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To brake or let prop windmill?

esc settings for electric power

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Martin Fraser13/03/2018 18:50:47
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18 forum posts
13 photos

Hi folks, What is the perceived wisdom for programming the esc on electric power? I'm tweaking some settings on a recent purchase of TT Yak 54 and wondering if a wind milling prop creates more drag than a braked prop.

T.I.A. Marty

Don Fry13/03/2018 18:54:01
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2399 forum posts
30 photos

Windmilling has more drag. Complicated fluid dynamics, behind me, but believe.

Don Fry13/03/2018 18:54:02
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2399 forum posts
30 photos

Windmilling has more drag. Complicated fluid dynamics, believe me, but believe.

God I hate predictive spellcheckers

Edited By Don Fry on 13/03/2018 18:54:57

Simon Chaddock13/03/2018 19:10:10
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5141 forum posts
2691 photos

A windmilling prop creates much more drag than a stationary one.

Whether you really notice the difference depends on the diameter and pitch of the prop and the aerodynamic performance of the plane.

For the average trainer the extra prop drag from windmilling is not likely to make very much difference but high performance gliders not only stop their props but fold them as well.

Tim Hooper14/03/2018 08:01:50
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2821 forum posts
2346 photos

The brake function only works for a second or two, and is designed solely for use on a folding prop. When the throttle back, the prop is braked to a halt and the blades can fold back because of the airflow.

The brake function makes little or no difference to a model fitted with a fixed prop.

Tim

Frank Skilbeck14/03/2018 08:16:03
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3995 forum posts
96 photos

Tim, that's not what I have found, I have an ASK11 motor glider with a non-folding prop (due to the cowl shape a folding prop cannot fold properly) and the ESC brake does stop the prop. It still turns over but only very slowly, if the brake is off it windmills and provides more drag.

Back to original question, on an aerobatic prop a windmilling prop provides useful drag, e.g. on the downward section of a loop etc and also for slowing the model on landing.

Ron Gray14/03/2018 08:31:12
851 forum posts
213 photos

Even on a foam Wot 4 it makes a difference, a windmilling prop certainly slows it down more than a stationary one.

Steve J14/03/2018 09:53:52
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647 forum posts
27 photos

I always have the brake on, mainly so that I can hear any failsafes.

Steve

Simon Chaddock14/03/2018 10:22:14
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5141 forum posts
2691 photos

As far as I am aware the ESC "brake" function simply shorts out the motor wires.

This causes a rotating motor to slow down pretty quickly - the same is used on many battery drills.

The fact that the motor wires remain "shorted" means even at very low revs the motor will be "stiff" to turn.

As shown here

**LINK**

It will remain in this condition as long as the ESC is powered.

Cuban814/03/2018 11:25:05
1950 forum posts
3 photos

Good way to strip the gears on a motor/gearbox prop drive.

Dickw14/03/2018 12:44:08
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321 forum posts
41 photos
Posted by Cuban8 on 14/03/2018 11:25:05:

Good way to strip the gears on a motor/gearbox prop drive.

Depends on the gearbox.

A good quality box with steel gears can quite happily stop an 18" prop from 8,000 rpm in fractions of a second without any problem. On the other hand I have stripped plastic gears.

Dick

Martin Fraser14/03/2018 18:37:08
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18 forum posts
13 photos

Ok folks, I'll keep it as is i.e. no brake - every little helps to slow down the approach speed!

Many thanks for the input, Marty

Peter Jenkins14/03/2018 18:58:37
988 forum posts
32 photos
Posted by Martin Fraser on 14/03/2018 18:37:08:

Ok folks, I'll keep it as is i.e. no brake - every little helps to slow down the approach speed!

Many thanks for the input, Marty

Hi Marty, at the risk of stating the obvious, the elevator controls your speed on approach. So, if you want to slow down, and you don't have obstructions on approach, then a flatter approach using power to maintain the required rate of descent, is the best way of proceeding.

Re braking in aerobatics when flying with electric motors, ESCs like the Jeti Spin 99, allow you to vary the amount and ramp up of prop braking so that you can adjust how much drag the braked prop provides and how quickly it ramps up. Remember, these are 21-22 inch props, so that's quite a lot of drag. This type of brake does not stop the prop dead as then there is virtually no braking. 3 blade props also increase drag over a 2 blade prop.

Martin Fraser15/03/2018 09:10:44
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18 forum posts
13 photos

Hi Peter, The strip at one of my clubs is quite short and is surrounded on all sides by tall hedges and trees, some of them pretty mature with an unnatural appetite for flying machines laugh. Landing on a calm day can be a challenge!

Cheers, Marty

Charles Smitheman15/03/2018 09:59:46
198 forum posts
17 photos

Hi Marty,

I have a Graupner Dandy (ancient simple glider with no airbrakes) with an electric motor and folder prop for launch. Often if it is too high on landing approach I OPEN the throttle slightly, this gets the prop spinning again, and the model can be seen to slow down and drop noticeably. It is then just a matter of shutting the throttle before touchdown to avoid prop damage. So yes, prop windmilling for steeper slower glide.

I often find it more fun flying a simple old model than the latest fancy gadget!

Charles

Erfolg15/03/2018 13:46:23
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10849 forum posts
1033 photos

Perhaps rather than worry about which condition generates the most drag, particularly on a non-glider type model, it is worth considering how a slow rotating prop can be useful. After all it is seldom that a power type plane has the motor switched of, other than when landing.

I expect that many have found, that just having the prop slowly rotating whilst under power, has the effect of acting like a low powered brake. In that you can slow the model down, if approaching the landing a little faster than you would prefer.

MattyB15/03/2018 16:20:13
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1763 forum posts
27 photos

I think this conversation is becoming a little more complex than it needs to be tbh. You can choose how to set the ESC purely based on the type of model and prop:

  • Gliders with big, fine pitched folding props and no undercarriage - Brake on (minimises drag in the glide, keeps the prop out of way on landing).
  • Any other fixed wing powered plane with a fixed prop - Brake off (creates useful drag at low revs that you can use on approach; won't stop the motor with the prop pointing downwards where it is most vulnerable on landing)
ASH.15/03/2018 17:46:57
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192 forum posts

To answer the OP's question: YES. A windmilling prop creates a lot needed drag to land. I fly ic mainly and it's very noticable. If you can programme an electric to 'idle' at minium revs then that is a big plus.

Good Luck

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