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Wing Design Questions  
User currently offlineANITIX87 From United States of America, joined Mar 2005, 3302 posts, RR: 13
Posted (6 years 9 months 2 weeks 6 days 7 hours ago) and read 4460 times:
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Hello, Everyone. If this is in the wrong forum, please let me know and feel free to move it.

I am in a Mechanics class where our final project is to build a forward-swept airfoil.

We have been given a fixed cross-section, length, chord, and angle of sweep to provide for some consistency in judging which group wins. The airfoil, if any of you are familiar with it, is the Gottingen 398 design.

My group has decided to build our wing (the internal components) out of balsa wood since it is light-weight and [relatively] strong. The skin will be a plastic mold with heat-activated adhesive that will stick and shrink the cover to the shape of the wing. My question is the following.

Since forward-swept wings have massive issues with torsion and stability, we need the wing to be well-reinforced. Which aircraft out there have the wings with the best lift:weight ratios and would any of you be able to recommend a specific layout of ribs, spars, and other elements to strengthen the wing while providing a lot of lift and not sacrificing a lot of weight?

I was thinking of looking at some aircraft that are specifically made to be high-lift, like gliders, and light-weight GA planes (Cessna 152?) but I don't know if this is the best approach.

Any advice?

TIS


www.stellaryear.com: Canon EOS 50D, Canon EOS 5DMkII, Sigma 50mm 1.4, Canon 24-70 2.8L II, Canon 100mm 2.8L, Canon 100-4
16 replies: All unread, jump to last
 
User currently offlineSoon7x7 From , joined Dec 1969, posts, RR:
Reply 1, posted (6 years 9 months 2 weeks 6 days 6 hours ago) and read 4444 times:

Quoting ANITIX87 (Thread starter):

The Blanik L-23...glider I fly has forward sweep...If I remember correctly it has 31/1 glide ratio..published any way,I'll have to dble check, that but it is Czhecklaslovokian(think that is how you spell it) Built...it is all metal 'cept control surfaces are rag, but flies sweet...Jerry


User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 2, posted (6 years 9 months 2 weeks 6 days 5 hours ago) and read 4424 times:

You weren't given a performance envelope? Aircraft weight, max G load and airspeed range?

You will want to design the wing with a structural limit load consistent with maximum G at maximum gross weight.

By the sound of it the aerodynamical design of the wing is pretty much fixed. Not a lot of variables to play with. I can see washout and dihedral, and that is about it. Both of those will mean a disadvantage.

What is the measure used to decide on the best design? Sounds like you will have to lock either the weight of the design or the lifting force it shall be able to generate and then use the other as the measure.

The latest in forward swept wing design, last I heard at least, is using materials with directional properties. That's composites which will have different properties in different directions, meaning they can be flexible if you try to bend them one way and rigid if you bend them another way. Wood is actually a rather good composite material from this perspective.

At low speeds, you will not see much of the main drawbacks of FSWs I'd say. Nor of the advantages...

Rgds,
/Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineANITIX87 From United States of America, joined Mar 2005, 3302 posts, RR: 13
Reply 3, posted (6 years 9 months 2 weeks 6 days 1 hour ago) and read 4402 times:
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Fred, thanks so much! Answers to some of the issues you brought up.

The wing will be an independent item. It will be cantilevered to a support and put into a wind tunnel where the speed will be increased either to 90MPH or until it breaks (which ever happens first). There will be force gauges all over the wing to measure various parameters. The lift:weight ratio, the torsion, and the speed of failure of the wing will all be taken into account to rate the wings.

The exterior shape of the wing is fixed but the internal structure is up for any kind of variation we can come up with. We will be given details about AOA and other parameters later on, but there can be no dihedral, gulling, camber, or any other kind of variation from the Gottingen airfoil.

Quoting FredT (Reply 2):
You will want to design the wing with a structural limit load consistent with maximum G at maximum gross weight.

What exactly does this mean?

Quoting FredT (Reply 2):

At low speeds, you will not see much of the main drawbacks of FSWs I'd say. Nor of the advantages...

I thought FSWs were very unstable at low speeds, no?

TIS



www.stellaryear.com: Canon EOS 50D, Canon EOS 5DMkII, Sigma 50mm 1.4, Canon 24-70 2.8L II, Canon 100mm 2.8L, Canon 100-4
User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 4, posted (6 years 9 months 2 weeks 5 days 19 hours ago) and read 4378 times:

Quoting ANITIX87 (Thread starter):
My group has decided to build our wing (the internal components) out of balsa wood since it is light-weight and [relatively] strong. The skin will be a plastic mold with heat-activated adhesive that will stick and shrink the cover to the shape of the wing. My question is the following.

