Flight Times

[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

[Hamtown009]

Our plane flew a 1:46 in a gym that is about 24 feet tall. Hit the ceiling and continued... Are there any tricks to maximize times in a plane? Our weight is 7.6 grams. We used the Freedom Flight Models 2010 kit. Any suggestions?

[jander14indoor]

Good progress so far. 1:46 is pretty good, your plane flys. Key to longer times becomes planned experimentaion and is where your flight log is critical to sort out what works, what doesn't.

No tricks though. Need to adjust trim to minimize drag vs lift (typically this means flying on edge of stall, with minimum difference between wing and tail angle of attack and rearward cg that's still stable). Need to match rubber to prop to maximize motor duration. Winding to torque vs turns is an advantage.

Of course losing that extra .6 grams will help, but there is more time in your current plane.

Give us some details of your current trim, prop and rubber and we can give some directional things to work on.

Jeff Anderson
Livonia, MI

[Draylon Fogg]

YAY PLANES ARE WORKING NOW!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

[Hamtown009]

Here are some specific details--- PROP: widest width - 3.5 cm length - 19.9 cm RUBBER: our best time was with a .095 rubber band at 80 winds after a dewind of two, trying to experiment with others too. What do you mean by asking us to measure the trim?

[jander14indoor]

That sounds like one of those fat, high pitch, Ikara props. The true experts (trust me, there are folks who outfly me consistently, but not by a lot) like them. But I've found fat, high pitch props to require real expertise to get the best out of them, and I don't use them without lowering the pitch significantly. When coaching students I find much faster progress with a lower pitch prop and longer thinner rubber. You might try that.

What's the multiplication of your winder? I don't know if 90 cranks is a lot or under wound for a 0.095 motor. Do you measure torque? How close is that to breaking your motor? As I've said in the past, I often see otherwise very good planes underperform because the students don't WIND their motors. The motor is your fuel tank as well as your engine. This is a duration event, if you don't put every bit of energy in that it can store and then get it all back out, someone who does will beat you everytime.

Trim. Where's your cg with respect to your wing (either leading or trailing edge)? What angle of attack are your wing and tail at? Prop offsets? Wing offsets? Wing twist? Tail offset? Etc. In flight, is your plane on the edge of a stall, or over stabil? Dutch roll? Flight attitude (nose or tail high, wings level, rolled into or out of turn). Also, etc.

Hope that helps,

Jeff Anderson
Livonia, MI