UAV Airfoil selection help

[ashray]

Hi
Building a UAV airframe to carry 20kg (for SAR at uni) and do short-field takeoffs. I have the following figures:
Weight:20kg
Span: 3.2m
CHORDroot:350
CHORDtip:220
AR:11
Wing loading: ~65oz/sq.ft
Winglets, Slotted flaps, Flaperons present
Fuse dia:250mm
Fuse length:2m
Powerplant:52cc Gas 2stroke/ 1 cyl
I had chosen a MH32 but need some expert guidance.

Thanks
Ashray

[builder]

You are asking for trouble using such a thin airfoil with the chord sizes you have chosen. Even at the root your wing would not be much thicker than an inch, and you would have to build a very strong spar to be able to handle the weights you are expecting. You should be looking at a 11 percent thick wing minimum, better to be in the 12- 15% range unless you increase the chord lengths, this would also have the added benefit of bringing your wing loading down.

Also; highly tapered wings may be sexy, but are not really necessary for good performance. The high amount of taper you have chosen increases your chance of tip stalling at low speeds - if you must have wing taper, I suggest you do not use less than 75% of the root chord for the tip.

A good place to start is maybe the NACA 4412 or 4415, if you have an airfoil program to compare similar airfoils, this would help you a lot in making a decision. Try "XFLR5" It's freeware.

[robin andrew]

Hi there, don't underestimate the bending forces at the wing centre because of the large span. We all agree with the other writer about 12% min wing thickness allowing a twin carbon firbre joiner tubes system. The second joiner is to allow the load of the flaps spar to be taken otherwise you get a twisting motion at the centre under load (sudden turns). Only a triful of diehedral and then the ailerons will work well; dont be afraid of a bit of fuselarge depth under the wing (side area) which helps well during banking flight, Best of luck, Robin Andrew Uk.

[Wendi Smol]

Hi Ashray

from your desription I gather that you want to compete in the outback challenge in Australia. I have not found a date for 2009, but think it will be again around September. So there is no cause to rush into building a dud.
In the 2008 comp. no one team actually succeeded to rescue the poor Outback-Joe.

I think you should get an experienced model designer-builder into your team.
Your estimates for the weight are way over the top. You are supposed to deliver a 0.5 liter bottle of cool drinking water to the guy (perhaps two) after you found him. And you have one hour for the task. Taking the load and the delivery system to weigh around 2 kg the total weight of your plane should be no more than 8 - 9 kg.
Otherwise I aggree with "builder" and Robin.
The best program to compare various profiles at different Re-numbers is in my opinion "Profili", specially if you take the cost into consideration.
Have you thought that your fuselage will have a volume of more than ten gallons - and the appropriate surface area (drag). For the equipment you will need a space of around 4"x4"x20" and the load will need around 4"x8"x 10" so the fuselage should be a lot smaller and lighter.

Wendi UK

[builder]

I agree with Wendi, if you are planning to use it for the outback challange your weight estimates are double what they should be. A good example of this would be Top Models "Miss Morava", an all up weight of 8 kilos for this large model.
http://www.topmodelcz.cz/index.php?&desktop_back=eshop&action_back=&id_back=&desktop=eshop&action=zbozi_detail&id=691
By the way, the airfoil they list is incorrect, the sample I have seen has a 15% thick airfoil, probably the Naca 4415, not the 4412 they have listed.
I have developed a 13% thick airfoil you can try out, it requires you have some building skills to produce a fully skinned wing with a fairly thin trailing edge to get the most out of it. An obechi sheeted vacuum bagged foam core wing with a good spar would do the trick.
The airfoil was designed with Profili. (also my favorite) I have been using it for a number of years and so far, I find my actual flying results are very close to the predicted data. If you are interested in the program, Stefano in Italy can give you a password to unlock the program for 10 euro.
Also, if you or other people are interested I can post the airfoil.

[Wendi Smol]

Ashray hasn't been here since the 3rd of Jan. Have I put him off?

Wendi UK

[airmcn_3]

Ashray hasn't been here since the 3rd of Jan. Have I put him off?

Wendi UK


This should not be a surprise; many people on here make one post and then disappear because they did not hear what they wanted. I am sure it was not you that turned him away just reality........

Chris

[Wendi Smol]

This should not be a surprise; many people on here make one post and then disappear because they did not hear what they wanted. I am sure it was not you that turned him away just reality........Chris

Hi

that seems to me quite illogical - why ask if you might not like the (quite sensible) answer?? :eek:

Wendi UK

[zitron]

Hi

that seems to me quite illogical - why ask if you might not like the (quite sensible) answer?? :eek:

Wendi UK


Your assumption that people are logical is in itself, illogical. ;)

Cheers,
-Z-

[airmcn_3]

Hi

that seems to me quite illogical - why ask if you might not like the (quite sensible) answer?? :eek:

Wendi UK


Sorry Wendi but I have been on here since this forum started; I am just speaking from experience of watching hundreds of individuals do the exact same thing........

