Am I in limits with this set up?

[david 7]

Hi all and happy new year, can any one advise me if i am in limits with this set up, the model is a dave boddington mentor trainer all up weight is 4.2 lbs or about 1900 grams, motor is emax 2815/09 1100kv 25-35 amp-75% , 46 amp 60 seconds,3900 4s lipo 25c, 60amp esc, 9x6 prop.

 

I have just run the set up on watt meter, coming out at 509 watts and about 35/36 amps at full throttle, the motor only gets warm.

 

My concern is iam very close to the motors max amps but will not be flying at full throttle only for take off, should i prop it down a bit and will 500watts be to much for a 35year old model to handle.

 

Any advise would be welcomed

 

thanks Dave

 

 

[Phil 9]

sounds fine as it is to me.

 

 

[Ben B]

Holy moly! 119 watts/lb. That will have some vertical performance despite an intrinsically draggy airframe.

 

Your propeller RPM will be 16500rpm on a fresh battery. With a 6" prop that will give you a pitch speed of 93.75 mph. Which seems like quite a lot! I expect a Mentor would have a Vmax well below that so you'll be wasting lots of energy trying to make an instrinsically not very fast plane go fast (which it won't).

 

The mentor originally was designed to be flown by .25-.40 engines. Something old like a OS40FP would do 16,000rpm and generally a 5" or 6" prop would be used. You probably wouldn't get 16,000 on a 6" prop though.

 

I'd be tempted to go for a 9*5 (personally I'd use a 9*4.5 electric APC). Slightly more reasonable pitch speed of 70mph.

 

[Bill_B]

I'd be tempted to go 3s with an APCe 10'' x 7'' and save some battery weight. With this prop you'll be drawing about 33A and have roughly 360W (1/2 HP) with the added advantage of a lighter AUW. The PWR will be about 90W/Lb which is more in keeping with the Mentor's original performance requirements.

A big prop will move a larger column of air and is generally more efficient than a small prop, even if turning at lower RPM.

[david 7]

Hi all maiden the mentor today conditions was not good with some gust of wind, the model flew well for 35 years old with a little bit of trimming but will have to sort out the

differential on the alierons, the power set up was ok but not very linear on the throttle control, was going to try a smaller prop to see if it helps, can this be adjusted with the speed controller settings, the model did not feel it had over 500 watts of power but that could be down to the gusty wind, but overal happy with it.

any advise on how to sort the throttle / power problem ie all or nothing would be welcomed.

thanks Dave.

[Tim Mackey]

The ESC response curve will be directly proportional to the output curve of the Tx - which is usually linear. Its possible on many Txs to alter this curve - I have done so on an indoor shockie which was a bit sluggish in 10 - 50% position. However, this wont affect the overall power available - which is something you complain of. I suspect your issue lies elsewhere.

Have you "trained" the ESC the throttle throws for your particualr Tx?

Personally, I would be using a larger diameter prop - and if that puts your currrent up too high, then you either have the wrong wind motor, or you will need to drop a cell or two on the battery. One of the prime benefits of leccy power is the ability to swing large props than the IC "equivalent" and these days, I think of a 9" diameter prop as very small.

Edited By Tim Mackey on 07/01/2012 17:53:04

[david 7]

Hi Tim, its me again just had a look at the motor and its rated to run on 3s lipos 11.1 volts, cant realy change the lipos as i have spent £120.00 on these, thats the reason why im running a smaller prop to keep it in its limits and i think thats why its not very linear on the throttle control, do you think i should change the motor to a bigger spec and run it on a bigger prop, or i could just live with it for now but it does have shed loads of power.

Dave.

[Tim Mackey]

Youre OP said you were running 4s.

[david 7]

Yes thats correct 4s 3900 25c on this motor, i just proped it down to be in the motors limits, not a very good way to go about it i was a bit unknowledgeable at the time when i

brought the motor, would the best advise be to replace the motor more suited to 4s lipos, or just live with it for now i need about 450watts for this model.

Thanks Dave

 

[Kntt]

David

Looking at the prop you are running it seems way to small

I would have tried a 12 x 6. I think you are under proped and it is maxing out at a low throttle setting giving you the impression of a none linear throttle which would be correct.

 

 

[Tim Mackey]

Well if you want 450 watts thats perfectly do-able on 3s , as it means around 42A.

However, if you wish to retain the 4s, then the same power will only need about 30A.

As you've already found out, because of the wind of your motor a small 9 x 6 prop is already pulling over 35A and producing 500+ Watts.

A bigger prop, ( which has been advised before ) will pull even more power and simply put...

There is no way out of this! - as you dont want to reduce the voltage.

The motor is rated to 46A though, so I would prop up a bit and use the wattmeter to stay within the 46A maximum at WOT.

These figures will be lower in the air, and for short bursts of full throttle should be fine.

I'd go up in diameter rather than pitch.

[Allan Bennett]

You might even want to reduce the prop pitch by 1" to allow you to go to larger diameter without exceeding the motor's 46A rating. Reducing pitch (while keeping amps the same by increasing pitch) is a bit like selecting a lower gear in your car -- it might give you a bit more throttle response.