RE: best airfoil for heavy lift airplane?  
View related threads: (in this forum | in all forums)

Tower Hobbies
Enter up to 4 keywords or Tower stock numbers
Logged in as Guest



Users viewing this topic: none
  Printable Version
       


Fei Bao 1/9 scale Sukhoi SU 27 Flanker - ARF
Seller:  rcjetmodels
Details:   $2,445.00   |  11/30/2008   |  Classified Ad
We will rotate YOUR AD in this spot if you select "Forum Featured" when placing or editing your ad!

All Forums >> RC Airplanes >> Aerodynamics >> RE: best airfoil for heavy lift airplane?
Page: <<   < prev  1 [2] 3   next >   >>  

Login
Message << Older Topic   Newer Topic >>
RE: best airfoil for heavy lift airplane? - 11/3/2006 4:38:46 AM   
Ralph Morris



Posts: 194
Joined: 11/29/2002
From: Glendale, AZ, USA
Status: offline 		The Orange Coast R/C Club (Santa Ana, CA) had the honor of hosting the SAE Weight-lifting aero-design contests at Mile Square Park in the mid-90s. The entries with radical undercambered airfoils didn't fly as well as the data would indicate. Those shapes, with drooping trailing edge and barn-door ailerons, had tremendous adverse yaw and not enough fin/rudder area to compensate. The down aileron would cause enough drag to slow that wing to a stall, so that full left aileron would result in a stall turn to the right!

The best airfoils, surprisingly, appeared to be less than 12 % thick and semi-symmetrical.




_____________________________

Keep 'em flying!

(in reply to Johng)
       Post #: 26

RE: best airfoil for heavy lift airplane? - 11/3/2006 3:50:10 PM   
Tall Paul



Posts: 4724
Joined: 6/23/2002
From: Palmdale, CA, USA
Status: offline 		On my SAE type planes, I went to 100% differential. No down aileron at all. Coupled rudder.

_____________________________

Sparky Paul
http://www.angelfire.com/indie/aerostuff

(in reply to Ralph Morris)
       Post #: 27

RE: best airfoil for heavy lift airplane? - 11/4/2006 3:53:44 AM   
Ralph Morris



Posts: 194
Joined: 11/29/2002
From: Glendale, AZ, USA
Status: offline 		Hi Paul; I bellieve we had this discussion back in the '90s. Here we are, deja vu all over again.

The best weight-lifting airplanes were those with no ailerons at all! Relatively small polyhedral "winglets" provided roll stability and coordinated turns with rudder only. This contest doesn't require any aerobatics, just take off, fly the pattern and land. You don't need ailerons for those maneuvers. Ailerons near the tip of high-aspect ratio wings at low speed results in tip stall and spiral instability (tailspin).

I was always surprised that canards didn't perform very well. Since the canard provides additional lift, as opposed to the negative component required of conventional elevators, the rule would seem to favor canards. Keep in mind that the size limit was based on the total area (shadow) of the aircraft.

There's theory, and the real world. These contests are probably the first real-world aerodynamic experience for the senior aerospace engineering students who participate. It was exciting to share their experience.




_____________________________

Keep 'em flying!

(in reply to Tall Paul)
       Post #: 28

RE: best airfoil for heavy lift airplane? - 11/4/2006 4:20:25 PM   
Tall Paul



Posts: 4724
Joined: 6/23/2002
From: Palmdale, CA, USA
Status: offline 		Ralph, none of the canards that came to the 2000-2003 SAE events at Lancaster managed a successful flight. Most of them couldn't take off at all.
This Open class managed a vertical displacement of a couple of feet, more due to a high wind gust than anything.
There was a tri-wing which crashed quite spectacularly, and a canard that after a week of try-crash-repair-try-crash-repair managed to get about 5 feet up, and then collapsed.


