Electric advice

[erikhaha]

I am just finishing up building my 1st scratch built electric and I have never flown electric before.

Here are the specs:

Wing span is 28" constant 7.5" chord
Wing area = 210 in^2
Symetrical airfoil
High wing, very similar lines to an ugly stick.
GWS 2.6:1 gearbox with a 400 motor
Flying weight minus the battery is 16 oz.

Now, I went to the hobby shop to buy a battery(preferably Nimh) and the woman said that the current 400 motor/gearbox combo would need a large Nimh battery, which would probably be too heavy.

She also said that I should probably use a 350 motor, with a 5.3:1 gearbox, and a 8.4 V 1050mAh Nimh battery.

Any suggestions as to what I should do?

Will the speed 400 draw too much current and is a wingloading of 20 oz/ft^2 too much?

Anyone who flys electric a lot your input would be appreciated.

Erik

[Wingin' Wayne]

My first scratchbuilt was similar to yours. Just a wing on a box. With that small of a wing area, weight is a major concern! If you can, go to the recommended 300, it will save weight. Best light battery for small 400 plane would be 2/3 AA 650, it would handle the amps from a 400 without going to a 1050 2/3A. If you go to the 300, try a 730AAA or the Crazy Max 5/4AAA, with 5/4AAA you can prop to 9 amps and have good power and a 5-6 minute flight.

[Lon Enloe]

Erik, please post pictures when you're finished building!

[erikhaha]

Sure thing Lon. I have to borrow my roomates digital camera, but I will try and get some posted tomorrow.

[erikhaha]

Here are a few pictures of my 1st scratch built. Like I said, the flying weight minus the battery is 16oz, but I would like to get it down to around 13-14oz

Also I am not sure how many amps this spd400, with the gearbox, will draw at various power settings. My ammeter only measures up to 2 amps, and i think at full power this setup may draw around 15+ amps.

[Lon Enloe]

Erik,

Thanks! I just wanted to compare notes on how you're mounting the motor and other items, how much structure you're putting into the fuselage, etc.

Might I ask, how many channels are you using? What type of structure is the wing?

I know that there are a lot of great designs available as kits these days, but there's just something about seeing your own creative efforts heading aloft--and coming back safely. Thankfully, scratchbuilding is not a dying art!

[vintage1]

speed 400 runs out of efficiency at 10A or so. People do push them to 12A, but its a waste of time.

Best regime is about 8-9A max,and cruise at 3-5.

Ue long can if 400 is not man enough.

[gouch]

Just to add to the questions, with a loading of about 20 oz as Erik said, would the 350 with 5.3:1 that was suggested not have the speed to keep the thing flying ?
My thoughts would be the motor Erik has fitted to that lovely :) fuselage would be better for that reason? Just curious.. :)

Cheers
Paul

[vintage1]

I don't think it would frankly.

Its hard to get more than 35mph pitch speed out of a C geared 350, exccept on 3s LIPO which is marginal in terms of brush bounce.


With a stall speed of about 18mph, this thing needs 45-60mph pitch speed.

[gouch]

Thanks vintage, all this hanging around shows i'm slowly getting it.. :D

What would be the max pitch speed of the 2.6 400 at it's most efficient setup?

[feihu]

Erik:

I have reviewed your post #1 carefully, and I have also seen the photo you posted.

Regarding your photo: I noticed you have no dihedral in the wings and you also appear to have no ailerons. It looks like you will have a movable rudder. If you plan to fly rudder with no ailerons and no dihedral, you're in for trouble because you will have no roll effect with rudder. You will need anywhere from 4 to 6 degrees per panel dihedral for that to work. I don't see ailerons on the wing, so if you do add strip ailerons, they need to be taken into account for computing wing area, but you'd be ok without dihedral. Or you can fly AET without rudder.

How did you get the flying weight if your plane is still incomplete (as in your photo) of 16 oz. If 16 oz is truly the weight, I have an 8cell 1100 mah Nimh battery pack that weighs 6.5 oz. Add that to your weight and you have a total weight of 22.5 oz. Your wing loading will be 22.5/1.46ft sq = 15.4 oz per sq ft. which is not too bad.

I assume that the motor you have is the the 400 w/ 2.6:1 gearing. Looked up the specs on the GWS/EPS400C-C motor and it says on 9.6v,w/9047 prop, 9.3amps,13.54 oz thrust,wgt 3.33 oz
For the GWS/EPS350C-CS motor, 9,6v,w/8060 prop, 8.6amps,13.3 oz thrust, wgt 2.22oz
If you go down to a 7 cell battery for the 350 motor, you might save another oz. for less thrust.
You will have to judge if it is worth saving 2 oz. The run time would be about 6 min either way.

I would guess that your plane w/ its wing loading will fly a bit faster than the average parkflyer. I would suggest a cg at 30% mac. and I do want to see you successful on your first scratch build. Would be glad to help you in any other if you so need.

feihu

[erikhaha]

Thanks for all the info guys, and Feihu I live in Tempe so we are in the same state.

As far as ailerons, I didn't include them in either the wing loading or the weight. Since I don't have a scale at home I had to go to the hobby store to use theirs and I forgot to bring the rudder, ailerons and landing gear. Since then I have tried to eliminate as much weight as possible.

Otherwise I will include a plot that I posted in another thread about duration. This plot includes another battery with a capacity of 1100 mAh. Feihu you said that with your 1100 mAh pack, my plane would probably fly for ~6 min. As you can see I plotted the duration for higher capacity packs, therefore their weight is also greater and the wing loading would also increase.

I guess my question is, would the increased wing loading from the larger capacity battery be worth the increased flying time by using a higher weight and capacity battery?

Also, I understand that as I draw more current the voltage will also decrease and that my calculations are purely analytical.

[feihu]

.......... I guess my question is, would the increased wing loading from the larger capacity battery be worth the increased flying time by using a higher weight and capacity battery?

Also, I understand that as I draw more current the voltage will also decrease and that my calculations are purely analytical.

Erik:

The esc controls the voltage going to the motor, and the motor characteristics, such as load, resistance, etc, determines the amperage. By flying at a reduced throttle setting, you can increase the flying time for a given battery.

Whether the increase in wing loading be worth the increased flying time depends many factors such as airplane flying characteristics, piloting ability, and your satisfaction with the flying time. As the wing loading goes up, so does the stall speed and the need for more power - which means that the plane will fly at a faster speed. How much (wing loading) is too much would depend on you and your ability. I would suggest you do the maiden flight with as light a wing loading (smallest battery) as possible to get your plane trimmed out and flyable the way you want; and then gradually increase the weight as desired.

feihu

[Sail 'n Soar]

Joining this discussion late, but it appears your motor/gearbox/wing loading/symmetrical airfoil combination will be a marginal flyer at best. With your wing loading and symmetrical foil your stall speed will be at least 23 miles per hour. If you go by the min prop speed factor of 2.5 for a no dihedral sport ship like yours, your prop speed should be at least 58 mph.
I don't believe any of your motor/gear box/battery combos have a prop speed in that range.