How to pick an electric motor for a model- Rob's DH 89.

[Frank v B]

Rob asked me what kind of motors should power his Dumas DeHaviland DH 89 model.
Questions:  I had 3 questions that affected the selection:
1) what is the estimated final weight of the plane?  - 16-18 oz
2) what is the max width of the combination cowl/engine nacelle?  That determined the max diameter of the bell on the motor.  He e-mailed me photos of a ruler on top of the plan.
3) What is the scale diameter of the props?  7"

Considerations in determining the power requirement:
- The full power system needed achieve about 100 watts (watts= amps X volts)
Guidelines- a power glider needs about 60 watts per pound, a Cub needs about 80 watts per pound, a P-51 needs about 100+ watts per pound.  This is a biplane and is very large volume (draggy) so that's why I aimed for 100 watts per pound.  There is nothing tougher than flying a scale model with limited power.  Proof is my Guillow's B-25 and DC-3

- The safest way to spin a 7" prop is by using a 2 cell motor. The 7" prop is on the large size for small motors. A third cell would shoot up the amperage exponentially.  The only way to control that is by going to a smaller prop which would look silly.

- a twin engine plane produces more thrust than an equal wattage single engine.  I suspect two 40 watt motors would equal the thrust of a single 100 watt motor. A single motor on 2 cells would have to draw about 5.4 amps (calculated 40 watts divided by 7.4 volts = 5.4 amps)

Dug into my collection of 100-ish motors and dug out two E-Flite 250's.  These are limited to 2 cell, swing a 7" prop and produce about 50 watts each.  They fit the diameter of the nacelle.

That was my recommendation and he bought it....literally!

If the plane flies well, it will be Rob's victory.  If it does not....it will be my fault.  Comes with the territory.

Frank

Photo 1- Rob's first motor would not fit the nacelle.  It is a 120 watt motor and would have ripped it apart.
Photo 2- the nacelle clearance.

[msatin]

Peter P, directed me to a very comprehensive website called eCalc.
You provide all the info (weight, wingspan, wing loading, etc), and it provides a recommendation for both motor & prop.
It's very inexpensive to use
https://www.ecalc.ch/calcmember/signup.php
Looking good Rob!

[Frank v B]

Mark,

I have heard about it.
Mine is a little bit like Grandma's recipe that uses a dash of this and a pinch of that.
E-calc is more like science class with ingredient measurements to 3 decimal places.
Prefer to live dangerously.  That way the victory is so much sweeter.
Thanks for posting it.

Frank

ps: small models need more feel to the recommendation.  Planes larger than 10 size are much easier to predict.