Airfoil Assistance

[Troy]

I want to get some second and third opinions before I start cutting foam.
I am building a "just for fun" model but I am going to throw some power into it and it may go kind of fast. I want a fairly stable planform so it comes down to a proper airfoil to make my life easier.

Basically, it's a flying plank with very low aspect ratio, constant chord, and hopefully reasonable to low surface loading.

Initial size is roughly:
24" span
34" chord

I am hoping to achieve 80-100mph depending on what power system I can scrounge up. This is for a fun event and I am throwing a twist to what other planes are going to be flown.

I know most deltas and planks use basic symmetrical foils for simplicity but I don't care if it has good inverted performance other than the occasional inverted fly by. I have been flying slope wings and planks long enough to know the proper airfoil can make a world of difference "right side up."

I have compufoil so I am foil savy, just need help in an area I haven't played in.

Any suggestions appreciated
This is what I am modeling it after:

[Sparky Paul]

Try one of Martin Heppler's reflexed foils intended for plank wings, which is what that thing is, and appears to be using.
http://www.mh-aerotools.de/airfoils/

[BMatthews]

For speed you want very low camber values. In fact what you want is a symetrical airfoil.

If you want to help extend the lower speed range with a lifting airfiol that is flying wing stable yet has a lower camber value to avoid drag at the high speed end. For that I would suggest just using a slightly thicker symetrical airfoil or go with the Eppler 186.

[Ollie]

For a plank wing you need airfoil with Cm>0.01. See:
http://www.aerodesign.de/english/profile/profile_s.htm
HS 130
JWL-065
EMX-07
Phönix
HS 2.0/8.0
"NEVER USE CM0=0 SECTIONS FOR FLYING PLANKS!!!"

http://naca.larc.nasa.gov/reports/1946/naca-rb-l5k02/

http://www.nasg.com/afdb/show-airfoil-e.phtml?id=922
http://www.nasg.com/afdb/show-airfoil-e.phtml?id=858
http://www.nasg.com/afdb/show-airfoil-e.phtml?id=1166
http://www.nasg.com/afdb/show-airfoil-e.phtml?id=1165

[Troy]

Well I've been looking at MH foils and others, and I understand I am going to need a foil with a reasonable amount of reflex (hence the low pitching moment). I have also used a few EH series for plank wings and swept flying wings.
I'm not sure a symmetrical foil is going to be as efficient as a thin, properly reflexed foil. A symmetrical airfoil needs to fly at a higher AOA to maintain lift so the effect is increases in drag, where as if you use a proper flying wing airfoil you fly with the wing with less AOA (and all other magical airflow tricks)to achieve the same lift. I guess my main concern is that with a plank with such low AR and in a relatively low Reynolds number I'm not sure what foil is best suited. My first pick is an MH 44 but since I will have somewhat of a horizontal stab on the car's "Wing" I may be able to get away with an MH 43 which has a low pitching moment and is a pretty well known airfoil for speed. I've also heard of a few guys using it on flying wings. That HS 130 also looks pretty promising.

[Sparky Paul]

Lots of power makes up for low Reynold's numbers. :)

[BMatthews]

At 100 mph the Reynolds number for a large chord like you'll be using is huge. Even at near the stalling speed your chord is going to ensure that the Rn stays higher than normal compared to many models. So you're not operating at a low Rn compared to most models.

At 100 mph the lift coefficient, and thus the angle of attack needed, will be quite close to 0. Most lifing airfoils at this speed will actually be operating at a negative angle of attack, but still positive lift, unless they have less than 0.5 to 1% camber. Under such a condition most 1.5% or higher camber airfoils have more drag at these very low lift coefficients than a symetrical airfoil has.

You can check it out for yourself by using a lovely little simulator called Foilsim. It's great fun and a fantastic learning tool and will soon tell you what sort of lift coefficients your model will fly at and if you know what the max Cl of your airfoil is you can get a pretty decent idea of the stall speed. You just gotta think about what the Foilsim is telling you.

Here ya go... http://www.grc.nasa.gov/WWW/K-12/airplane/foil2.html . You may need to download and install Java but it's well worth it to get this to run. It's a great tool for figuring out how airfoils work and how the lift-drag charts relate to our actual flying. The airfoil they show is just a generic one but the effect of camber on the lift and to some extent the thickness is pretty much directly applicable to our airfoil selections.

But frankly good luck on the 100 mph goal unless you fit an big honkin' engine that is known for it's power. The low aspect ratio and the fences plus oddball tail will produce enough drag that I suspect 60 to 70 mph is far more realistic and even that will take a pretty decent engine to do that speed.

Our typical models fly a lot slower than most folks realize.

Anyhow, good luck with the lawnmower or other oddball. They can be lots of fun.