Attachments
Click to see fullsize image.
Click for fullsize 		Click to see fullsize image.
Click for fullsize 		Click to see fullsize image.
Click for fullsize


_____________________________

Sparky Paul
http://www.angelfire.com/indie/aerostuff

(in reply to Ralph Morris)
       Post #: 29

RE: best airfoil for heavy lift airplane? - 11/4/2006 6:35:16 PM   
Johng



Posts: 1824
Joined: 1/24/2002
From: Deland, FL, USA
Status: offline
I guess the "best" airplane is subjective. One year Embry-Riddle did win with a dihedral wing airplane. If they'd have had to deal with a crosswind the story would have been different. If you judge "best" as having the most capability under real world conditions, all the best that I've seen have had ailerons. They allow the tuning of the camber of the airfoil, more precise flying, and greater ability to deal with cross winds. Improper setup and use should not be blamed on the device.

As for canards, it certainly is a case of "theory and the real world" The diffence often being the observer's misunderstanding of the theory. There is no required down force on a standard configuration. This is determined by the CG location and the tail design. It is entirely possible to have a lifting tail design that is completely stable. It has nothing to do with the airfoil of the stab either. As to why canards are not good competitors for weight lifting, an old professor had a great answer for us when we were students and thought he knew something that hadn't been thought of (CANARDS!): "If canards are good for a payload carrying plane, why doesn't the airforce have a canard version of the C-5?" And in the 15 years since I heard that, the C-17 has been developed in a completely standard configuration, and the C-130 - a design that originated in the 40's, is still being revised and built new - with no great changes to the basic configuration. Plus, there are no payload lifting STOL airplanes of any type or size, operational, anywhere in the world.

THe reason is that lifting ability for a canard tends to counteract stability. Either a canard that is built stable enough to fly will be very inefficient at high loads, or a canard build to lift loads efficiently will be very difficult to keep stable. This is what happens at the SAE. THey design to lift, then the thing is too unstable to make it around the pattern. It's a long story involving realitve wing loading, aspect ratios, etc that determine the relative lift curves and stall points of the two wings. But that is what I think, and I'm a fan of canards in the right circumstance. I've flown a canard sailplane before, and it's pretty cool.


_____________________________

John

(in reply to Tall Paul)
       Post #: 30

RE: best airfoil for heavy lift airplane? - 11/5/2006 4:12:54 AM   
Ralph Morris



Posts: 194
Joined: 11/29/2002
From: Glendale, AZ, USA
Status: offline 		Thanks for the photos, Paul, and thanks for the comments, John. The canard is such a neat idea, I'm not surprised that the Wright brothers employed it. It wasn't until Glenn Curtiss ran his airplane into a fence (the dummy put wheels on it!) and broke off the canard, that he hinged an 'elevator' to the rear horizontal stabilizer. Why, if it hadn't been for that accident, maybe most airplanes today would be canards, and a 'conventional' arrangement would be the oddity.

John, there are aircraft that fly with lifting stabilators, but they're computer-controlled, like the F-15. You can be assured that the stabilizer on a C-17 is not lifting.


_____________________________

Keep 'em flying!

(in reply to Johng)
       Post #: 31

RE: best airfoil for heavy lift airplane? - 11/5/2006 5:33:07 AM   
dick Hanson



Posts: 10264
Joined: 12/12/2001
From: slc, UT, USA
Status: offline 		There are very good reasons that canards are not favored for very stable high load use

the obvious point is that in 100 years the tractor setup with a big wing up front - teeny wing on the back- keeps on being the most efficient -reliable - easiest to adjust setup.
build and fly a few of each -you will see why.
I once even tried pusher pattern planes - another great flop .
My favorite is the canard models which "flew great" --until you let them get into a slip or some situation which allowed the plane to rotate -with the nose moving sideways .
Flat spin here we come.
lots of the military canards also had some nasty habits
Notice that Glenn Curtiss and his "headless" stayed headless?
also, arrows have stabilizers at rear and cg well ahead of CP

