Building techniques

[danyalukin]

Thanks for the info! I already have very good instructions on making rolled motorsticks from the book "Building and Flying indoor model airplanes" by ron williams, the only question was if it is worth the time trying. I guess i should try that...
I understand that all those nuances are to complicated to judge strictly anyways...
Thanks!

[jander14indoor]

From your nationals time, respectable, you have room to improve without getting too exotic.

Was your plane right at 7.0 gms? If you were far off, you can drop a lot of time just from the weight drop. Can you build right to a target weight everytime? If not, make sure you develop a technique to do that as you try more complicated things.
Is your plane nice and stiff? If not, work on wood selection. That alone can make surprising differences in plane trimability and consistent performance.
Can you wind close to max and back off just enough to skim the ceiling at any site you fly? In other words, how hard to you make the rubber work?
Does your trim approach level wing and very slightly nose down tail? Is everything controlling the turn done in balance and minimally? You'd be surprised how fast you can add drag and kill duration with wonky trim settings that fly, but...

Of course, if you are like me, and the building is the challenge, have at. My hollow motorstick is a rectangle just because I wanted to see if I could. I cover with grocery bags just to see if I can and still hit competitive times.

But don't underestimate the vital importance of carefully planned and executed test and evaluation! I couldn't examine the build questions unless I could do that well first.

Oh, and comment on rolled motor sticks. Ron's book is the bible! But the sticks he's building are for 1-2 gm airplanes. Make sure you beef up accordingly. I'd target 1.5-1.8 gm for even a hollow motorstick in Wright Stuff.

Jeff Anderson
Livonia, MI

[danyalukin]

Thanks, yeah, the problem i faced is at times a bowing motor stick, or one too heavy to keep at the minimum... Could you suggest the best density of the balsa for the motor stick and tail boom? Because when at school we order the supplies, the tech coach and i are not really sure what to buy... And im also not sure what to use for the wing (the spars). could you help me with the densities?
~daniil

[jander14indoor]

For solid motor stick I like 5-6 lb/ft3 balsa and use a short, large cross section stick for stiffness. We're talking 1/4 by 3/8 or more cross section here. Overall target weight for the stick is around 1.8 to 2.0 gm in your overall weight budget. Use the largest piece of low density balsa that hits that weight. You should also make several sticks and check for stiffness. Use the stiffest, throw out the rest.

For a hollow stick use maybe 7 lb/ft3 density balsa, A-grain. Again, make several and pick the best couple to build with. Same overall target weight.

For tail boom, use your lightest stuff, stiffness is important here, but less so than the stick.

If you are having trouble with stick bowing, make sure the issue is with the stick, not the prop hanger. I think a lot of problems folks attributed to stick bowing last year was really the Ikara prop hanger that comes with the Ikara props being too flimsy.

Wing spars can be much denser, say 8-9 lb/ft3 and still hit target.

Note, what you really need to do is to set up a bill of material for every part in your plane. Use this to build to weight by intention. I use a spreadsheet. Each row is a part. The columns have quantity, part dimensions, calculated volume, planned density, calculated weight. Include EVERY part! Prop wire, covering, GLUE! You'll have to estimate glue, use .5 to 1.0 gm till you have data. For the prop just use its actual weight, don't measure and figure density or anything. Now add up all the part weights to get the total plane weight, it should be 6.5 to 6.8 gms (just under the limit you'll add weight as you repair your plane, add clay to just be over limit when flying). If you are way off, change the density till you are at target. Then when you build, have a scale next to you and weigh every part to make sure its at the target weight (and associated strength/stiffness!).

Comment on buying balsa. Most places don't get too specific. And when you strip your sticks they'll vary hugely across the sheet depending on which growth ring it falls on. Get some contest grade balsa (density below 6-7 lb/ft3) and some regular balsa. Get extra, you'll need it. As your building skills (wood evaluation skills in this case) become advanced, more of the balsa becomes craft or practice material and less competitive quality.

Thanks,

Jeff Anderson
Livonia, MI

[calgoddard]

Jeff:

I agree, building a WS plane with a rolled motor stick should only be attempted by very experienced flyers that have already excelled at all other aspects of this hobby. Experimenting with rolled motor sticks is really for someone thinking about eventually competing in F1D or F1M. But they should probably try building a Penny Plane first.

