Flight Times

[Draylon Fogg]

ok so i guess i didn't clarify this right but im asking about my second plane, the 1:23 time is a different plane that is our main one im asking about my other two so here are the actual demensions of the second plane, wing length is 39.8cm, cord is 10.6cm, tail length is 25.scm, cord is 7.2, tail distance from wing is 37.1cm,fuelsilage length is 37.5 cm, prop span is 19.8cm. by what you are saying im doing really bad but even when i make one that is cm by cm to a plane that is 7.2 grams i still end up at 11g.

[jander14indoor]

First I'm not saying you are doing anything bad. But it does look like there's some confusion between us. Sorry.

OK, lets see if I understand this. Last years (or your primary) plane weighed 7.2 gms and flew. New planes to same dimension weighs 11 grams? Is that the issue?

I'll stop there till I really understand, sorry.

Jeff Anderson
Livonia, MI

[Draylon Fogg]

I know you not but Im just getting really frustrated. Ok so we have plans for a plane that is 7.2 grams and our current one that looks exactly like it is 11.

[jander14indoor]

OK, so you are asking why you aren't hitting target weight.

Do you weigh wood or parts as you build? Do you have a bill of materials showing what each part should weigh? If not, you need to start doing that.

Balsa, as you probably already know, is a hugely variable material. Density varies from below 3 lb/ft3 (rare) to 12 lb/ft3 (common) 20+ lb/ft3 (somewhat common). So two parts of exactly the same size can vary in weight by almost 700%! If you don't select your wood carefully, the same 'looking plane can go from 7 gm to 15 gm in a heartbeat. For Wright Stuff, the bulk of the wood will be in the range of 5-9 lb/ft3 for a 7 gm plane, but even that will leave a huge weight range if you don't stay aware of it.

OK, so how do you manage it.

Many (most?) SO plans are only half complete for this purpose. They describe all the parts and their dimensions, but at best give recommended density wood to cut pieces from. Unfortunately, even if you buy the right density wood, when you cut strips the density varies across the sheet, and two seemingly identical pieces can easily vary by two or three to one. Again, leaping from that 7 gm plane to a 14 gm plane in a heartbeat. Now selecting your wood by average density will get you closer, it still leaves a lot of variability.

To minimize variability you make a bill of materials and figure out target weights for the parts. (BTW, I'm an engineer at Ford Motor Company, and a BIG part of my job is just this! Fuel economy means weight is critical in cars, and if I can't tell my bosses what the engine or transmission we're designing weighs before its ever built I get my head handed to me. And it BETTER be right when built!)
- Your bill of materials should list each piece of the plane, from wood to prop wire to prop to covering to glue, everything. Each part should have a target weight. They must sum to slightly less than you desired weight.
- You get the target weights by using the dimensions to figure volume, multiply by recommended density (for steel parts just look up steel density) to get a target weight.
- For some parts, like the prop, simply weight it.
- For covering look up a area density or weigh a sample piece and calculate it. Multiply by the covering area.
- For glue, make an estimate. A very conservative starting point is 1.0 gm. Good builders will use less than about .3 gm. Use your judgement.
- If final weight doesn't equal 6.5 to 7.0 gm, increase or decrease density or piece sizes till your bill of materials total is in that range. When you have a choice, find a lighter covering first to lose weight. Strengthen the motor stick, wing posts or wing spars if you are underweight (you'll get there). Lighten the tail boom, tail spars, and ribs to lose weight.

Now, when you start building, make sure you have a good scale next to you. Weigh the parts you select for each use. Too heavy (heavy plane) or too light (weak plane) put it back and select another.

If you do this rigorously you can build a plane to within .1 to .2 gm of target every time! If you use at least PART of this info, you should be able to stay within a gram or so of your target.

Let me know if I'm starting to get at what you need?

Jeff Anderson
Livonia, MI

[blue cobra]

What jander said is completely true, but I think that if you are significantly over weight, you are using way too much of something. I personally don't find the densities of my pieces (though I know I should) and my most recent plane, including bass for the wing posts and a few other components, came out to 6.93 grams (but do NOT use bass until you can build light, then you probably still don't want to). I think that if you use "regular" sizes of balsa; about 1/8x1/4 for MS, 1/8x1/8 for boom, and 1/16x1/16 for your wing and stabs, and don't go crazy on the glue, your plane should come out under 7 grams. I'm not trying to disregard what jander said, I just feel that it is very easy to build to 7g and making a bill of materials might be better when you are a bit more experienced and can start worrying about consistency.

[jander14indoor]

IF you have a stock of good, light balsa and light covering, yes hitting 7 gm or below is easy. WS planes are actually fairly heavy for their size.

BUT, without a plan or experience it is pure random chance. I think a good BOM is even MORE important for those flyers moving beyond the very basics. Its the fastest way to get experience and reinforce it.

