|Dec 23, 2003, 10:57 PM|
Doing the Math on AP Slow Stick
Here is the weight of my Slow Stick with Pencam SD:
Fuse w/motor, prop, and servos = 11oz
Wing = 4oz
Total Weight=23.5oz or 666 grams (yikes, not the Satan Stik)
If 30 watts is required for an aircraft of 1 lbs (or 454 grams) to take flight then 1 watt=15 grams. So my AP aircraft would require 44 watts of power just to get airborne.
If 50 watts is required for comfortable flight performance of a 1 Lbs aircraft then 1 watt=9 grams. So my AP aircraft would require 73 watts of power.
According to the GWS tables, a 400 size motor with gearing of 4.5:1, and a prop of 12x8, operated at 9.6 volts would produce 100 watts of power. This should be more than enough power for my AP Slow Stick.
Is my math right on this? Am I missing something in my calculations.
|Dec 24, 2003, 12:10 AM|
You've won FIRST PRIZE in the OVERTHINKING contest!
Holy cow, man! Buy the 400, pick a geatbox (F,G,H) and FLY the THING! Try out some different props, 12x8, 13x6.5, 13x9 and see what makes you smile!
I know you're just trying to get it right before you buy, but you are definitely in the ballpark with the motor, box, battery, and prop. A little experimentation with props and you'll hit the homerun.
|Dec 24, 2003, 03:08 AM|
Re: Doing the Math on AP Slow Stick
I went with a 400 but there was something wrong with my gearbox, it chattered a lot and wouldn't fly very high at all. It had plenty of power but I just couldn't get it to fly right after a lot of tinkering, so I bit the bullet and went brushless. Some may think it's kind of odd flying a $35 plane with a $150 motor/esc, but the power and longevity gains have all been worth it. Incidentally, my AUW is about 25 oz. with my Aiptek.
|Dec 24, 2003, 07:26 AM|
I already purchased the motor and prop, I just want to get the right battery. They can be expensive so I don't want to make the wrong decision.
I only showed the math because up until this point I was just trying to solve the problem without understanding the underlying math involved with electrical flight. At some point in this hobby, if you ever want to advance, you have to sit down and teach yourself the mathmatical formulas required to be successful.
Oh, and an EE degree would not hurt either.
|Dec 24, 2003, 01:24 PM|
I agree you need to understand the physics behind your various motor/gb/battery/prop choices if you want to be able to choose the best combinations. You've done the math and are in the ballpark now. The rubber hits the road, though, when reality clashes with the theoretical stuff on paper. The math gets you close, then actual readings with tachometers and whattmeters gets you the best possible combo of prop/battery/gearbox. Your geographic elevation, motor and battery "real world" idiosyncracies, ambient temperature, etc. now come into play.
Have you got a tach and a whattmeter? Essential, IMO.
|Dec 24, 2003, 10:13 PM|
Shaving a Few Grams
For your wing, skip the Iron Crosses, substitute 1/8 pultruded CF tube for the kit's solid fiberglass spars, substitute 1/8 ID tube brass for the aluminum joiners, use a thin smear of polyurethane glue to attach spars to the foam and in the center section. 3.05 oz IIRC.
I did this with one 48" length of CF tubing (~$5), running the LE short by one "rib" bay on each side.
Oh, and if you go with an 8x1100 Hecell pack, figure 6 oz instead of 5. Drop it to maybe 4 oz using LiPo's.
Other important ratios: $$ per megapixel, pictures per megabyte, and spousal nagging per $$.
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