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Old Today, 01:18 AM
Balsa&Tissue
payne9999's Avatar
United States, OR, Beaverton
Joined Jan 2011
2,489 Posts
Quote:
Originally Posted by Robert R View Post
Bart and Dave, I really enjoy visiting your build and design threads. Thank you for your kind evaluation of this build. I like the way it is turning out as well. .

That being said I have tried and failed to achieve extreme light weight. Before I cut one piece of balsa I had hoped to have an all up wight of 40 oz or 1134 gm. Dreamer!

A more clear eyed estimate later in the build hoped for 63 aounces

With much of the major structures completed the current estimated. weight is 74.5 oz or 2112 gm.

This is almost twice my earliest projected weight and 17% over my more realistic early estimate.

Is this a disaster? I don't think so . By the formulas at http://www.ef-uk.net/data/wcl.htm At 74 ounces the WCL(wing cubic loading) is 7.9 and at 80 oz it is 8.6

The full scale WCL is 5.9 empty and WCL is 9 at gross weight. So unless it gains more weight yet, it should fly like a full scale 247 near gross weight. Dave clued me in to WCL so if my plane doesn't fly well, I am going to blame Dave.

Where did all the weight come from. The ply and epoxy used to join the wing panels and more ply yet to compensate for the cut out in the aft spar for wheel clearance cost 3 oz, I didn't include nacelles 2.4 oz (estimated) , motor mount x 2 = 8 oz (estimated) and I over estimated the power system weight by 2 ounces, 3 oz on the empenage . which is where 14.4 oz ounces of extra weight could come from.

Robert
Robert,

Some of my multi engine projects have had a WCL well into the high teens. Only time and a maiden flight will tell. However, the power to weight ratio of an antique twin probably quite limited in cargo carrying capability would lead to a full scale cube loading that is quite low.

what is your thrust to weight ratio? Btw, I only base my opinions on the multi engine builds I have done and the relative cube loading calcs. So far a cube loading calc always seems to yield good data on the way the plane will behave. I also tend to look for a little more thrust than a single engine project due to the draggy nature of multi engine planes.

Dave
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Old Today, 02:25 AM
This is a fine fiddly business
Robert R's Avatar
United States, WA, Marysville
Joined May 2009
1,345 Posts
The Headsup RC data indicates that I should be getting about 35 oz of thrust per motor, so 70 oz of thrust total. I think AUW should be 74-80oz. That is a thrust to weight ratio of about 0.875 to 1. I think that should be plenty.

Robert


http://www.headsuphobby.com/Firepowe...otor-E-640.htm
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Old Today, 08:42 AM
I hate spray cans
don olsen's Avatar
United States, NM, Rio Rancho
Joined Jul 2007
1,133 Posts
Robert
OMG !!!!There must be something wrong with your scale! The Stinson I just got done with came in at just under 5 lbs (80 oz) so your 247 with no sheeting + glue +lighter covering (I used Ultacote ) It just isn't possible that your Boeing will come out the same as my Stinson. They are approx. the same size planes so yours missing what I added, It has to be a significant wt difference .
BTW after I read your post I ran out and weighed mine again 3 scales agree 5lbs I have to envision yours at 20% less minumum
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Old Today, 11:55 AM
This is a fine fiddly business
Robert R's Avatar
United States, WA, Marysville
Joined May 2009
1,345 Posts
Don,
Thanks, I think you may be right. Back with rechecked information later. Perhaps 64 oz is closer to correct. If so WCL would be 7 which would be wonderful. Back later.
Robert
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