hawk426

Working on an R/C plane design, and so far I've been able to get rib spacing and just about everything else settled... but I can't seem to find any guidelines for where the stringers need to be placed to keep the microlite covering from warping in flight (microlite because weight is a concern). Any suggested reading, or recommendations from experience? I'd rather avoid sheeting if I can help it, since that'd be rather heavy compared to a few sticks of balsa between each rib.

Relevant (I think) specs: Wingspan is 62", chord is 10" (2.5" of which is control surfaces), and it's intended max speed is ~70mph. Ribs are currently spaced 5" apart, with a fuselage that takes up the center 2". And currently the LE is a 1/8" stringer that I plan on sanding down to match the airfoil.

Thanks in advance for any help.

longdan

Sounds like a bit of a weak design to be honest - a 70 mph plane with a 1/8" leading edge and 5" rib spacing? Sounds like some kind of lightweight 3D plane.

When you say 'stringers' do you mean spars? How many 'stringers' are you planning on putting in the wing? Is it a D-tube wing? What type of design is it? Need a bit more info before any advice can be offered.

cutaway

1/32 sheeting doesn't weigh much, and the airfoil shape integrity will be vastly improved...I'm just saying.

I've built a number of very light C/L planes with 1/32 sheeting and will attest to its ability to survive 70mph. Super light is great, and there's nothing wrong with building a plane to fly rather than survive impacts, but as a practical matter it also needs to be sturdy enough to handle/transport unless you build a transport box for it like the indoor rubber powered microfilm guys do.

BMatthews

I agree with longdan. You're structure as described is far too weak to expect those sort of speeds to not produce a problem. I'm the first to agree with you that "Light Is Right and Lighter Is Righter" but I would never builds a wing with as light a structure as you're describing for a model that is intended for even half the speed you're intending.

Double up on the ribs for starters. 5 inches apart is insufficient to supply enough structural rigidity and strength for holding the leading edge, spars and trailing edges together well enough to not be too flimsy. Especially on a faster moving model that'll be hitting over 35 to 40 mph. It's also not enough to support the covering well so you run the risk of it drumming or fluttering like a clarinet reed at speed.

Higher speeds means that you will have greater G loads when you pull full up for snappy looping maneuvers or sharp direction changes at speed. This requires a well engineered spar. The normal way to produce a strong and rigid spar is to form an I or box section with strong flat upper and lower caps joined by vertical grain webbing to lock the two caps together. The ideal way is to put the webbing in between the spar caps to form an I shape. But a lot of us "cheat" a little and glue the webbing to the vertical faces to produce a ] shape instead. Or in some cases such as the first 1/4 of the span use front and rear webs to form a full box section where the stresses are the highest.

Ailerons that wide generally imply a 3D model that flies at slower speeds much of the time. Full span ailerons of that width WILL be prone to flutter at higher speeds. But if you're actually using conventional partial span ailerons out in the tips then it's a lot more tolerable. Still, 25% of the chord is pretty darn big for ailerons on a higher speed sort of model. The response is going to be pretty twitchy unless you use low rates for the higher speeds and then cut to high rate for the slow stuff.

Such a structure can still be kept very light so don't feel like you're overbuilding the model. What you're doing by making it to our suggestions would be building a wing that is still light for the size and purpose but that will be strong enough to actually survive more than the first flight.


I have to also wonder about your numbers you gave. By asking the questions the way you've presented them I strongly suspect that you have not done much flying or designing up to now. Could you suggest more information on why you think your model will fly this fast and what you want it to do? The speed you're considering implies a racing model or fast sport model. But the questions about construction and choice of covering implies a slow park flyer. Some more information on the model and power package would aid us in better understanding what you are hopeing for the model to do and point you in the best direction with a bit more clarity.