Besides the complexities of rolling the sheet balsa around a suitable mandrel with waxpaper backing, a special jig is required to slice the rolled and cured sheet longitudinally before gluing the seam. Then the rear motor hook and prop bearing have to be carefully attached to web-reinforced ends of the rolled stick. Then a special jig is needed to properly drill the diametric holes for the tissue tubes so they are perfecly aligned. But before that, you have to know where to mount the tissue tubes to get the correct CG - as there is no way to change the CG by moving the main wing once the tissue tubes are installed. This can easily add another ten to twelve hours of building time, if you take into account the time needed to construct the jigs. Most flyers would be better off using this time to practice with a WS plane with a solid motor stick.

Over the years I have been involved in WS, I have seen about 20+ WS planes built and flown by our students, including one with a rolled motor stick. The best plane of them all did 4:04 in the 2008 Division C WS competion in a trecherous pyramid flying site with a large central suspended score board and 35 feet "flyable" ceiling on either side provided your plane circled tightly. That plane had a SOLID motor stick. Another plane had a geodetic wing rib configuration on both the main wing and stab primarily for a different look. Be aware though, if you simply angle your standard ribs, you will effectivly increase the camber. It is tricky to get the ends of the ribs correctly angled (mitered) where they join each other and the spar. Geodetic wing construction is not worth the trouble for WS, unless you are a very experienced builder and love the look. It can be very effective for certain outdoor planes where additional strength is needed.

BTW Jeff, I would be interested in your thoughts about whether the trailing edge spar of the main wing of a WS airplane should be made of lighter balsa, e.g. 5 - 6#, while using heavier balsa for the leading edge spar, e.g. 8#.

[danyalukin]

Thanks so much!

For a hollow stick use maybe 7 lb/ft3 density balsa, A-grain. Again, make several and pick the best couple to build with. Same overall target weight.

Are you sure that it's better to use A grain and not C grain? Ron William's book says to use C grain, and my feeling is that it is better if the grain runs parallel to the stick...

And for the spars, I used basswood last year and sanded it for it to be more aerodynamic, decreasing the cross section 1.5 times... Is it more efficient to use heavy balsa though?
And we order from the A2Z corporation...It has a pretty good selection.
Also, I had found a place with a rubber "kit" with the most common rubber sizes for SciO:
http://www.aeroracers.com/cgi-bin/shopp ... add=action
Do you think it is a good one to order? What i liked about it is ALOT of rubber (75 ft)

My other question is the advantage of vertical stabilizers on the wing's ends (not sure what are their names). Why might it be nesessary to have them?

And again, thanks for all that help!

~Daniil

[jander14indoor]

Jeff:
<SNIP>BTW Jeff, I would be interested in your thoughts about whether the trailing edge spar of the main wing of a WS airplane should be made of lighter balsa, e.g. 5 - 6#, while using heavier balsa for the leading edge spar, e.g. 8#.

Well you certainly don't want the reverse, likely to get wing flutter that way.

But I have to ask, what are you trying to achieve with differential spars? Saving weight? That kind of trick isn't really needed in WS. Increase wash in during flight for stability? Why not use a stiffer rear spar and build in the wash you want?

Personally, I like stiff planes that hold consistent settings for WS. I know the advanced classes like EZB and F1D take advantage of natural flex to help control the non-linearity of rubber power. But those things are so light they are going to flex and you might as well plan it in to take advantage of it. WS planes can be very stiff and still hit target weight. I think stiff planes are so much easier to fly that its a better way to go for all but a very few (if any) advanced competitors.

Thanks so much!

For a hollow stick use maybe 7 lb/ft3 density balsa, A-grain. Again, make several and pick the best couple to build with. Same overall target weight.

Are you sure that it's better to use A grain and not C grain? Ron William's book says to use C grain, and my feeling is that it is better if the grain runs parallel to the stick...

And for the spars, I used basswood last year and sanded it for it to be more aerodynamic, decreasing the cross section 1.5 times... Is it more efficient to use heavy balsa though?
And we order from the A2Z corporation...It has a pretty good selection.
Also, I had found a place with a rubber "kit" with the most common rubber sizes for SciO:
http://www.aeroracers.com/cgi-bin/shopp ... add=action
Do you think it is a good one to order? What i liked about it is ALOT of rubber (75 ft)

My other question is the advantage of vertical stabilizers on the wing's ends (not sure what are their names). Why might it be nesessary to have them?

