on a pitch about half the diameter of the diameter, ie 16 by 8, 16 inch diameter, 8 inches forward by the soup analogy above. 
The interrelationship between diameter and pitch is different for electric and fuel burning power plants. You will have to stipulate which type unit you want an answer for.

Edited May 28, 2021 by Don Fry

[Mitchell Howard]

52 minutes ago, Don Fry said:

Another way, pitch is like gears in a car. 
The plane that is a good candidate for a small pitch is a slow flyer. 1st gear, low ratio, low speed, lots of power. A racing aircraft, large pitch, lots of speed, won’t go uphill without badly loosing speed.

As a VERY rough rule of thumb, most ordinary models run on a pitch about half the diameter of the diameter, ie 16 by 8, 16 inch diameter, 8 inches forward by the soup analogy above. 
The interrelationship between diameter and pitch is different for electric and fuel burning power plants. You will have to stipulate which type unit you want an answer for.

 

How does the load that a prop puts on an electric motor differ from the load the same prop puts on a glow engine?

 

Does a 12x8 on electric not require the same amount of energy to drive on a glow engine?

 

Is there an offset rule one must apply?

[Steve Houghton 1]

Funnily enough I asked this question on a Facebook group page last night as I have a large motor glider. The recommended prop for the motor was either 16 x 8 or 15 x 10. I'm not wanting speed but a better climb. The response I got was a larger prop and smaller pitch will give more thrust, the other gives less thrust but higher speed, and he too used the car gears analogy. So I've fitted the 16 x 8 which I will try tomorrow. 

[Denis Watkins]

1 hour ago, Mitchell Howard said:

 

How does the load that a prop puts on an electric motor differ from the load the same prop puts on a glow engine?

 

Does a 12x8 on electric not require the same amount of energy to drive on a glow engine?

 

Is there an offset rule one must apply?

 

If you overload a glow motor with an inch of prop, it does still run more slowly maybe, and a little quieter.

It is why the leccy boys use an amp meter.

If you over load a leccy motor by an inch of prop, you will likely have a fire

Or at least a much shorter time in the air

[Mitchell Howard]

I've had a Ripmax Quantum 40 on a trainer with a 12x8 prop. There's no minimum prop recommendation I can see for this, only Ripmax suggesting 12x9W (W?) is equivalent to a .40 2-stroke.

 

Having found the stock issues with Ripmax aren't ending anytime soon, I'm aware of the Overlander Thumper motors, and they have a motor almost identical but with a 13x10 minimum prop size.

 

@Steve - Judging by the numbers, there's going to be more thrust and speed on a 13x10 vs a 12x8. Will have to get on eCalc.

 

What will happen if I fit the smaller prop? There's still a load there on a 12x8, so it won't be running free (high current).

[Mitchell Howard]

3 minutes ago, Denis Watkins said:

 

If you overload a glow motor with an inch of prop, it does still run more slowly maybe, and a little quieter.

It is why the leccy boys use an amp meter.

If you over load a leccy motor by an inch of prop, you will likely have a fire

Or at least a much shorter time in the air

 

My watt meter arrived yesterday, much tinkering to be done!

[Bob Cotsford]

18 minutes ago, Mitchell Howard said:

What will happen if I fit the smaller prop? There's still a load there on a 12x8, so it won't be running free (high current).

There's no real lower limit with electric motors, they will just draw less current and may be be a little less efficient (ie a higher %ge of the electric power is motor losses rather than propeller power).  Motor speed is set by the supply voltage.   Glow motors have minimum prop sizes as they will overrev.

[Dickw]

22 minutes ago, Mitchell Howard said:

I've had a Ripmax Quantum 40 on a trainer with a 12x8 prop. There's no minimum prop recommendation I can see for this, only Ripmax suggesting 12x9W (W?) is equivalent to a .40 2-stroke.

 

Having found the stock issues with Ripmax aren't ending anytime soon, I'm aware of the Overlander Thumper motors, and they have a motor almost identical but with a 13x10 minimum prop size.

 

@Steve - Judging by the numbers, there's going to be more thrust and speed on a 13x10 vs a 12x8. Will have to get on eCalc.

 

What will happen if I fit the smaller prop? There's still a load there on a 12x8, so it won't be running free (high current).

If you fit a smaller prop the rpm will go up and the current will go down, but you will have less thrust/speed.

 

Dick

[Simon Chaddock]

Mitchell

To answer your question directly at the same rpm the power required for a particular prop will be exactly the same be it being driven by glow, electric or anything else.

Basically an electric motor always tries to run at the rpm determined by the applied voltage so it will draw more and more amps trying to achieve that rpm until something fails and with electrics a failure is usually permanent.

 

Note there is a difference between IC and electric props. IC props have to be much more robust to withstand the big torque reversals between compression and firing stokes. Electric torque is by comparison extremely smooth so the prop can be tailored for higher aerodynamic efficiency but would be to weak for IC.

