Presently a battery Nickel/Cadmium (Ni/Cd) common to both motors : 2400 mAh, 7 elements (8.4 V). Autonomy is relatively low (4 to 5 minutes), and it might be largely preferable to use 3000 mAh Nickel Metal Hydride (NiMh) elements. Though it is needed that your e charger be able to fill this kind of batteries.
Propulsion motor runs a propeller relatively large for the given power. It is so needed to have a low rotating speed with a low pitch.
Current with 8.4 V is around 15 to 20A with ferrite motor, so a power on shaft of around 18 * 8 * 0.75 = 108 W
Propeller is pushing. It is very difficult to find left turning propellers, especially with a low pitch. So there is two solutions :
. Use a reducer with a gear which reverse rotation direction, typically a boat reducer.
. To turn motor in reverse side. An electric motor does have some sort of 'ignition advance', so it is needed to be able to adjust this advance. Because if it turns reverse direction with it's original adjustment, there is efficiency reduction and considerable sparkling. Only good quality motors allow this sort of adjustment. Standard motors like Graupner ‘speed’ normally does not have any 'ignition advance' and are able to run reversed.
This is solution i've used, because i've not found appropriate left turning propeller.
Propulsion motorisation i'm using is a good quality ferrite motor size 500, but i have absolutely no technical data about it. It's equipped with a epicycloidal gear reducer 3.8 :1. When loaded, estimated speed down the reducer is about 6500 rpm for 20A, so about 24700 rpm on motor. It is so a very fast motor, with an unloaded esitmated speed of around 30 000 rpm.
Whatever it's load (tested under 20 and 40 A), this motor have very low heating. We can then suppose that' it's efficiency is one of the best among ferrite motors, so around 80%, reducer included.
Propeller right turning, 12" x 4.5"
Please note that while testing on lifting fan, current is same as speed gear race 500 (with a gear ratio of 2.8:1)
This is with that information i've based my estimations, which are not guaranteed...
It is possible to use simple gear reducer (reversing), ratio 4:1 to 5:1, with a speed 500 race or equivalent. In that case the propeller pitch must be higher, around 8".
Thi propeller needs the widest possible blades
Lifting motor is a Graupner speed gear 500 race 2.8 :1, 7.2V .
Reducer maintain rotation direction.
Unloaded speed is 8000 rpm.
At best efficiency speed 7000 trs/min with current 14 A (voltage ?)
It is used with a maximum current of 8A, so an estimated speed of 7500 rpm down reducer.
Please note that when we look these numbers, a speed 500E with direct drive may be used, with 9 to 10 elements, but with what efficiency ?
Lifting fan is issued from ‘computer type’ fan size 120 x 120 mm, 12V
Fan used: Papst Multifan 4312M (120 mm x 120 mm)
Dismount the fan and drill plastic riveting, fan came easily.
As it is not possible to change the fan, it is imperative to have exact gear reduction.
Max power estimated is :
8 V * 8A * 0.73 = 47 W at 7500 rpm.
For both motors, smaller motors may be used, allowing a weight gain, but we have to take care of global efficiency. As used, both motors may be near from their best efficiency.
Speed 500 race have it's best efficiency (76% including reducer) at 14 A.
Speed gear 500 race may be used for propulsion, but with reversed rotation.
To improve autonomy, which is a bit low, we may use 8 elements packs, but it may be difficult to find 8.4V motors instead of 7.2V
It is possible to use slow turning brushless motors (with external rotors) but price will increase significantly.
Another possibility is to use battery packs 3000 mAh NiMH, if your charger does allow it. Current being relatively low (< 25 A mean), does allow NiMH to work without any troubles.