[Ollie]

With the aspect ratio of 0.7, the wing is very, very draggy. The induced drag near stall and induced angle attack are very, very high, like ~30 degrees. At slow speed the induced drag is about ten times the airfoil drag! You fly on the prop more than the wing. If you fly with the power off it will glide like a brick. Don't worry about airfoil! Play around with aspect ratio and angle of attack. See:
http://aero.stanford.edu/WingCalc.html

[Troy]

You guys are definately helping me put things into perspective so I appreciate it. I am use to building more conventional and sleeker airframes so I knew I would need help with the little monster. I am not so worried about low speed drag issues as I already know this is going to land like the Space Shuttle. And your right, there is no replacement for displacement...or watts in this case. There will be no shortage of power and I plan to keep things as light as possible. That's why I am building my own version, the original uses some fairly heavy building methods and I have to change a few things to make my set up fit. I am also trying to minimize on certain aspects of the design to lower the ugly drag. None the less, it should be interesting and entertaining.
Thank you for the links! I enjoy playing with those programs.

[BMatthews]

Watts? obviously electric then.... Forget about speed in that case. Instead look at the performance of that flying hydroplane for guidance on what your lawnmower is going to fly like. You want speed and efficiency? Build something that looks like a Speed400 pylon racer. YOu want fun and unique? Go the way you are but be prepared to give up speed and efficiency.... and build it light so the motor can haul it around.

[Troy]

Watts? obviously electric then.... Forget about speed in that case.
I'm not looking to break any speed records with this, it's all in fun. :) And trust me, I have very humble expectations on an airframe like this but if I have to make it a flying welder I will. I fly F5D which regularly hits speeds of 180-200mph so I get my adrenaline pumping there. That's at about 700-750 watts. This is for a foamie speed event where the majority will be flying Zagis and Projetti type planes. It's going to take about 600 watts or so to make things entertaining. I guess my ambition is more of a tongue in cheek approach to the event. It's very commonplace to see zagis on steroids but this ought to give myself and my flying buddies something to laugh about. Looking back at the first time I saw the flying Hydro leap off the water and fly, I almost fell out of my chair laughing.

[Tom Harper]

Troy,

Go here:

http://members.aol.com/slicklynne/pavreport.pdf

for more than you ever wanted to know about low aspect ratio craft. Barnaby Wainfan is the guru.

[BMatthews]

If you overpower it to that level I want to be there with a video camera to what it roll uncontrollabley thanks to the super low aspect ratio.... :D

Seriously, good luck with it. It's not the path I would take but it certainly will be flipant enough to make the other guys laugh.

[uriah]

Troy, I am working on a similar aircraft, the X-43B...
NASA fixed the roll problem BMatthews mentioned by adding forward canards and swept-wings. (see attachement) as the fusalage is already a lifting body, one the X-43 and your design, added wings produce more lift and stabalization.

are you really going for the "flying racecar" look??? if so i would add larger verticle stabalizors!

[raptor22]

My choise would be the PW51. It is basically a higher speed and less lift version of the popular MH45.

--Alex

[Troy]

I have been tweaking the design so it isn't an untamed beast to fly. I have added a little more span to increase aspect ratio and the profile (of the car) is not like the original RC version above. It has less side area towards the nose and the verticals are more substantial. It is based roughly off of this GTP car. I don't have my CAD drawings handy but it has more airplane features than car features. It's going to be a 40 foot model; it looks like a car from 40 feet away! ;)

[uriah]

40 FT LONG!!!!!! :eek:

WHAT IN BLOODY HELL IS GONNA POWER IT FOR UNDER A MILLION???

[Troy]

No, no, no
it looks like a car from 40 feet away! ;)

[BMatthews]

40feet? I dunno, that sounds like a higher standard than I'd be working to. How about "stand way back and squint hard" scale? Or "Horse Scale"... as in " a man on horseback at a fair clip" sort of scale.... :D

[Troy]

How do you like them apples!!??
http://www.rcgroups.com/forums/attachment.php?attachmentid=820866

She flew quite nicely and had more than enough room to prop up. I had more issues with motors the last two weeks than the airframe. It glides in like a dream and like the caption says, the only entry to taxi out and taxi back in from speed runs. It has an insane roll rate but is fairly stable at speed. Once it settles in for landing it floats on ground effect for a bit before lightly touching down on the mains.

[raptor22]

I saw it at MWE. It looks suprising car-like; much more than I would have expected.

Good job.

--Alex

[Troy]

I am glad to hear that as I had to redesign the cockpit so it was more scale like in appearance. I believe that if I had omitted the landing gear and hand launched I could have gained another 10-15mph but I wanted it to ROG and act like a car on the ground. BUT, I reached my objective and surpassed it and that was the goal. I am going to kick the power system down a notch and fly it as a sport model for longer flights. I am also working on an all Depron version as we speak which will have even more scale appearance.
I hadn't flown it on the new power system till the speed runs and the in flight speed was deceptive because it ran quieter on direct drive vs. my previous geared version that had a nice snarl to it. I also didn't dive very steep because I didn't want to over-speed the depron tail surfaces. I am almost as excited about the 105mph run as my 205mph run with the F3D on Sunday. :D