And again, thanks for all that help!

~Daniil

C-grain is stiff perpendicular to the grain and harder to roll. For the VERY thin stuff used in F1D you can do it. For the 1/32 thick stuff for SO and relatively small diameter, you might get in trouble with the grain splitting. Try it if you like, but you may need a larger diameter to make it work.
For both A and C grain the grain will run parallel to the cylinder axis.

If you are getting to weight with basswood and your spars are stiff enough, great, but don't sacrifice weight for strength. And basswood isn't very good wood mechanically speaking. You'd be better with spruce if you want a stronger wood, or carefully selected balsa. Instead of thin, you might try wedge shaped balsa for better stiffness and aerodynamics.

That's a nice range of sizes in that rubber kit, but pretty pricey and not actually a lot of rubber. Only 12.5 ft of each size or 3-4 motors each. Barely enough to figure out which size is best, not enough to support an season of testing and competition. That much money will buy you a 1/2 pound each of super sport at www.faimodelsupply.com in 0.096 and 0.083 for over 150 motors of each size. Plenty for a season of testing.

Finally, vertical fences on wings. Well, you need some form of dihedral for a stable wing. For a flat wing, as long as the fence is mostly above the wing it serves that purpose and is needed. For a normal dihedral wing you don't need them for stability, MIGHT gain efficiency by controlling the tip vortices. But to do that right is tricky.

Frankly they got started as a way to cover wings flat with tissue and still have effective dihedral. That problem went away when they let you cover with other materials, but the tips have stayed around. Partly because some very good flyers like them, but I'm not convinced they are 'better'.

Thanks,

Jeff Anderson
Livonia, MI

[danyalukin]

That's a nice range of sizes in that rubber kit, but pretty pricey and not actually a lot of rubber. Only 12.5 ft of each size or 3-4 motors each. Barely enough to figure out which size is best, not enough to support an season of testing and competition. That much money will buy you a 1/2 pound each of super sport at http://www.faimodelsupply.com in 0.096 and 0.083 for over 150 motors of each size. Plenty for a season of testing.

So 1/2 pound of a rubber is 150 motors, if i understood you correctly? Should we than order 1/2 pound of 3/32 (.093") and 1/2 "New 2007 SciO" .083"?
Could you possibly know if the site is up to date and they have those motor sizes?

Thanks in advance!
Daniil

[jander14indoor]

Just checking my math, 1 lb = 454 gm, divide by two, divide by 1.5 gm gives... Well you do the math, exercise left to the student. 150 motors might be a little optimistic with waste and all, but far more than 20 or so motors you get in that kit.

As to how up-to-date the website is, the owner isn't real notorious for how web-savvy he is, but the rubber prices are generally fairly accurate as that's his main product. FAI Model supply is the source of Tan Super Sport after all, everyone else is a reseller. Anyway, always best to call or e-mail him before you order.

As to sizes, last year I had best success on slightly smaller rubber with the prop I had, 0.075 or so. But I like low pitch props which use skinnier rubber. Larger sizes will work OK. Need to set up the prop different, more pitch.

At some point you might want to look for a friendly indoor flier with a rubber stripper who can help you strip to custom sizes, or consider investing in a stripper. Unfortunately that's the one device there is no good way to cheap out on so I hate to lead students down that path. Last I checked prices were in $150 to $175 range.

Heck, ask FAI if they'll strip thinner than 0.083 that was optimum a few years back. Even if you pay a slight premium you'll save a lot over buying a stripper.

Note, get the Super Sport, not Sport, more energy storage.

Jeff Anderson
Livonia, MI

[danyalukin]

I see...Also, the A2Z Corp. has the the Tan super sport (i assume they resell it) and it is already cut to the sizes .095, .090, .085, .080, and we will not need anything greater or less than that, Im sure. Our team needs to start building soon, because we want to be more competitive this year, but the Fai model supply store is closing (9/12 to 9/23) so we are planning to order from A2Z.
The question is, will 3/32 (.093) be too strong, and Which sizes to choose from those listed above?

Also, Is ''TAN Indoor Contest Rubber ''(in catalog) the same quality as the ''Super Sport Rubber''? Are those just different names?
Those are the questions i have, and please keep in mind taht the .083 is not available at A2Z.

Thank you!
Daniil