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Author Topic: Propellors - what is the effect of changing pitch?  (Read 724 times)
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Jack Plane
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« on: December 03, 2017, 02:28:41 PM »

Trying to understand prop-pitch a little better - since someone in Nijmegen suggested the blades of a plastic prop can be carefully twisted by finger at the thickest part next to the hub to marginally alter its pitch, but I've forgotten which way was suggested (increase or decrease) and why.

So, taking the example of a Peanut going for maximum duration indoors, what would be the consequences of slightly increasing or decreasing prop-pitch?
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mike
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« Reply #1 on: December 03, 2017, 02:49:05 PM »

Warning!  You are opening a very large can of worms....

In ultra simple terms:
If you don't change the rubber motor, increasing pitch (more blade incidence) will, in general, make for a lower prop rpm and less thrust for the same turns.  If the model is climbing too fast and running out of turns 'early', this may help.

Less pitch will increase rpm and thrust - might be a solution if you're landing with 'too many' turns left in the motor.

It's possible to have too low a pitch - the prop spins round and uses all the motor torque in blade drag as the model flys too fast to give the prop any 'bite'.
Too much pitch and the prop tends to be 'stalled' because the model does not accelerate to sufficient speed to make the prop efficient.
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lincoln
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« Reply #2 on: December 04, 2017, 02:27:02 AM »

It's my understanding that props are most efficient with fairly high pitch to diameter ratios, like 1.5 or 2 to 1. However, with draggy models, it may be necessary to use less pitch to avoid stalling the blades.
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Yak 52
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« Reply #3 on: December 04, 2017, 03:51:14 AM »

Jon,

Starting at the beginning - by prop pitch we are talking about the angle that the propeller blades are mounted on the hub. Fine pitch refers to a lower angle, coarse pitch refers to a greater angle. In this case by 'more pitch' we mean coarser ie twisting the blades to a greater angle.

In full sized aviation, fine pitch is used for take offs because it is more efficient at slow airspeeds with coarse pitch being used for the higher speed cruising flight. It could be likened to gears on a car engine: low pitch is like 1st gear - good for getting going but you don't want to be in 1st down the motorway Smiley by the same token coarse pitch is efficient for cruising but can have some problems if used for taking off - like trying to pull away in 2nd or 3rd gear (possible but not efficient!)

http://www.pilotfriend.com/training/flight_training/fxd_wing/images/50.jpg

Because the blade is twisted its convenient to ignore the actual angles and define pitch in terms of how far the prop would in theory screw itself through the air in one revolution with no slip. In other words a prop with 7" pitch would move 7" forwards in one revolution.

We then get a sense of a props qualities by talking about pitch to diameter or P/D ratio. So if the above prop is 14" diameter with 7" pitch it has a P/D of 0.5 - fairly low pitch. Props for electric power tend to be around 0.5 but props for rubber power have higher P/D ratios being 'square' or 'over square' meaning P/D's of greater than 1, in other words quite a lot of pitch. Commercial plastic FF props like Peck tend to be on the lower pitch side for rubber power hence the advice to twist a bit more in.


http://avstop.com/ac/flighttrainghandbook/image9kh.jpg

The prop blades act just like small rotating wings in the sense that they need some angle of attack on the air to make 'lift' which becomes thrust. As you can see from the picture, this angle depends on the forward speed of the aircraft and the rpm of the prop.

If you increase the prop RPM you get a greater angle of attack and more thrust. But as airspeed increases the blade aoa decreases and thrust reduces. This means that for a fixed pitch prop there is a particular airspeed for best efficiency but also an 'envelope' where the prop works at all. A low pitch prop will have a limited top speed but a very high pitch prop may be very inefficient or even stalled at zero airspeed in an ROG situation and require hand launching.

In rubber power we have plenty of torque available but we want to keep the RPM as low as possible because we have a finite number of turns. The lower the RPM the more duration we get from said turns. To keep the RPM down we go for the biggest diameter and highest pitch prop we can get away with... but being careful to avoid stalled blades. It also improves rubber powered duration to reduce airspeed so a lower wing loading and slower stall speed is better.

So the advice you received probably means that you could get a bit more duration from a bit more pitch. If the pitch is too low then RPM will be higher than it needs to be and you are 'wasting' turns and duration suffers. It's a matter of balancing lowest RPM possible against the inefficiencies of too much pitch. Maybe you need to change up from 4th to 5th Smiley

Jon
Propellors - what is the effect of changing pitch?
Propellors - what is the effect of changing pitch?
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Pete Fardell
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« Reply #4 on: December 04, 2017, 04:17:45 AM »

Jon, as someone who's never had a good grasp of prop pitch and related theory I've found that that clear synopsis extremely helpful. Thanks! (I've saved it for future reference. Cheesy)
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Jack Plane
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« Reply #5 on: December 04, 2017, 06:17:07 AM »

Mike, Linc, Jon - thanks so much for these inputs, I now understand these basics with much more clarity.

