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Hi-Performance and Sailplanes RC hotliners, electric pylon racers, F5B, F5D, sailplanes and gliders

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Old 10-02-2012, 03:47 AM   #1
Beemerider
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Default Performance difference fixed vs folding prop?

What is the performance difference between a fixed 10X6 vs folding prop of the same size?

I'm thinking in terms of current and power. Which is more efficient? I'm trying to make sure I use the right combination of prop and battery for my HobbyKing Efair sailplane. AUW-21.5oz.

My motor is an Emax CF2822 1300KV, flew it initially with a 3s 2200, 8X4 fixed prop. WOT it would fly out of my hand and go vertical. I don't really want that kind of performance. Thinking of a 2s and folding prop. Motor data info says I can maybe go up to a 10X6. I just ran it up on the bench with a 2s1800mAh and 10X6 folding prop. Showed it was pulling just under 8A and 65 watts. Thrust felt pretty substantial. Tomorrow's a rain day so can't try this out till Wednesday. Does this combo of prop and battery seem ok? I don't want a barn burner--I like getting it up to altitude and then shutting the motor off and ride the currents as long as I can.

I'm still learning about all this so please overlook my newness! But I can say this......I am sure having fun!!
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Old 10-02-2012, 08:07 AM   #2
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No idea about the performance difference between a fixed and folding prop when running. The big difference will be drag when the motor is stopped and the model is gliding, in which case the folder will be superior (less drag). Folders are also less likely to break on landing if you have no undercarriage.

As for the power set up, it could be OK but might be a little marginal. If your setup can pull some more amps the you could try a 10x8. Try it and see, that's half the fun of electrics- experimenting.

Don't forget you do have a throttle that is proportional.
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Old 04-24-2013, 03:00 AM   #3
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The folding props and "fixed" type rarely have the same blade shapes and the shape difference will cause a difference in performance.

Most folding props that I have seen have wider blades near the hub compared to the same label size "fixed" prop. The tips tend to be narrower. Its hard to predict the end result in performance difference.
There is probably a crossover RPM with one being less load below that rpm and the other being less load above it. But that's a guess.

Then there is the issue of the hub spacing. Almost all makers of folding props sell assorted hubs for use in different spinner diameters. If you do not use the EXACT hub the prop is labeled (and designed) for then the pitch and diameter numbers on the prop become essentially meaningless. More hub dia = more total prop dia and higher effective pitch. The further you are from the design hub diameter the worse things get as the pitch curve of the blades will be incorrect. Instead of being "true pitch" the full dia it will be different pitches everywhere.

Folding props are harder to balance as you have to balance the hub, then affix the blades and lock them in place at the same alignment to the hub and balance again. If the blades move then the balancing won't work. If you had to insert washers to friction-lock the blades then you have to be sure that the washers are not driving the balance off.

Folding props help enough with the glide of a powered sailplane that it is worth the effort.
But don't expect to test the load on your motor with a non-folding prop and have the same dia/pitch labeled folder give even close to the same result.

Compare the actual diameter of the assembled folding prop (fully extended) with the non-folding. I doubt they will be the same even if using the design hub...
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Old 04-27-2013, 04:32 PM   #4
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Beemerider
Folding props are less efficient and heavier than an equivalent fixed one.
I have put both on the same plane and the difference is quite noticeable but as has been pointed out the benefit of being folded when gliding far outweighs the loss in climb rate on any plane that has any sort of glide performance.
The higher the performance of the glider the bigger the benefit of a folding prop.
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Old 05-21-2013, 05:26 AM   #5
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If it is a glider, you want a folder, regardless of whether it is more or less efficient.

And you want an ESC that has a brake.

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Old 05-21-2013, 05:46 AM   #6
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Originally Posted by quorneng View Post
Beemerider
Folding props are less efficient and heavier than an equivalent fixed one.
I have put both on the same plane and the difference is quite noticeable but as has been pointed out the benefit of being folded when gliding far outweighs the loss in climb rate on any plane that has any sort of glide performance.
The higher the performance of the glider the bigger the benefit of a folding prop.
Agreed:
Having a big diameter, slow turning prop windmilling in front of your sailplane is kind of like bolting an aluminum pie plate of the same diameter to your motor when motor power is off!

As for the brake, I didn't have one on my two 10 foot Viking sailplanes of the mid 1980's. Those 15 inch diameter folding props never did fold, until an electronic brake was added to the Astroflight 40 geared brush type motor.

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Old 05-21-2013, 07:01 AM   #7
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I solved the lack of brake (way back when ESCs did not have brake) by adding a servo to push a bent pushrod against the spinner backplate.
It doesn't take much friction to stop the prop and get it to fold.

I later learned how to wire in a servo actuated switch to put a resistor between the + and - leads. This does exactly what the ESC brake does... uses the motor windings trying to act as a generator to slow the motor. This folds the prop nicely.
Good way to blow an ESC if you forget to turn the brake off though...
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