Airframe weight vs Power system weight

[raabid]

Every time I make a little progress in this sport, I learn something else and take 4 steps back. So now I'm working on my first scratch built plane and I'm wondering if there is a ratio of the weight of the plane to the motor, gearbox and batteries? I had heard that the weight should be half and half but that didn't work right in my experimentations so far. Couple of 10 feet flights. LOL Oh well, part of the fun. I know Ecalc would tell me what I need but it won't tell me why, if that makes sense as to what I'm trying to find out.

The plane I'm making is a semi profile 3d plane with a 12in by 30in constant chord w/ symmterical airfoil.

[Ollie]

For 3D flying, the static thrust should equal or exceed the overall weight. This is so that the plane will have acceptable vertical performance. In vertical flight the thrust is equal and opposite to weight and drag. The wing produces no lift in this flight attitude. The plane will accelerate vertically if thrust exceeds weight untill weight and drag equal thrust.

Measure the static thrust. Subtract the weight of the motor, batteries, ESC, prop and the rest of the radio gear. What ever is left is the maximum allowable weight of the airframe. If there isn't enough left for the airframe, it means that you need to get a motor with a better power to weight ratio, a lighter battery, lighter receiver and servos or increase the thrust for the same weight with a more efficient prop or more powerful motor. All this improvement in the power system means much higher cost. If the cost is unacceptable then you need to consider a smaller airframe. You have picked one of the most difficult and expensive design problems by choosing a 3D capability using electric power.

[raabid]

Ok, there has to be a little more to it than that, otherwise I couls take a huge brushless system and put it on a little guillows freeflight jobber.

And is it really that expensive? Most 3d brushless arf's are around 6-700 dollars AU, and I'm cutting out 100-250 dollars by building the plane myself.

[Ollie]

Of course, there is more to it than that. I just gave you the description of the limiting case of vertical flight because vertical flight is one 3D objective. Armed with a gross weight that covers propulsion, control and airframe, you can calculate the wing area needed for a stalling speed you select, based on the maximum lift coefficient of the airfoil.

The bigger the wing, the lower the stalling speed. So low stalling speed is in conflict with good vertical performance because of the weight and drag of the wing. This illustrates that the art of design is to find a happy balance between many conflicting objectives of which I have discussed only one.

Finding the proper balance between conflicting objectives requires that various aspects of performance, cost, building effort, etc. be quantified and then matched to the plane's mission objective. The mission objective for a 3D model requires that many aspects of flight must be quantified so that performance calculations can be compared to objectives to see how well you are succeeding. This difficult analytical approach avoids a lot of trial and error like building and crashing.

If you don't want to go to all the trouble of learning how to do an analytical approach to design, you can copy someone else's successful design with minor variations. If you do want to learn how to do analytical design you can get a copy of The Basics of R/C Model Aircraft Design by Andy Lennon, and published by the publishers of Model Airplane News. There are various computer programs like Moto Calc that will crank out the calculations for you. The process is to start with a seed design, calculate the performance, modify the design and see if the modification improves which aspects of performance and which it hurts. After many iterations of this process, you can evolve what you are after. Perhaps that is why so few modelers enjoy the design process. It takes a lot of effort and some talent to be good at it.

[Viper Pilot]

raabid,

Check out the Quiet Flier Special Edition just released (kinda xpensive) but worth the excellant article for determing a power system for any plane.

There are several typo's, but the explainations are thourough and easy to follow.

VP

[raabid]

Thanks for the book reference. I find the challenge of doing it myself alot more enjoyable, but I did need a starting place. I will probably pick up a copy of ecalc along the way but it wouldn't have solved my questions.

By vertical performance do you mean the ability to accelerate vertically or just hang there?
The wing's done and the fuse is 50% done, I was trying to ascertain a weight / thrust / wattage number for my drive train, based on my experiences I'd like about 250 to 300W, and around 2 to 1 thrust. I'm figuring the dry weight of the plane to be around 16-20 oz and the AUW 32-40oz.

The experiments, have been in tweaking my foamies, I wouldn't dream of just slapping a motor on my baby. I've worked too long and hard putting her together. I do keep trying to find more power for my Crazy max but my guesstimates never seem to work out.

There is a bright light though, in that my flying buddy just picked up a whattmeter, so we can see what kind of power these motors are putting out before I give them a toss.

Thanks for the advice past and future, and I'll keep my eye out for that new QF mag, I have the last few copies. I guess I don't have the latest one.

[Ollie]

See:
http://www.rcgroups.com/forums/showthread.php?s=&threadid=100908
for a simple thrust measurment set up using a postal scale.

Hovering is minimum vertical performance for a 3D model in my opinion. Vertical acceleration from a hover would be better. So, you should be looking for a motor-prop (gearbox?) combination that will produce more than 32 ounces of static thrust.