So (trying to prise only one worm at a time out of the can for a good examination then putting it back before letting another one out!) a couple of elaborations:

Prop diameter
The same kind chap suggested that switching to a 6" prop and hand-launching should give greater duration.  (I didn't as the ceiling in Nijmegen wasn't particularly high and I preferred therefore to use the available 'height' by sticking with my cut-down prop and flying ROG - thus gaining the 10 bonus secs as well.)  This is consistent with Jon's explanation above, where increasing the pitch and/or the diameter would (within limits) increase the duration of a rubber powered model.  But increasing just one or the other would increase or decrease the pitch-to-diameter ratio (greater pitch increases P/D, greater diameter reduces it).  I assume that a balance is simply found in practice?

Blade chord
Again, trying to leave all other variables constant, what would be the effect of increasing or decreasing the chord of the prop-blades?  I ask this question particularly because I notice that my smaller Gasparin CO2 motors (G24, G28, G43 etc) come with wooden props of considerably narrower chord (almost akin to that of full-size aircraft), whereas the larger motors (G63, G120, G160 etc) come with plastic props of wider chord (closer to props for rubber).  What's going on here?
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Yak 52
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« Reply #6 on: December 04, 2017, 02:54:37 PM »

Cheers Pete Smiley  I also meant to say that Mikes post was spot on in terms of practical application.

If you don't change the rubber motor, increasing pitch (more blade incidence) will, in general, make for a lower prop rpm and less thrust for the same turns.  If the model is climbing too fast and running out of turns 'early', this may help.

This makes sense because the energy available in the motor is fixed. Slower RPM means a longer run of slightly less 'energy' ie thrust. Where as if you speed up the RPM you have the same total energy but get more of it at a time. Rubber power is a bit like the water behind a Hydro-electric dam. You can have a small hole and a long, low powered run. Or a big hole and plenty of flow that doesn't last so long Smiley Increasing the pitch makes the hole a bit smaller.


Prop diameter

....But increasing just one or the other would increase or decrease the pitch-to-diameter ratio (greater pitch increases P/D, greater diameter reduces it).

Yes, and conversely cutting the tips off a prop will change the P/D ratio to give effectively 'more'  pitch. So if you have 5.5" prop clearance, a cut down 6" will be better than a stock 5".


 
I assume that a balance is simply found in practice?

Yeah mostly, but I would also highly recommend Hepcat's Prop Picker Excel calculator if you like fiddling around with the numbers a bit. It takes some basic parameters like model weight and wing area and allows you to predict duration and climb thrust from different motor and prop combinations. I turned up to the Indoor Nats in 2013 with an unflown Kit Scale model and the motor prediction was spot on (5/32" pre-stripped on the basis of the prediction.)


Blade chord
Again, trying to leave all other variables constant, what would be the effect of increasing or decreasing the chord of the prop-blades?  I ask this question particularly because I notice that my smaller Gasparin CO2 motors (G24, G28, G43 etc) come with wooden props of considerably narrower chord (almost akin to that of full-size aircraft), whereas the larger motors (G63, G120, G160 etc) come with plastic props of wider chord (closer to props for rubber).  What's going on here?

Blade chord and area is a tricky one. Roughly speaking bigger blade area (and also more blades) will absorb more power, which is better for rubber power (smaller hole in the dam). You can see also see this in practice in the big multi blade props on late version Spitfires with the same prop clearance but many more horses under the cowl.

But... more blades and bigger blade areas are less efficient (blades have an aspect ratio just like wings ie thin is good) so there is an optimum. Reynolds numbers play a part too especially at lower RPM's as the bigger chord has a higher Reynolds number which helps a bit. It really is a balancing act and simply whacking a bigger blade on probably won't get you much.

Increasing blade diameter is much more effective because you get to influence so much more air. This would have been a better way to get the best out of the aforementioned Spits had it not been for the practical clearance issues.

I don't know what the RPM of CO2 motors is but I'm guessing it's higher than rubber models and I'd think the small motors have relatively less torque available? Whereas the bigger motors probably have more torque available and so can handle lower RPM? On a side note electric motors are comparatively high on revs but low on torque so the props reflect this (low pitch, smaller blades.)
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Jack Plane
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« Reply #7 on: December 05, 2017, 06:27:10 AM »

Jon, thank you again for taking the time to expand on my questions so effectively!  Smiley

Where, by the way, can John Barker's Prop Picker spreadsheet be found?
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Yak 52
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« Reply #8 on: December 05, 2017, 07:20:31 AM »

Where, by the way, can John Barker's Prop Picker spreadsheet be found?

If you'd like to PM me your email address I'll send you a copy.

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