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Old 01-11-2013, 12:53 AM   #1
domtech809
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Default Building a 7lb RC Plane!

Hello,

I'm going to be building an RC plane that will weight approximately 7lb with all the equipment, including motor, battery, esc, servos etc.

I've built a few smaller kits before, but never my own scratch design. I'm having some difficulty trying to determine what size of wing I will need and basically just the overall dimensions. I did some math on some searches I did online and "general rules" for wing loading and calculated that I will need approximately 5ft^2 of wing area to be at about 22oz/ft^2 wing loading. If anyone can point me in the right direction I will appreciate it. I will also need to figure out how much thrust I'm going to need to keep it in the air at a not so fast speed, maybe 10mph stall speed or something like that. Any info will be appreciated. Although I've read a lot of articles about this it always good to hear what others have to say and recommend.
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Old 01-11-2013, 01:13 AM   #2
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Originally Posted by domtech809 View Post
Hello,

I'm going to be building an RC plane that will weight approximately 7lb with all the equipment, including motor, battery, esc, servos etc.

I've built a few smaller kits before, but never my own scratch design. I'm having some difficulty trying to determine what size of wing I will need and basically just the overall dimensions. I did some math on some searches I did online and "general rules" for wing loading and calculated that I will need approximately 5ft^2 of wing area to be at about 22oz/ft^2 wing loading. If anyone can point me in the right direction I will appreciate it. I will also need to figure out how much thrust I'm going to need to keep it in the air at a not so fast speed, maybe 10mph stall speed or something like that. Any info will be appreciated. Although I've read a lot of articles about this it always good to hear what others have to say and recommend.
Hey, welcome to wattflyer!

Most of my models are in the 7-8 pound range. And, all of them fly very well.

You will need a model with about 650-750 square inches of wing, and about 1000 watts of power for very good performance. One program that will be useful for your project is www.wattflyer.com, free for 30 days, then $39. With the proper quality electric power components, it is quite easy to electrify a model with electric power, and have a model with all the flying capabilities of a glow powered model. The only real issue is shorter flying times. On these larger models, 6-8 minutes of flying time is reasonable.

In fact, today I just did the maiden flights on my Hanger 9 Kantana model with 850 Square inch wing, 8 pounds weight, powered by a Hacker A50-12S motor, 16X8 APC-E propeller, and a 6S2P A123 battery pack. That A123 pack would be similar to a LiPo 4400 Mah battery pack with five cells.

With this setup, the motor pulled 62 Amps on the ground, with 16.9 Volts and 1130 watts. The motor turned that 16 inch propeller at 7500 RPM on the ground. The Castle Creations ESC flight data was downloaded, it showed the current dropped to about 37 amps at wide open throttle, turning the prop at 8200 RPM.

This power system hauls that model straight up, and keeps going. The Kantana is a very nice model, and is very easy to convert to electric power.

For what it's worth, it will be very difficult to build a model with a 10 MPH stall speed. At a 6 pound weight, you'd need perhaps 2000 square inches of wing.

As for information on electric power on medium sized models, perhaps these threads will be useful?

Thread on 70 size glow engine conversion to electric
http://www.wattflyer.com/forums/showthread.php?t=45222

Hacker 6S2P A123 powered Models
http://www.wattflyer.com/forums/showthread.php?t=44686

AEAJR's Site on Electric Power
http://www.wattflyer.com/forums/showthread.php?t=18521

DennyV
Retired and the days are just too short, busier than ever!
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Old 01-11-2013, 01:26 AM   #3
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Hmmm... coming at it from another direction, I see that if you started with, say, a Parkzone T-28 and scaled it up on all axes by 1.5, it would go from a 44" wingspan to a 66". It's mass would increase by the cube of the scale. 1.5^3 is 3.375, so it's mass would increase from 30oz to 30*3.375 or 101.25oz. (7lbs is 112 oz.)

Scaling up by 1.6 gives a wingspan of 70" and a mass of 123 oz.

So a T-28 with a 68" wingspan would weigh about 7lbs.

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Old 01-11-2013, 06:41 AM   #4
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You seem to be approaching things opposite to what most people would do when designing a plane.

Most would pick a scale subject or an idea of the look off the plane they had in mind and what they wanted the model for (speed, aerobatics, general sports, e-glider etc).... Then they would decide how big to make it based on practicality, transport, budget, available hardware etc... then the weight would simply be what it was. The weight itself would rarely be the place that you started out.

Why is the weight of 7lb so important?
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Old 02-22-2013, 09:49 PM   #5
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How is your project comming? Interested to hear if you have made any progress.

Long Island Silent Flyers
www.lisf.org
Eastern Soaring League
www.flyesl.org
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Old 02-23-2013, 04:10 PM   #6
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Mine came in at a little under 8 pounds with two lipo packs and flys slower than a big trainer .


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Old 02-23-2013, 08:23 PM   #7
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That's a great lookin' plane.

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Old 02-23-2013, 08:39 PM   #8
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Your stall and speed characteristics will generally be governed by wing area, airfoil, and weight.

Pick an airfloil, sounds like you want a floaty plane, i'd stick with either a undercambered wing, or a flat bottom style. A broader leading edge will mush the stall and make it a bit more predictable, and the less weight you have, the floatier it will fly.

Hope this helps, and good luck.
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Old 02-23-2013, 09:41 PM   #9
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If you have a few kits under your belt and talking about scrach building I bought my plans from IVAN .Check out his flying weights of his desighns and wing loadings and you will see why i chose to buy his plans. There is a bigger twin otter kit and a smaller one with a link to buy a laser short kit but the rest like my short solent 480 has none .http://www.ivansplans.com/ When Ivan drew most of these he flew them with c size batterys and brushed motors so with lipos and brushless they even come out lighter than he said. BTW i bought his bigger twin otter and the lazer cut kit plus his big albitross seaplane for a build coming soon. This is a video of a Ivans martin mars and mine short flys just like this, a big trainer . The mars is 130inch wing span . joe http://www.youtube.com/watch?v=TlJgU3rkJeo
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