hawk426

Sorry I was a light on details... Lets see:
First, thanks for the suggestions. Yes, I'm new at designing things, hence the questions... Aero Engr. classes help, but they don't go very in-depth on things like this, especially about things of this scale or with these materials. As for your questions:
-In terms of nomenclature, yes by stringers I mean spars/longerons. The main spar is an I-beam, 1 1/4" wide x 1" tall, w/ 1/8" thick webbing. From the stress #s I've calculated, it should be plenty sufficient to carry the flight loads intended. The pieces I'm asking about are generally intended to prevent skin deformation.
-Yes, the ailerons are only partial span on the outboard end. The rest is flaps (yes, that's huge... it's necessary. ).
-To explain the discrepancy in purposes, this plane is being designed for a competition ([link=http://www.aiaadbf.org]AIAA Design, Build, & Fly[/link]) - one mission requires the team to carry a relatively heavy payload (but has no time requirement) while another has a time requirement, but no payload. Hence, the numbers the team I'm on came up with look wacky, because they have to be able to handle two very different performance profiles. The 70mph speed is a preliminary speed we're aiming for based on the electric current limitations imposed in the contest rules. Since this is a competition design, I don't want to give away too much just in case our competitors can read... so yeah. The hand-launch requirement is why the wings are massive compared to what this would be if it could take off normally: it's not going to be that heavy (sorry, no weight #s as that's one of the main components of our score [sm=47_47.gif]), but we can't use any kind of device to assist in launching the plane so there are higher takeoff lift requirements than would be normal for the plane size we're designing.
-Re-reading things (and actually looking at a ruler and thinking things through [sm=72_72.gif]) I do agree that the LE spar is probably going to be too small, and I'll probably wind up bumping that up to at least 1/4".

Overall, I guess I'll have to re-work the model I have and see how it works out strength and weight-wise if I sheet from the LE to the spar and shrink the rib spacing. I'll probably wind up building a couple different test sections to check out different configurations anyways, but your suggestions are very much appreciated, as you all undoubtedly have more experience with this than I do.

Lnewqban

Some good reading here:

http://www.airfieldmodels.com/inform...nstruction.htm

I recommend you to study some published manuals for kits of similar dimensions to your plane's:

http://manuals.hobbico.com/gpm/gpma0952-manual.pdf

http://manuals.hobbico.com/gpm/gpma0968-manual.pdf

http://manuals.hobbico.com/top/topa0210-manual.pdf

http://manuals.hobbico.com/gpm/gpma0236-manual.pdf

http://manuals.hobbico.com/top/topa0155-manual.pdf

BMatthews

Sorry to tell you this but you're not the first one to come online with this topic.

http://www.rcuniverse.com/forum/m_99...tm.htm#9970684

The discussion was primarily generic so there's nothing in the other thread giving away anything of significance. But I think you'll find that the thread will serve as a reality check for your team on where the priorities shoudl be placed. In particular the part about scrutinizing and drafting the biggest, burliest and longest throwing football scholarship guy you can find for launch duty....

Be careful with spar caps of such a wide size. Support from the webbing only extends out from the edge and into the caps so far. Making them that wide with front and back webs means that the middle third of the cap is not really well supported. And if it's a single I web in the middle then the outer third on the edges again isn't fully supported and is mostly just dead weight. You'd be better off with a higher tensile and compressive material of narrower section that is more properley supported by the web being under more of the material. Or split the spar into two with one at around 25% of the chord and the rear at around 65%.

Your desire to keep the wing as light as possible shows that you've got your priorities in the right place. Similarly the size you've picked for the wing speaks volumes about recognition of some of the factors related to having to hand launch the model. Although by limiting the span you're hamstringing your efforts. Going a bit bigger to a 6 to 6.5 foot span is still going to produce a model that is hand launchable. And the extra wing area will be welcome at lowering the stall speed to allow for more load being able to be carried. The added weight for the tips is really minor and the gains to be had at the critical launch time are far higher than any speed disadvantages that would come with this modification.

hawk426

Lnewqban: thanks for the suggestions and links. To be honest, I have been semi-conciously drawing ideas for the design I have from the general layout from some of the models I've built, but didn't really think about going out and looking through other plane's construction manuals/plans.

