Estimating weight of a plane

[DBono99]

Hi All I am going to build a downsized version of a plane called the Miss Texas..This a low wing sport type plane with a WS 89" weight of 8 3/4 lbs for the full size, wing cord 13".. Is there any way of estimating the weight of my downsized version that will have a 50" WS.


Thanks Dan

[BMatthews]

Not really unless you're willing to put a heap of work into the measurements and calculations. To do it you'd need to measure the surface area of all the individual balsa and other wood parts in the plans and come up with a total cubic foot number for each type of wood. Covering would then be based on measurements of how many square feet of area is needed to cover the model's exterior and then compare that to charts of weights for various coverings. It's not a direct scale sort of deal that if the big one is X amount then the small one will be Y amount.

Often structure doesn't scale well over a larger range. You'll want to study the resulting design and adapt here and there to ensure an adequitely strong structure but without using too much wood.

THis is the Chuck Cunningham design Miss Texas? If so I recall that it was a real lumberyard and a lot of thinning of stock and sheeting sizes will be in order. Assuming it comes in at around 480 sq inches for the smaller version you'll want to keep the ready to fly weight to under 3 lbs if you want to retain the gentle manners the full size one is supposed to have. In fact I'd recomend trying to keep it closer to 2.5 lbs. To do that in many areas you'll want to skip the sheeting altogether and go for open structure or use turbulator spars or stringers instead of sheeting. Also you'll want to pick your wood so that other than load bearing items like wing spars most of the wood is closer to the 6 to 7 lb/cuft density value rather than 10 or more. Harder balsa is often in the 10 to 12 lb/cuft range and adds up the weight fast.

[vintage1]

If you scaled the balsa down exactly as well, weight varies as the cube of the linear dimensions, so your weight would be 8.75 x (50/89)^3= 1.55lb

Now that is not unreasonable for a lighly built 50" plane. OTOH I've seen 40 powered lumps crack 5lb at that span.

I agree iwth Bmthews that a target weight of 2.5lbs is what to aim for. And something like 250W of electric power, or a .25 2-stroke.

[green66]

First subtract all the non-scalable stuff eg radio gear, motor, etc then apply the cubic factor (.177) per vintage1 to get new airframe weight then add the weight of all the non-scaled gear that will go in the 50" plane.

But, per above this calc assumes that everything will be scaled. Assume you're not going to use balsa sheets 50/89ths as thick as on the bigger plane :rolleyes:

With that, I'd factor the 89" airframe weight by something more, eg .25, maybe even .33..... also considering that a smaller plane may require a higher power/weight ratio to have similar flight characteristics as the bigger one.

[DBono99]

Hi All Thanks for the great info.. My target AUW was 3 lbs. Power system Hyperion 3019/10/Titan 50 amp ESC..I'm using this same set up on my downsized Cloud Dancer, AUW 45 oz..Apc e prop 10x5 36amps 365 watts at WOT..

Thanks Again Dan

[BMatthews]

Dan, I would respectfully suggest that your power option is going to produce a model that is not really in keeping with the character of the original. The big Miss Texas was a slower flying gentle sort of looking model. The numbers you're quoting are more typical of a hot flying sport model. If you want to go that way then fine. But if you're looking at a more gentle sort of model that has a flying speed typical of a trainer but with sharper and more sporty control responses then I think you've missed the target.

For that same power option I'd suggest scaling down to around 550 to 600 sq inches and aim at modifying the structure as neede to keep the same or even less weight. Build it more like an old timer free flighter if required. Or if you've got your heart set on the 50 inch 480 sq inch size then consider using a smaller and lighter power package and again reduce the overall target weight by modifying the structure.

With your numbers it will certainly fly and fly well but it will be more of a speedy model with a typically sporty model landing speed. It all comes down to what you're wanting it to fly like. If my ramblings don't match your expectations then carry on.

[DBono99]

Hi BM I'm not too sure as too what the AUW will be, so there fore I tend to over power some.I can always throttle back..Here are some of the numbers I get.
Wingspan 49 inches
Wing root chord 7 1/8 inches
Wing tip chord 7 1/8 inches
Average Wing chord 7 1/8 inches

Model Weight 48 ounces
Wing Area 350 sq.in sq.dm
Total Wing Loading 19.7oz/sq.ft

Thanks Dan

PS I can always swap out the Hyperion 3019/10 for a 3013

[DBono99]

Hi Bruce Do you have any recomendations for the Electric set up in the Miss Texas..Any one else would like to chime in with thier thoughts on a power system for this plane. What I'm looking for is a nice relaxing plane to fly..NO 3D or speed is needed..More like a Sunday flyer type..

Thanks Dan

[vintage1]

Use a 2000mAh 3s LIPO pack and shoot for an efficient 20A-25A or so.

Something outrunnerish with a KV about 1000 and a 2808 or 2217 sort of stator size will fall in that class.

Probably 8-9" prop. 8x6 or 9x6 I'd guess.

[DBono99]

Vinatge: Do you know anything about the Hyperion electric motors..I've used 3 of them in my projects and I'm quite happy with there performance..so far.
I will check with my distributor and see if I can come close to your specs.

Thanks Dan

[DBono99]

Hi All
Here are 2 pics of the Miss Texas..Today I put every thing on a scale, except for the wing, came out too 2 lbs. So I'm figuring on maybe 3 lbs or a little over, No more then 3 1/2 lbs..Still haven't a clue about the control throws..

Dan

[DBono99]

Another pic

[DBono99]

Hi All
Happy New Years to everyone..I'm rethinking my choice for the motor for the Miss Texas..I just might go down to next size Hyperion motor, a 3013/14..Should give me my needed 250 watts..I put Miss Texas and all the parts on scale and came away with 2 lbs or 32 ozs with OUT the wing..So I should BE UNDER 3 lbs or 48oz...
http://www.allerc.com/z3013spec.htm

The 3013/14 with either a 10x5 or a 10x7 APC e should give me the watts I need..The Miss Texas should fly fine with 80 w/lb.


Thanks Dan

[vintage1]

I fly a 60" 2lb 10oz machine on 180W or so of speed 600 brushed!!!..plenty for fun.

60W/lb is adequate for flying on the wing. Should loop from level if tuned properly.

[DBono99]

Hi Today I ordered the NEW motor for the Miss Texas..Hyperion 3013/16 Kv's 985..Should give me 250 watts with NO problems..I'll save the 3019/10 for the Swallow.
http://www.allerc.com/product_info.php?cPath=7_76_77&products_id=1672

Thanks Dan