Since forward-swept wings have massive issues with torsion and stability, we need the wing to be well-reinforced. Which aircraft out there have the wings with the best lift:weight ratios and would any of you be able to recommend a specific layout of ribs, spars, and other elements to strengthen the wing while providing a lot of lift and not sacrificing a lot of weight?

For starters, I would recommend you use something other than plastic for the skin. Spars primarily carry bending loads and ribs carry aero loads (and shorten the buckling length of stringers if you use them) but virtually all of the torsional resistance is provided by the skin. A straight plastic will probably have such a low G that your torsional rigidity will suffer. A composite with the fibers at +/- 45 degrees will give you far higher torsional rigidity.

Balsa is certainly a good choice as a core material, although honeycomb would probably be great if you can get it. Depending on the size you have to build, I have seen foam-core CFRP skin wings (no internal reinforcement) with amazing lift/weight.

Quoting ANITIX87 (Reply 3):

I thought FSWs were very unstable at low speeds, no?

FSW's, unless you do some very strange things to make the wing highly anisotropic, tend to increase angle of attack with twist, which is an unstable mode. That's why torsional rigidity is so important. As far as I know, it's not particularly related to low speed.

FSW aircraft also tend to be statically unstable but, for the setup your describing, that's not really an issue you have to deal with.

Tom.


User currently offlineANITIX87 From United States of America, joined Mar 2005, 3302 posts, RR: 13
Reply 5, posted (6 years 9 months 2 weeks 5 days 12 hours ago) and read 4343 times:
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Thanks for all the great advice, guys.

If any of you are in the industry, is there any way I can get your information (name, position, company, contact, etc) so that I can cite you guys in my report as if I interviewed you? It doesn't have to be on the forum, a PM would be fine, and only if you're willing, I realize it's a random request and not one you have to comply with at all. It would just be a cool additional source to have.

Quoting Tdscanuck (Reply 4):

For starters, I would recommend you use something other than plastic for the skin.

We have no choice in this matter, I'm afraid. It has to be the certain clear plastic he assigns so that the wing's internal structure can be evident for analysis and so that all the groups have the same surface. He did warn us about buckling due to the skin shrinking. I don't know if plastic has directional fibers, but if so, what I may be able to do is rotate the plastic 45 degrees to attempt to align the fibers at an angle? Just thinking out loud.

Quoting Tdscanuck (Reply 4):
FSW's, unless you do some very strange things to make the wing highly anisotropic, tend to increase angle of attack with twist, which is an unstable mode. That's why torsional rigidity is so important.

Firstly, what does "anisotropic" mean? Also, to deal with the issue of torsion and increase AOA with twist, we are planning on building up the front of the wing a lot (even considered a completely solid D-box up front) to move the shear center up towards the 1/4 chord point to deal with the moment caused by the lift on the wing. Would this work?

TIS



www.stellaryear.com: Canon EOS 50D, Canon EOS 5DMkII, Sigma 50mm 1.4, Canon 24-70 2.8L II, Canon 100mm 2.8L, Canon 100-4
User currently offlineWPIAeroGuy From United States of America, joined exactly 7 years ago today! , 240 posts, RR: 0
Reply 6, posted (6 years 9 months 2 weeks 5 days 10 hours ago) and read 4323 times:

Isotropic means material properties are the same in every direction, anisotropic means that the properties vary. Fiberous materials are generally anisotropic, like wood. Balsa is much easier to bend when the bend is along the grain rather than perpendicular to it.

Sounds like you're following the same building structure as an R/C airplane, including the heat-shrink convering. From every plane I've build the LE an TE are sheeted with balsa, only from about the main spar back to an 1inch off the leading edge is open ribbing. Those wings are incredibly strong if built well, even R/C jets which push 200 mph and more are made of balsa. I've seen lots of planes go down due to structural failure, however every one involving the wing was due to the entire wing separating from the fuselage where it is bolted on. Granted, I don't recall any FSW aircraft, but I know there are some. If you are being judged on weight, get a good cyanoacrylate CA instead of epoxy as its much lighter and will wick into the wood. If you want to look at wing designs look up plans for R/C aerobatic aircraft such as an Extra 300, Cap 232, or Edge 540. The forces on those wings are insane (it took 5 years of 1080*/s snap rolls to finally break the bolts on my Cap 232). I'm sure there are plenty of plans online, but it sounds like the quality of your build is going to be more important than a design as those parameters are pretty much chosen.



