pusher prop help.

[sierra_bravo]

Hello,
I am going to build a ft versa wing as a pusher but i have a question about my motor and prop. I am going to using a turnigy D2826-10 which is 1400kv motor and I am going to use a 3 cell battery. My prop is a 7x5. My question is if it will be able to have enough power to fly.
Thank you in advance.
-Sam

 

[mjmccarron]

You may find it under propped. An 8x4 or 8x6 might be more in line with that motor and 3s setup. As far as power goes, the 2826 on 3s is great.

Mike

 

[sierra_bravo]

Hello,
I was going to use a 8x6 prop originally but when I checked the recommended prop for the motor it said 7x4. I will go with the 8x6 then. Also the plans for the ft versa say to use a 8x6 prop. I did have one more question. I have been reading about lipo batterys and I wanted to know if I am checking the battery while charging do I still have to put it in a lipo bag or is it fine without a bag.

Thank you.
Sam

 

[mjmccarron]

The lip bag is an extra measure of safety but it's fine to pull it out to check it. It's mainly for if you aren't right there to catch a problem. 99.9% of the time you could charge outside the bag. The cost of a new house or garage is much more than a lip bag though.

 

[sierra_bravo]

Hello,
Thanks I will always be babying my lipo but I just wanted to make sure I dont have to use a lipo bag.

 

[mjmccarron]

I've been flying LiPo's for about 7 years and never had an incident. I baby my batteries too. It only takes one fire to ruin your whole day though. Common sense and a good routine go a long way to preventing disaster.

 

[joshuabardwell]

I have that same motor, and ran it with an 8x6 on a PopWing for months with no problem at all.

For what it's worth, eCalc estimates 12 amps with a 7x5; 19 amps with an 8x6; 24 amps with a 9x6. The 8x6 estimate is pretty close to what I actually measured--and bear in mind that the prop will unload in flight and the current will drop off. If you were hurting for thrust, I think you could probably run a 9x6 and get away with it, as long as you didn't go flat out too much. But the 8x6 makes about 1 kg of thrust, which should be plenty for a Versa with an expected AUW of around 600-800 grams. The 7x5 is estimated to make 750 grams of thrust, which, again, should be enough to fly off of, but you'll have a better time hand launching with more thrust.

 

[sierra_bravo]

Hello,
After reading all the responds I will be going with the 7x5 prop. I don't to risk hurting the motor for a bit more thrust but I do want to fly at a good speed. Thanks everyone for the help.

 

[Foam Addict]

Good call! If you find your self wanting more speed or thrust, try and APC 7x4 or 6x5.5 on 4 cells (the 7x4 is significantly more efficient than the 7x5). These are ballistic setups, but they also run very cool.

 

[joshuabardwell]

For what it's worth, eCalc says that a 7x4 on 4S exceeds the motor's power rating. You're only pulling 17 amps, but around 240 watts. The motor is rated for 205 watts. To complete the analysis, eCalc estimates a little over 1 kg of thrust and a pitch speed of 63 mph. By comparison, an 8x6 on 3S gives about the same thrust and pitch speed, pulling 18 amps at 200 watts. So, in as much as we trust eCalc's estimates, it seems like there is not much advantage to a 7x4 on 4S compared to an 8x6 on 3S.

 

[sierra_bravo]

Thanks everyone for the help. I will posting photos of the build in my other thread.
thanks
-sam

 

[Foam Addict]

For what it's worth, eCalc says that a 7x4 on 4S exceeds the motor's power rating. You're only pulling 17 amps, but around 240 watts. The motor is rated for 205 watts. To complete the analysis, eCalc estimates a little over 1 kg of thrust and a pitch speed of 63 mph. By comparison, an 8x6 on 3S gives about the same thrust and pitch speed, pulling 18 amps at 200 watts. So, in as much as we trust eCalc's estimates, it seems like there is not much advantage to a 7x4 on 4S compared to an 8x6 on 3S.

Hey JBW, based on my experiments, I don't think that the watt reading is a big concern. As long as the current draw doesn't exceed 18 amps, you should be fine, any additional watts you get from extra voltage are more or less free.

 

[joshuabardwell]

Hey JBW, based on my experiments, I don't think that the watt reading is a big concern. As long as the current draw doesn't exceed 18 amps, you should be fine, any additional watts you get from extra voltage are more or less free.

That's always been my hunch as well, but then why does eCalc flag it?

 

[Foam Addict]

I don't know, my guess is that its because Ecalc is a relatively simplistic program, though there are some very nice components to it. I might try Drivecalc, it's an incredible piece of software, though it has some weaknesses as well.