 

 

 

 

[Don Fry]

Michael, in electric, the motor KV says how much it revs per volt. Put a small prop on it, it goes at the KV times volts. Does not do much thrust, or use much battery. Put the overlarge prop on, it goes round just as fast, and uses a lot more power. If too much, it can melt, overheat batteries.

Now I have a slightly overpowered biplane. Powered by a 75 size 2 stoke, and in electric terms has about 4 to 500 watts per pound. The airframe is slow, high drag. It cruises vertically at just under half power, at terminal velocity. Reduce power it slows, until at about quarter power it will stop, and hang on the prop. Open it up and it’s gets to terminal velocity in a couple of seconds. Leaving the throttle open is a waste of fuel. It goes no faster, drag kills the power. I come back to just under half throttle.

It has a standard power prop on it. If I dropped an inch of pitch, the acceleration would be more extreem, top speed the same. If I added a couple of inches of pitch,  I overload the motor, and it drops off the top of the power curve. And it would be more docile, but sluggish to accelerate.

Do the same with an electric motor, less pitch same revs, less power. Nicer aircraft in fairness, it can be a handful. More pitch same revs, more amps, and power. As it already has a drag dictated top speed, my vertical velocity velocity is achieved at less than half throttle and a bit more less, making throttle response very 

 course.

 

 

[Dickw]

10 minutes ago, Don Fry said:

Michael, in electric, the motor KV says how much it revs per volt. Put a small prop on it, it goes at the KV times volts. Does not do much thrust, or use much battery. Put the overlarge prop on, it goes round just as fast, and uses a lot more power. If too much, it can melt, overheat batteries.

...............................................................

 

 

Don

I understand what you are trying to convey in the interest of simplicity, but in the interests of accuracy I must point out that the rpm will drop as the load increases.

 

For the same motor and battery, the power is related to the current drawn and the current drawn is related to the rpm - the higher the rpm the lower the current. A small prop needs less power so will reach high revs at low current. A large prop needs more power, which requires more current, and the only way that can happen with our electric motors is if the revs drop.

The kV times volts figure is what the motor wants to rev at, but, how close it gets depends on how light the load is.

 

Dick

[Frank Skilbeck]

Plus as the amps go up, the voltage at the battery terminals goes down due to the internal resistance in the battery.

[Mitchell Howard]

If I'm understanding correctly (and I think I'm piecing the puzzle together now), the following statements should be true;

1. For a given LiPo, ESC, Motor, airframe, the prop at say 11x9, 12x8, 13x7, 14x6 etc would yield the same motor power output (as read by a watt meter) but would be respectively lower speed higher thrust, thru higher speed lower thrust depending on where you set your diameter-to-pitch balance.
2. To increase power draw (i.e. to make use of spare pack capacity), for a given LiPo, ESC, Motor, airframe;
   1. the prop at 11x10, 12x9, 13x8, 14x6 would use more amps than above but would only give more thrust.
      1. because the pitch is increased without reduction in diameter.
   2. the prop at 12x9, 13x8, 14x7, 15x6 would use more amps than above but would give more speed.
      1. because the diameter is increased without reduction in pitch.

 

Is there any evidence to suggest that 1" off the diameter and added to pitch yields the same power draw? Or is it less convenient than that, i.e. 1.5" etc. there must be a ratio.

 

Edited May 28, 2021 by Mitchell Howard

[Denis Watkins]

41 minutes ago, Mitchell Howard said:

is there any evidence to suggest that 1" off the diameter and added to pitch yields the same power draw? Or is it less convenient than that, i.e. 1.5" etc. there must be a ratio

 

Difficult, but you could chart your own estimates.

To be true factor, the numbers would change with rpm, temperature, pressure, humidity, height above sea level etc etc 

The number you are chasing is difficult to remain linear.

Nothing to stop your ball park 1.5

[Don Fry]

Dick W, you are correct, simplification does not tell full story. But the relationship, revs to power is not linear.
On electric, smaller prop less power, bigger prop more power.

On IC, under optimal size prop, less power, over optimal size prop, less power.

Michael, an increase in diameter of an inch loads the motor more than an increase in pitch by an inch.

And it depends on the size of motor you use. I have a working relationship for 20 cc motors, but it’s different working relationship to my 10 cc motors. 
Suck and see, use a wattmeter. Just to throw another spanner in, a telemetry current sensor is better to fine tune in the air. On the ground, a full power prop run stalls the prop, which increases load, and amps, in the air the unstalled  prop gives its real world loads.
 

Edited May 28, 2021 by Don Fry
I hate spellcheckers, idiot things.

[Fun Flyer]

Not wishing to hi-jack this, but since we're on the subject of props can someone explain the relationship between two and three blade props.  For example, if I have a 10x5 three blade, what would be the equivalent two blade?