Jeff Anderson
Livonia, MI

[3slh9]

I am the coach of the 11g plane and we got our design and wood from a wonderful gentleman in our community who builds planes as a hobby. It has been very frustrating since we are building it exactly to plan and his plane was 7 grams and ours is 11. I have watched my student build these planes and he is so careful about how he is building them that I can only think that the wood is the problem. We got the 1 min 23 seconds using the plane from last year which is right at 7 grams.

I finally broke down and bought a Leading Edge 2010 kit which is very similar to the design we have been using. Do you think this would be a good one to create a BOM from? I just got it yesterday so it is still in pieces. I have a scale that can measure to 0.005 grams so I feel that we can get a fairly good measurement on the weights.

We really have a competitive region and state as even at our invitationals, winning times have been over 2 min 30 seconds and we have seen over 3 minutes during practice flights and this is in a gym with a 28 foot ceiling.

Thank you for your recommendations.

[jander14indoor]

Note, I'm making an assumption your students building skills are good and glue isn't excessive, but this can add weight FAST. Do you have any data on how much glue you use?

The Leading Edge is a perfectly fine design to work from.

But you don't build the Bill of Materials from the kit. You build it from the plan. Calculate the ideal piece part weight from volume (length, width, thickness on the plan) and density (look up, measure or estimate) to target 6.5 to 6.8 gm (leave a few tenths for clay ballast to adjust center of gravity). Adjust the woods estimated density to hit the total weight target. Note, I do cheat and weigh the prop due to its complicated geometry, but that's the only part I do that for.

As a starting point, see the table below from one of last years designs.

THEN you start selecting parts (either from the kit or from scratch) and weighing to find the correct parts to build to your target density.

With the scale you mentioned, that's plenty of resolution for all but the smallest parts. I tend to select sets of ribs (they are so light individual ones are at the limit of the scales usefulness) and weigh them, reselecting till I find a bunch that weigh the target amount.

Note, when I'm really serious, I record the weight of wood used, and calculate weight added by glue as I go. Lets me adjust the BOM if I find one part breaking too often. That's how I arrived at that 0.5 gm estimate on the below BOM, it used to be 1.0 gm.

The following table is cut from my Excel spreadsheet where its MUCH easier to play with the numbers and adjust. Its for a particular design, so don't use it blindly as a guide.
Not, I don't weigh parts to thousandths, only hundredths. And I only measure to thousands of an inch. And I mix units TERRIBLY, sorry, I grew up with the english system and engineer in metric, deal with it.

Code: Select all

Finny 09	by Bill Gowen							
PLANNING DATA								
Part Name                                 qty	length	width	thickness	volume (in3)    rec density (lb/ft3)	wt (grams)	
wing spars	                                2	15.5	0.0938	0.0625	0.182	                8	             0.382	
wing ribs	                                7	4	0.0625	0.0625	0.109	                8	             0.230	
stab spars                 	2	7.75	0.0312	0.0313	0.0151      	5	             0.020	
stab ribs                    	5	2.25	0.0312	0.0313	0.0110      	5	             0.014	
wing fins	                                2	7	0.0625	0.0625	0.0547	                6	             0.086	
stab fins	                                2	4.5	0.313	0.188	0.527        	6	             0.831	
motor stick	                1	10	0.375	0.25	0.938	                8	             1.970	
tail boom	                                1	12	0.188	0.125	0.281	                5	             0.369	
wire prop hook	                1	1.5	0.02		0.000471   	489.6	             0.061	
rubber hook,wire	                1	0.75	0.02		0.000236   	489.6	             0.030	
wing posts	                                2	4	0.0625	0.0625	0.0313	                10	             0.082	
prop							                                                             1.750	
groc bag covering, wing	1	15.5	4		62	                8	             0.320	gm/m2
groc bag covering, horz stab	1	7.75	2.31		17.9	                8	             0.092	gm/m2
groc bag covering, wing fin	2	4	2		16	                8 	             0.083	gm/m2
groc bag covering, stab fin	2	2.25	1.5		6.75	                8	             0.035	gm/m2
glue, tissue tubes, etc(est)							                             0.500	
total wt estimate							                                             6.856	

ARGGHH, the columns look right in edit, but bunches in preview, oh well, I'll post now and try to sort it out later.
Double ARGGHH..., even after I post, when I pull back up to reedit the spacing is right, is there a tab command I'm missing????

Jeff Anderson
Livonia, MI

[3slh9]

I understand now. I am new at this so thank you for your help. I will have my students start working on this first thing tomorrow. We are only a few weeks away from regionals and don't have a competitive plane yet so I am a little stressed.

[Draylon Fogg]

ok so weighting them is working out very well, i never thought of doing that before. thank you