I never said the question was not a good one, what i said is when some individuals come on here and don’t get the answer they want they magically disappear never to be seen again.

Cheers,

Chris

[rich smith]

Some are just intimidated by "strong" opinions. I can empty a room in minutes :).

FYI Raskins 4-40 undercambered has no equal in terms of lift/weight and short field.

[Wendi Smol]

Sorry Wendi but I have been on here since this forum started; I am just speaking from experience of watching hundreds of individuals do the exact same thing........
I never said the question was not a good one, what i said is when some individuals come on here and don’t get the answer they want they magically disappear never to be seen again. Cheers, Chris

Hi Chris

as you may have noticed I'm quite new here and still have to figure out how this board ticks ;) . Must be more than just the sum of it's contributers.
Ashray has been here on the 17th, so perhaps he is waiting for us to come up with an answer to his problem instead of only putting his "image" onto the scrap.
I found that there are quite a few aspiring students in the various sub forums here who would like to produce a credible UAV. Most of them seem to have a mechatronic (what a word!) background but lack design capability in the air frame and particularly propulsion areas.
It's exactly the other way round in our little team: we have not enough expertise in the electronic sector, which is one of the reasons for me to have a look around here.
Rich Smith, you have to forgive me but who is (or was) Mr (or Mrs) Raskins?
It may well be possible that that profile "has no equal in terms of lift/weight and short field" (pretty nonsensical wording - yet I know what you mean :rolleyes: ). But under-cambered? That will not cut the corner for this application. This is not going to be an A1 free flight glider :D !
What one really needs here is a profile which is thick enough to accommodate a decent spar, a high Cl at low Re-numbers and a low Cd at small Cl and high Re-numbers. We are also looking for a benign stall at all Re-numbers between say 120000 and 500000. For everyday use it must also be immune against dust and rain and it would be beneficial if flaps could be used.

This is not a strong opinion but a technical necessity.

All the best

Wendi UK

[airmcn_3]

Hi Chris

as you may have noticed I'm quite new here and still have to figure out how this board ticks ;) . Must be more than just the sum of it's contributers.
Ashray has been here on the 17th, so perhaps he is waiting for us to come up with an answer to his problem instead of only putting his "image" onto the scrap.
I found that there are quite a few aspiring students in the various sub forums here who would like to produce a credible UAV. Most of them seem to have a mechatronic (what a word!) background but lack design capability in the air frame and particularly propulsion areas.
It's exactly the other way round in our little team: we have not enough expertise in the electronic sector, which is one of the reasons for me to have a look around here.
Rich Smith, you have to forgive me but who is (or was) Mr (or Mrs) Raskins?
It may well be possible that that profile "has no equal in terms of lift/weight and short field" (pretty nonsensical wording - yet I know what you mean :rolleyes: ). But under-cambered? That will not cut the corner for this application. This is not going to be an A1 free flight glider :D !
What one really needs here is a profile which is thick enough to accommodate a decent spar, a high Cl at low Re-numbers and a low Cd at small Cl and high Re-numbers. We are also looking for a benign stall at all Re-numbers between say 120000 and 500000. For everyday use it must also be immune against dust and rain and it would be beneficial if flaps could be used.

This is not a strong opinion but a technical necessity.

All the best

Wendi UK


Wendi,

No problem. I do understand where you are coming from. Hopefully he will come back so we can give him some valuable input; there are a lot of individuals here with a substantial background in Aerodynamics and UAV integration. Just have to learn how to weed out the crap from the good.

Your suggestions/comments are good ones and have a lot of truth to them. I may be wrong but I am not 100% sure he has intentions of competing in the OBC. As I said hopefully he will come back and be successful in his adventure.

Good luck with your projects.

Chris

[rich smith]

My response was mostly for the OP who inquired about weight and short field.

The late Jef Raskin, developer of the Macintosh Computer, was also a brilliant aerodynamicist who helped dispell the myth that planes fly via Bernouli principle with his work on Coanda Effect.

He also did exhaustive studies of model airplane airfoils and determined an undercambered profile of 4% thickness at 40% of chord cannot be improved upon for Reynolds Numbers and speeds we are concerned with. I've also applied his ideas to full size plane design with great success.

Rich Smith, you have to forgive me but who is (or was) Mr (or Mrs) Raskins?
It may well be possible that that profile "has no equal in terms of lift/weight and short field" (pretty nonsensical wording - yet I know what you mean :rolleyes: ). But under-cambered? That will not cut the corner for this application.Wendi UK