[Don Fry]

11x5, or 10x6. 
Don’t know why, but it a ballpark rule. Either one up, pitch or diameter, three blades to two. Two to three, other way. Poetry.

Ignore comments earlier post, re pitch to diameter. There are no definitive RULES. 
Rules of thumb, suck and see if it suits.

BTW, I own a box full of props, bought over the years. As in a small suitcase.

[Denis Watkins]

12 hours ago, Fun Flyer said:

Not wishing to hi-jack this, but since we're on the subject of props can someone explain the relationship between two and three blade props.  For example, if I have a 10x5 three blade, what would be the equivalent two blade?

Just 1" smaller Fun man, is the general rule, so a 10 x 4 for 3 blade

But not without penalties as the 3 blade often turns out less efficient as the 3rd blade disrupts the 2 blades efforts and can be extra noise

But 3 and 4 blade can look very nice, and solve ground clearance problems.

 

[Engine Doctor]

Another way to look at prop pitch is that a prop with an 8 inch pitch theoretically moves / travels 8 inches of air per revolution, whereas a 4 inch pitch will move/  travel 4 inches per rev. This give more or less pull or torque effect hence the higher low gear analogy. So at 10k rpm a lot more air/ distance is travelled/ moved . You do the maths.

A drawback to higher pitch props with IC models is that when landing even at idle they can still pull enough air to make for very fast landing.

I had a Flair Leo and fitted a YS 140  but to fit a prop that would suit the engine ( not allow over revving ) and just give enough ground clearance I had to fit a 14 x14  prop.

When fitting the engine I never realised the Leo had such short undercarriage legs.The plane flew great but my first landing with that set up took 6 attempts as even with extended approach and engine at idle it was still incredibly fast. The 140 was removed and replaced with a 90 the prop was a 14 x 7 and the landings were / are much easier/ slower and manageable now.

[Martin Harris - Moderator]

Another reason for considering a multi bladed prop is to reduce tip speed.  This can make all the difference getting a model through a noise test and in the case of a fast revving engine or motor, may help keep the propeller within its safe operating limit. 
 

The inch per blade reduction is a rule of thumb which works well in sport sized models but remember that if you also change the make, there are considerable differences in performance between various makes. 

[Martin Harris - Moderator]

36 minutes ago, Engine Doctor said:

Another way to look at prop pitch is that a prop with an 8 inch pitch theoretically moves / travels 8 inches of air per revolution, whereas a 4 inch pitch will move/  travel 4 inches per rev. This give more or less pull or torque effect hence the higher low gear analogy. So at 10k rpm a lot more air/ distance is travelled/ moved . You do the maths.

A drawback to higher pitch props with IC models is that when landing even at idle they can still pull enough air to make for very fast landing.

I had a Flair Leo and fitted a YS 140  but to fit a prop that would suit the engine ( not allow over revving ) and just give enough ground clearance I had to fit a 14 x14  prop.

When fitting the engine I never realised the Leo had such short undercarriage legs.The plane flew great but my first landing with that set up took 6 attempts as even with extended approach and engine at idle it was still incredibly fast. The 140 was removed and replaced with a 90 the prop was a 14 x 7 and the landings were / are much easier/ slower and manageable now.

Reminds me of when someone brought a Shoestring racer up to the club fitted with a pumped OS engine and high pitched prop. His fixation with not allowing the idle to go as low as possible for “reliability” didn’t help when it came to landing and after several failed attempts I was asked to get it down for him. 
 

By using a longer diagonal run, I just got it to stop in the field and we started discussing the model, which he was keen to keep with the high pitched prop for some unremembered reason and I mentioned the high angle of attack “back of the drag curve” technique.  To my horror, he seized on this and asked me to demonstrate it...

 

 I took it off and started to experiment at height, finding it unexpectedly benign so started slower and slower approaches, using sufficient power to balance the drag. With the owner’s encouragement, I eventually got the approach so steep and slow that the resulting landing used barely half the runway length - although the model looked perilously nose high throughout the approach. 
 

This satisfied the owner but it was the last time he brought it to the field. Whether he flew it elsewhere and whether he attempted the same technique was never mentioned...

[Basil]

Gentlemen, such a lot to take in.I have tried to extract a number of rules('of thumb' perhaps.)Is there a figure that can be  called fine, or are the figures related to the Diam vs pitch.So 8 is fine for 16, is 8 .What about a 12"diam or 20" daim. Are  the figures 12x8, 20x8, or more likely 12x6 & 20x10. Not wanting to make things more complicated but still trying to put it together. Is this worse than motor size ratings Kv etc , I think so.What ever I have learned a great deal from your comments, and some to come no doubt.

Perhaps I should have said I am looking at slow scale(82" span)hence fine pitch.

Bas

Edited May 29, 2021 by Basil
add spec

[PatMc]

There is no relationship between pitch and diameter when taken as the only parameters.