BMatthews: Interesting thread, thanks for the link. Unfortunately, our school doesn't really have a football team... maybe one of the hockey guys would suffice?[sm=lol.gif] Somewhat nice to see that the hand-launch requirement gave others a headache too; it took us about almost a month to nail down a way to deal with that.
As for the wing span, unfortunately while I agree a larger wing would aid in takeoff we also have to break the whole plane down into something that'll fit inside a commercially available carry-on suitcase (not an "equipment case") w/ total outside dimensions (L+W+H) of 45", with a max of 22" for any one dimension. On the positive side, that makes it easier to get the plane to competition than last year, when the box was a 4ft x 2ft x 2ft. On the negative side, based on those limitations we can't really increase the plane's dimensions from where it is without inventing a bag of holding .
In regards to the spar caps, I'm not entirely sure I'd agree that the edges are unsupported, since the ribs will act as a full-width webbing every 5 inches as its designed right now. That, and if we're maneuvering the plane hard enough that buckling occurs over a distance of 5 inches I'm pretty sure we've exceeded our originial design conditions.
In general though, thank you for the kind words. It's good to hear we're going in something resembling the proper direction.

To anyone else who replies: Thank you for your suggestions... I might reply eventually, but I kinda need to stop working on the fun project and get back to working on the less fun homework and other projects. At this point, I think I've got a pretty good idea on what we need to do and we'll probably wind up building a bunch of test sections to see what works best for what we're doing. Thank you to those who've replied so far for the advice, as its given me several ideas for how to go forward that I wouldn't have gotten on my own. Hopefully, in a few months we'll have something positive to post about (aka the plane wasn't a huge lawn dart), and maybe eventually some more detailed specifications and pictures.

BMatthews

Ah, but 5 inches is a lot of a gap where the wood or other material can bow inwards under the buckling or tensile pressures as the wing flexes. The spar caps are bending at the same time they are being loaded in tension and compression after all. This can lead to the material failing before you know it.

Having said this I don't know what you're using for the spar caps since you didn't mention that. All I'll suggest is that you make up samples of the spar and bend them to destruction and see if this long unsupported issue is going to trap you.

And as I mentioned already you'll want to double the number of ribs to aid in better supporting the physical parts of the wing as well as supporting the covering better to help ensure a truer airfoil shape.

If you don't use sheeting for the wing to best support the covering and provide for a more true airfoil shape at least use mutiple turbulator spars that will aid in supporting the covering instead of ending up with a "starved dog" look. Especially with a 5 inch rib spacing the airfoil between the ribs would resemble a short triangle far more than the true airfoil it is supposed to be. More ribs and adding sheeting or turbulator spars will greatly aid in keeping the airfoil shape much more accurate.

On my own contest designs where lift counts and weight is important I still go with ribs spaced no further apart than 2 inches. And I tend to prefer 1.5 inches AND still use turbulator spars. The wing is important to the performance so it's worth budgeting a little more weight allowance for its structure to ensure that it's optimum.

iron eagel

If you wanted a longer span and the size restriction is an issue, I have a Bellanca airbus (also called the flying "W") which has the 10 foot wing built in three sections. There is a center section and two wing panels that join to it, that is an approach you could use to get a longer span and still keep it transportable in the space available. I join the center section to the fuselage and then put on the balance of the right and left wings. The flaps are in the center section. and wires are run out to plug in the ailerons that are in each wing panel. While it take a bit of time to put together each wing section is just over 3 feet in length. If you wanted to be able to get more span and still transport it this may be an alternative.

I agree with Mr Matthews, the 5 inch wide bays will be an issue unless you spar caps and webs are bullet proof, as well as sheathed with no less than 3/32ths balsa if payload is a factor, and the major structure of the wing is balsa (spars, ribs, webs).

jonastecknare

How about you go another way and cut the wing from Pink foam an hollow it properly! Some carbonstrips as spars. You should be able to avoid the "profiledeformation" of a builtup wing!
Jonas

Laird SS

I too was wondering why nobody had suggested the use of carbon fibre spars. They are incredibly strong and light.