FT Flyer Target Weight

[loopwhile1]

Hello Friends,
I have a beginner question for building my first scratch build.

What should be the target weight for my FT Flyer?

Motor: 24gram Brushless Outrunner 1500kv
Prop: Slow fly 8x4.5
Battery: 3S 1000mah
Esc: Hobbyking SS Series 18-20A ESC
Radio: Old futuba attack 4 AM radio

My current weight is around 12-13 oz (340-360 g). Is this too heavy for my motor? The plane was hand launched and had difficulty gaining altitude. It just slowly glided to the ground even on full throttle.

Any help would be very appreciated. I can provide more information about the parts if needed.

Thanks a lot!
Loopwhile1

Things I've tried:
- Recalibrated ESC
- Checked prop orientation

 

[Craftydan]

loopwhile1,

a good rule of thumb for regular everyday sport flying is 100W/lb

running a 3S(or roughly 12v) and the motor you have should draw at most 7.5 amps. That yields roughly a max of 85 watt, so you're at about 108W/lb max. With the right prop it should be happy in the air, unfortunately, it sounds like you're motor's not drawing that. I've gotten plenty of thrust out of an 8x4.5 on a 1300kv motor -- it seems like you've *over* proped the motor on the 1500.

Try this:

attach the prop and restrain the airframe.
spin up the motor slowly.
listen to the motor and feel the vibe and prop-wash as you throttle up.

Does the pitch smoothly slide up as you move the throttle? does it get to a point about 1/2 -3/4 where the whirr suddenly switch to buzz? does the vibe in the airframe go up noticeably when it does? does the propwash drop off? If so, I think you might be spinning that prop too fast and you're getting flutter at higher RPMs.

Another thing to check, does the motor spin freely? It will click a bit around the magnets, but if you give it a quick spin, it should windmill freely for a few moments.

 

[loopwhile1]

Hello Dan,

1. Right about the half way throttle mark (may be sooner) the whir does change into a buzz and there is a lot of vibration. There is also a noticeable sound difference (whir => buzz). I am using a prop saver, could that make a difference?

2. The motor spins freely and seems free of debris.

What would you recommend as a solution for this?


PS: Thank you very much for your reply. I really appreciate it!

 

[xuzme720]

Actually, the prop saver might be causing some of this. Make sure your bands are tight. They should be fairly hard to install to give the prop the security it needs to stay centered and straight. If your bands are loose, sometimes doubling up (using 2) can help until you can find a more secure band material. Surgical tubing has been recommended many times.
Also, have you balanced the prop? An unbalanced prop can create unnecessary vibration and rob you of a lot of your top end.
Finally, if none of these help, try a shallower prop. If you are still getting the same result, it's likely that your motor is creating a scenario where the prop is actually spinning faster than it's angle of attack can move air and it then stalls, just like a wing can high speed stall. The prop profile is an airfoil, after all, and is subject to the same aerodynamic forces as a wing. If it is the prop stalling, until you can get shallower props, one option is to adjust your throttle endpoints to just inside where it's not stalling/buzzing. This is not a great option and should only be used as a stop-gap until you can get the correct props, but should get you into the air without risking accidentally wandering into the loss of power zone the stalled prop will create.

 

[Craftydan]

Actually I was going to say the props are a likely on the cheap (weak) side. They're fluttering, and likely if they weren't they'd draw too much current at WOT.

first, I agree -- get a stronger prop saver band. Surgical or exercise latex tubing works well --I use "Theraband" brand blue tubing. If it's easy to pull off, it's easy for the motor to throw it off.

If that doesn't fix it (I don't think it will work), if you can balance the props, trim it by an inch, balance, and try again. A 7" prop will probably give you plenty of thrust on a 1500kv motor with 3S.

In the future, do buy cheap props, but don't buy the cheapest props. They break *way* too easy and flatten out and flutter at medium RPMs.

 

[xuzme720]

Yeah, I went back and just realized you're using 4.5" pitch props. For some reason I thought you were using higher pitched props. With that low a pitch, Dan has probably nailed it. If you can pull on one of the prop blades and it flexes pretty easily, what he has described is likely happening and they will never give you the power they should. Those really inexpensive props are good for lower RPM's but you are well into the middle to higher range for slow-fly prop RPM. Try to find a stiffer prop to use with this Kv motor.

This is of course provided that the other suggestions do not work.