-WPIAeroGuy
User currently offlineANITIX87 From United States of America, joined Mar 2005, 3302 posts, RR: 13
Reply 7, posted (6 years 9 months 2 weeks 4 days 23 hours ago) and read 4254 times:
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WPIAeroGuy (which I assume means Worcester Polytechnic, the school I almost went to) thanks so much. I checked out those models and I learned some very interesting things.

I also searched for FSW R/C or flying scale models and found nothing. They might just be too unstable for light-weight R/C aircraft since drag is proportionally greater at smaller sizes, which could increase torsion.

TIS



www.stellaryear.com: Canon EOS 50D, Canon EOS 5DMkII, Sigma 50mm 1.4, Canon 24-70 2.8L II, Canon 100mm 2.8L, Canon 100-4
User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 8, posted (6 years 9 months 2 weeks 4 days 21 hours ago) and read 4241 times:

Quoting ANITIX87 (Reply 5):
Quoting Tdscanuck (Reply 4):

For starters, I would recommend you use something other than plastic for the skin.

We have no choice in this matter, I'm afraid. It has to be the certain clear plastic he assigns so that the wing's internal structure can be evident for analysis and so that all the groups have the same surface. He did warn us about buckling due to the skin shrinking. I don't know if plastic has directional fibers, but if so, what I may be able to do is rotate the plastic 45 degrees to attempt to align the fibers at an angle? Just thinking out loud.

Bummer, but that's real engineering...some constraints you just have to live with.

A cool project to optimize under this constraint would be to do some basic material properties tests on the plastic film and find out if it's actually isotropic or if it's got a bias. If it does, then definitely align the film so that the stiffest tensile direction is at +/- 45 degrees to the wing's neutral axis. You might also be able to find some way of doing the heat shrink to enhance the properties in one direction over another.

Quoting ANITIX87 (Reply 5):
Quoting Tdscanuck (Reply 4):
FSW's, unless you do some very strange things to make the wing highly anisotropic, tend to increase angle of attack with twist, which is an unstable mode. That's why torsional rigidity is so important.

Firstly, what does "anisotropic" mean? Also, to deal with the issue of torsion and increase AOA with twist, we are planning on building up the front of the wing a lot (even considered a completely solid D-box up front) to move the shear center up towards the 1/4 chord point to deal with the moment caused by the lift on the wing. Would this work?

WPIAeroGuy beat me to it...anisotropic just means "not isotropic." Most basic material classes deal only with isotropic materials...you can characterize everything with just two material constants (E & G) and the properties don't depend on orientation. However, most of the really interesting aero materials are highly anisotropic (up to 36 unique material constants), which means it's not just material choice, but material orientation, that matters.

Tom.


User currently offline2H4 From United States of America, joined Oct 2004, 8955 posts, RR: 60
Reply 9, posted (6 years 9 months 2 weeks 4 days 19 hours ago) and read 4216 times:
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Quoting FredT (Reply 2):
At low speeds, you will not see much of the main drawbacks of FSWs I'd say. Nor of the advantages...

Would he not see the advantage of aileron authority into the stall?

2H4



Intentionally Left Blank
User currently offlineANITIX87 From United States of America, joined Mar 2005, 3302 posts, RR: 13
Reply 10, posted (6 years 9 months 2 weeks 4 days 18 hours ago) and read 4214 times:
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Quoting 2H4 (Reply 9):
Would he not see the advantage of aileron authority into the stall?

Probably, as FSW help lower stall speed, but our airfoils won't have ailerons, flaps, or anything else to change the camber on the wing.

TIS



www.stellaryear.com: Canon EOS 50D, Canon EOS 5DMkII, Sigma 50mm 1.4, Canon 24-70 2.8L II, Canon 100mm 2.8L, Canon 100-4
User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 11, posted (6 years 9 months 2 weeks 4 days 10 hours ago) and read 4174 times:

Quoting ANITIX87 (Reply 5):
Also, to deal with the issue of torsion and increase AOA with twist, we are planning on building up the front of the wing a lot (even considered a completely solid D-box up front) to move the shear center up towards the 1/4 chord point to deal with the moment caused by the lift on the wing. Would this work?

Sorry, I forgot to answer this part last night.

That certainly sounds like it would help. If you can get the shear center ahead of the center of pressure then torsion will want to decrease AoA with increasing loading, which is a good thing.

Tom.


