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RPM Conundrum

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SIMON CRAGG09/06/2019 20:12:53
576 forum posts
5 photos

Our club hold regular electric glider "one design" events with the HK Phoenix 2000. Everything stock, and all using a zippy 3s 1300 lipo. The events are always well attended due to the "level playing field" of all having exactly the same set up.

However, we tried recording the max rpm of each model today.

The range was from 7800 to 8200 rpm.

Although 400 rpm is not a lot, it makes a difference on a 30 second climb out.

Is the difference just due to some motors being better than others?

We used the same tacho throughout.

All batteries fully charged and balanced.

Is there some way of increasing the rpm, so that we are all at 8200?

Great competition either way!

Trevor09/06/2019 20:16:50
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454 forum posts
57 photos

Could be some variation between the motors but more likely differences in internal resistance of the batteries.

Richard Harris09/06/2019 20:20:20
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2174 forum posts
2023 photos

Simon,

You could flash the ESC's so they are rpm limited. They do this in E2K pylon racing so that everything is even making it all down to pilot skill.

Rich

Chris Walby09/06/2019 21:53:46
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1280 forum posts
315 photos

My vote is with Trevor,

Example, I use standard, nanotech, and Graphene lipo and the battery voltage at full throttle drops less wit the Graphene hence the motor turns faster and absorbs more power.

As Scotty would say "You cannot change the laws of physics" and at least some of the additional consumed power should be turned into more speed.

On a different tack, if you wanted to bend the rules then reducing the models drag would probably produce a greater climb rate... especially if the model lost some weight...

The new lightweight gilder depending on the event might not work as well as its heavier brother...swings and roundabouts!

PatMc09/06/2019 21:57:30
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4436 forum posts
538 photos

IMO the RPM variation noted in the OP is remarkably low.

I think you'll find that 5% variation in the motor's quoted kv figure is reasonable across most, if not all, cheap brushless motors.
Add in the variations to be expected in the batteries, even when brand new. Then take into account age, usage & differing charge regimes individual batteries will have had, again 5% performance would seem reasonable.
Even the props will have some variation during manufacture &/or storage.

I don't know whether or not Richard's suggestion would be possible with the ESC's supplied in the kit but another method would be altitude limiters set to cut motors at a specified altitude or time whichever occurred first.
Of course the extra cost would have to be considered as that could kill the popularity of a low key fun comp.
HK used to sell a suitable device but don't seem to have stocked it for some time now. 

Edited By PatMc on 09/06/2019 22:01:05

SIMON CRAGG10/06/2019 02:58:05
576 forum posts
5 photos

Thanks for the comments.

I think we will have to live with the variation, and just concentrate on the set up / trimming and flying as smoothly as possible.

I have tried adjusting the motor timing (no difference), balancing the batteries (same) and virtually everything else I can think off, without success!.

Height limiters are a good idea, which we use in other events, but the P2K comp is to be kept as simple and cheap as possible, to encourage as many entrants as possible without additional cost,

Gordon Tarling10/06/2019 08:50:42
237 forum posts
4 photos

Most likely just down to variations in rpm/V and batterry age/internal resistance, as already noted. If it's within the rules, warm the battery up to around 40C in a battery 'oven' just before flying - it'll make a difference. Don't go over 45C though.

Gordon Tarling10/06/2019 08:50:43
237 forum posts
4 photos

Sorry - dup post.

Edited By Gordon Tarling on 10/06/2019 08:51:07

PeterF10/06/2019 08:53:23
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545 forum posts
739 photos

I had an IC twin once that ran 10x6 APC props, I tested 7 props one after the other on one engine and found something like a +/-200 rpm difference at 10,600 rpm, so even props can have a little variability.

BackinBlack10/06/2019 12:45:57
128 forum posts
2 photos

+/- 2.5%, not bad when you consider all the manufacturing, calibration and process variables.

Propeller

Motor

ESC

Wiring

Connectors

Batteries

Rx conversion/processing

Tx calibration/processing

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