DeepEndofPool
New member
Math day:
Let's start with the good news. I wasn't calculating wing area correctly (I didn't include the wing root) so the wing area is a little bigger and the wing loading a little smaller than I thought.
Here is the wing area calc from ecalc.ch/cgcalc.
And here is the wing loading, cubic loading and stall speed assuming the same weight, 2600g, as estimated in an earlier post.
Wing loading is down from 78 to 73.
The next thing to tackle (numbers wise) is the props. The distance from the inboard nacelle center line to the outside of the fuselage is 95.6mm (3.75"). The distance between the nacelles CL to CL is 181.2mm (7.13"). So the max prop diameter is 7". For authenticity's sake I'd like to use a three bladed prop. MasterAirscrew has 3-blade props in 5, 6 and 7 inches. If anyone has other suggestions, please speak up.
Assume the 3-blade 6x4 for now. eCalc says that if those props spin at 18,000 RPM it will generate a total of 4100g of thrust and will pull the plane through the air at 61 mph.
I would like to use a 3S battery setup if possible. The reason is that a 3S battery of a given mAh rating will be smaller and lighter than a 4S or 6S battery of the same mAh capacity. I'm not expecting size problems, but this bird is already heavy. If I can save some battery weight that would help.
Fiddling with these values in eCalc it looks like the motors need to be around 350 watts maximum and around 250 watts continuous and have a KV of about 2200 with a 3S battery.
One other motor parameter that's a non-issue is the motor diameter. The inside of the nacelle is 59 mm in diameter so a wide selection of motors will fit. Depth is currently 29 mm but that can be easily modified if necessary.
So I have motor power, motor size (max) and KVs for different number of cells. Using the db built into eCalc I found the RCinPower QAV 2206-2200 which seems to have the best performance balance.
The last parameter to look at is the total drive weight. Again, fiddling with eCalc different choices swing the drive weight anywhere from 700 g to over 1600 g. My earlier estimate of total weight was based on a drive weight of about 1200 g, but tuning the setup in eCalc that number comes down to 723 g, a savings of 477 g.
Reducing the weight by 477 g also reduces the wing loading. The new wing loading is now 60 g/dm2 which is less than 3DPrintLabs' P-38.
Interestingly enough if the props are changed to 2 blade (still 6.0x4.0) the plane picks up 4 MPH on the top end and the motors run 40 W less power, but thrust to weight drops from about 1.9 to 1.7.
Of course this is all just pushing numbers around in a model. I'd be very interested in people's feedback of where I've done something that won't work in reality.
Thanks.
Let's start with the good news. I wasn't calculating wing area correctly (I didn't include the wing root) so the wing area is a little bigger and the wing loading a little smaller than I thought.
Here is the wing area calc from ecalc.ch/cgcalc.
And here is the wing loading, cubic loading and stall speed assuming the same weight, 2600g, as estimated in an earlier post.
Wing loading is down from 78 to 73.
The next thing to tackle (numbers wise) is the props. The distance from the inboard nacelle center line to the outside of the fuselage is 95.6mm (3.75"). The distance between the nacelles CL to CL is 181.2mm (7.13"). So the max prop diameter is 7". For authenticity's sake I'd like to use a three bladed prop. MasterAirscrew has 3-blade props in 5, 6 and 7 inches. If anyone has other suggestions, please speak up.
Assume the 3-blade 6x4 for now. eCalc says that if those props spin at 18,000 RPM it will generate a total of 4100g of thrust and will pull the plane through the air at 61 mph.
I would like to use a 3S battery setup if possible. The reason is that a 3S battery of a given mAh rating will be smaller and lighter than a 4S or 6S battery of the same mAh capacity. I'm not expecting size problems, but this bird is already heavy. If I can save some battery weight that would help.
Fiddling with these values in eCalc it looks like the motors need to be around 350 watts maximum and around 250 watts continuous and have a KV of about 2200 with a 3S battery.
One other motor parameter that's a non-issue is the motor diameter. The inside of the nacelle is 59 mm in diameter so a wide selection of motors will fit. Depth is currently 29 mm but that can be easily modified if necessary.
So I have motor power, motor size (max) and KVs for different number of cells. Using the db built into eCalc I found the RCinPower QAV 2206-2200 which seems to have the best performance balance.
The last parameter to look at is the total drive weight. Again, fiddling with eCalc different choices swing the drive weight anywhere from 700 g to over 1600 g. My earlier estimate of total weight was based on a drive weight of about 1200 g, but tuning the setup in eCalc that number comes down to 723 g, a savings of 477 g.
Reducing the weight by 477 g also reduces the wing loading. The new wing loading is now 60 g/dm2 which is less than 3DPrintLabs' P-38.
Interestingly enough if the props are changed to 2 blade (still 6.0x4.0) the plane picks up 4 MPH on the top end and the motors run 40 W less power, but thrust to weight drops from about 1.9 to 1.7.
Of course this is all just pushing numbers around in a model. I'd be very interested in people's feedback of where I've done something that won't work in reality.
Thanks.
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