One thing that I have experienced so far when building planes is that the motor you want, is just out of stock. So this time I want to avoid that and have selected and ordered a motor (2 of them) up front. Props, ESC and battery will have to wait, but they are much less critical to the actual design. Of course selecting a motor also means choosing the prop and battery, its just that there is some leeway in changing these components at the last possible moment (or even after the build). Not so with the motor. Dimensions and mounting holes need to be part of the design.
With that in mind, my first visit was to the Hacker website were they have a nice list for motor selection. From that list it was simple to select an engine/prop combination: select sport/scale, then weight 4500gr/2 = 2250gr yields two choices: A30-10XL (prop 14x7, 3s1p) and A30-12XL (14x7, 4s1p).
That was not hard at all! well, maybe not so fast… this engine costs €95, that is quite a lot of money…
Turnigy engines are usually much cheaper, so I used the ‘specs’ of the A30 to select a corresponding Turnigy engine. Initially my choice was the Aerodrive 3542. And I posted this choice to the RCGroups forum. I quickly learned that simply comparing sizes & power is not enough ;-)
In the thread you can see my initial ignorance in this matter. After some more searching I found two resources, one the Electric Motor Handbook and the other ecalc an online calculator for motor/prop calculations. I read the handbook and bought a subscription to the calculator website. I did need some more pointers which I got from users scirocco and Ron on RCGroups. Then things started to come together and was able to narrow down my selection.
My final try was another motor, the Turnigy 4240-740 and with a very nice 4 blade prop (scale!) that has an adjustable pitch. The results from that can be seen in the following screen shots:
(for full size: right-click -> show image in new tab or window)
What is not visible in the screenshots is that this battery/motor combination can support a wider range of propellors than the other motor’s I tried. And since the Ramoser 4-blade is pitch-adjustable that should give me plenty to experiment with. Btw: a plain-old 12x9 2 blade prop can be expected to perform even better that the 4 blade. But the 4-blade is more scale like.
The blue lines warns that the prop is operating in ‘stall’ mode at stand and low speeds. But at only 4km/h this effect is gone. And since flying is always at higher speeds, this is no concern at all. It does reduce the trust at stand-still a little, but quite likely not enough to be of concern. So why do this? that is for better throttle-response in flight. The air when leaving the prop is at higher speeds this way. This means that changes in throttle have more (air)mass to work with, resulting in a better behaviour of the model. Especially at higher speeds.
The motor(s) are ordered now, and should arrive in a few weeks (at most).