User currently offlineANITIX87 From United States of America, joined Mar 2005, 3302 posts, RR: 13
Reply 12, posted (6 years 9 months 2 weeks 4 days 5 hours ago) and read 4141 times:
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Quoting Tdscanuck (Reply 11):
That certainly sounds like it would help. If you can get the shear center ahead of the center of pressure then torsion will want to decrease AoA with increasing loading, which is a good thing.

Ok, so we'll probably go with a front-heavy design.

I spoke to my professor again and he gave me the following information, if this changes anything.

The plastic covering is isotropic so will give no benefit if used in different directions.

The wing will be tests at a positive AOA of at most 20 degrees, apparently, though it will probably be less. This requires a wing that is also very strong along the span to resist bending.

My group does have a very simple preliminary design, and if I can find a good scanner I'll post the image here a little later.

TIS



www.stellaryear.com: Canon EOS 50D, Canon EOS 5DMkII, Sigma 50mm 1.4, Canon 24-70 2.8L II, Canon 100mm 2.8L, Canon 100-4
User currently offlineWPIAeroGuy From United States of America, joined exactly 7 years ago today! , 240 posts, RR: 0
Reply 13, posted (6 years 9 months 2 weeks 3 days 19 hours ago) and read 4114 times:

Another suggestion: using heat-shrink covering such as Monokote can be a real pain in the ass if its the first time you're using it. I'd go to your local hobby shop and see if someone can help you with the covering part (if thats allowed in the assignment) because otherwise your most likely going to end up with a lot of patches and wrinkles and overlaps, all which will degrade your peformance. Even he can give you a demonstration that would be helpful, and you're most likely going to rip it all off several times before you get it right. Also, it does very little in regards to strength, at best it keeps fuel from soaking into the wood and helps keep all your crashed pieces of balsa stuck together Smile Good luck on your project, I'm interested to see how it turns out.

-WPIAeroGuy



-WPIAeroGuy
User currently offlineFlipdewaf From United Kingdom, joined Jul 2006, 1568 posts, RR: 1
Reply 14, posted (6 years 9 months 2 weeks 3 days 1 hour ago) and read 4071 times:
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Id go with the 45% thing but im not sure that the plastic will have a direction in which it has higher tensile strength will it? Id use simple stiffners in the inside of the skin. First you'll want to do some calcs to work out the forces you are going to be dealing with and then integrate the forces and all that jazz to get the bending moment and compression forces (in the upper skin id assume for this) and derive equations for the skin buckling load and stiffner buckling load and then force these to equal the max compression load and get iterating with your equations on excel to find the lightest possible that will work, then make sure it wont go in yield and euler buckling. Check it out in Megson, there'll be some good stuff in there.

Fred


User currently offlineANITIX87 From United States of America, joined Mar 2005, 3302 posts, RR: 13
Reply 15, posted (6 years 8 months 2 weeks 5 days 20 hours ago) and read 3846 times:
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Ok so we finalized our design, built it, and tested it today. I'll get the wing back next week and will post photos of it. The lightest wing in the group was 20 grams, the heaviest was 48 grams. Ours came in at a respectable 30 grams, and, based on the dimensions (40cm long, 12cm chord) was pretty light, especially considering how much glue we used. We tested the wing at a 10 degree AOA, and the tunnel was set at 45MPH. The leading edge of the wing deflected 0.7 cm, and the trailing edge deflected 0.8cm, resulting in virtually zero twist (we had the second-best wing in that regard, as one group's wing was so heavy it didn't budge at all). Overall our wing performed admirably, though there were some problems, such as...

Quoting WPIAeroGuy (Reply 13):
because otherwise your most likely going to end up with a lot of patches and wrinkles and overlaps, all which will degrade your peformance

Our Monokote was terribly applied. I was the only one in the group who'd done it before, but the night of the Monokote application, I had a music rehearsal and my group members ironed it on incorrectly, resulting in wrinkles everywhere. Thankfully, the wrinkles only occurred at the top of the wing, which may have added a great deal of down-force on the airfoil.

Overall, we are quite happy with the wing, and wanted to thank everyone for your great advice. Again, I will post shots of it sometime before December 14th.

TIS



www.stellaryear.com: Canon EOS 50D, Canon EOS 5DMkII, Sigma 50mm 1.4, Canon 24-70 2.8L II, Canon 100mm 2.8L, Canon 100-4
User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 16, posted (6 years 8 months 2 weeks 5 days 19 hours ago) and read 3835 times:



Quoting ANITIX87 (Reply 15):
Overall, we are quite happy with the wing, and wanted to thank everyone for your great advice. Again, I will post shots of it sometime before December 14th.

Congratulations on what sounds like a pretty good performance! Seeing the photos would be fantastic.

Tom.


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