< Message edited by dick Hanson -- 11/5/2006 5:35:55 AM >

_____________________________

I am watching you
Libby

(in reply to Ralph Morris)
       Post #: 32

RE: best airfoil for heavy lift airplane? - 11/5/2006 1:34:40 PM   
Johng



Posts: 1824
Joined: 1/24/2002
From: Deland, FL, USA
Status: offline
If you read up on the Wright Bros, you will find that they didn't choose the canard from a comparison of different configurations. They assumed that configuration - and that the airplane had to be naturally unstable. The pilot was continuously making large, fast corrections to the canard to keep his nose out of the dirt. It was assumed that planes were naturally hard to fly and it was left up to the pilot to be enough of a man to handle it. It wasn't the perfect design. Few 'firsts' are anything close.

Took a few years, and Curtis added some practicality with the tail and the addition of ailerons.

As for the lift on the C-17 tail, prove it Many large jets are trimmed with aft CG by pumping fuel into the stab, to force it to be a lifting stab during cruise for efficiency. I've flown plenty of models with a lifting tail. Any aerobat trimmed to fly upright & inverted with no elevator change - as many are - have lifting tails. Any of the old free-flight models with huge stabs are lifting tails. It's not as tricky as you think, and definitely doesn't require computer control. What was I saying about misapplication of theory?....

_____________________________

John

(in reply to Ralph Morris)
       Post #: 33

RE: best airfoil for heavy lift airplane? - 11/5/2006 4:28:32 PM   
Tall Paul



Posts: 4724
Joined: 6/23/2002
From: Palmdale, CA, USA
Status: offline 		All tails lift!
Some lift down.

_____________________________

Sparky Paul
http://www.angelfire.com/indie/aerostuff

(in reply to Johng)
       Post #: 34

RE: best airfoil for heavy lift airplane? - 11/6/2006 12:54:39 AM   
da Rock



Posts: 7006
Joined: 10/11/2005
From: western, NC, USA
Status: offline 		If you can, look at the airfoil on the stab/elevator of a C130 Herky. It's cambered. Downward.


BTW, simply moving a CG aft when in flight by pumping fuel anywhere proves nothing about whether or not the horizontal tail suddenly starts lifting or not. About the only thing that would prove that it causes the tail to start carrying load would be a look at the designers' drawings, or maybe the designer's testimony. If you have no idea where the NP is on the ac to begin with and no idea what the tail is doing to begin with and no idea where the CG is to begin with relative to the NP, you have no idea what moving it aft would do.

Many large jets are trimmed in cruise by pumping fuel aft, but it might be to simply move the CG closer to the NP, nothing more. The farther forw

(in reply to Tall Paul)
       Post #: 35

RE: best airfoil for heavy lift airplane? - 11/6/2006 1:00:08 AM   
da Rock



Posts: 7006
Joined: 10/11/2005
From: western, NC, USA
Status: offline 		Many large jets are trimmed in cruise by pumping fuel aft, but it might be to simply move the CG closer to the NP, nothing more.

The farther ahead of the NP the CG is, the more the tail has to lift down to balance the aircraft. All the down force produced to counterbalance that noseheavy condition winds up adding to the load the wing has to carry. Add to that load and the airplane has to cruise at a higher AOA. Higher AOA results in higher drag. Higher drag means more fuel burned.

Any noseheavy airplane will be operating less efficiently it would if it wasn't noseheavy. They move the CG aft when they can to gain efficiency.

(in reply to da Rock)
       Post #: 36

RE: best airfoil for heavy lift airplane? - 11/6/2006 1:00:25 AM   
Tall Paul



Posts: 4724
Joined: 6/23/2002
From: Palmdale, CA, USA
Status: offline 		The Tristar has a similar "negative cambered" horizontal.
I believe it's a 5-digit NACA shape.
Large planes with short tail moments, like the Tristar, C-130.. have negative camber due to the short tail moment.. the longer momented airplanes like A-300s probably have symmetrical horizontals.

_____________________________

Sparky Paul
http://www.angelfire.com/indie/aerostuff

(in reply to da Rock)
       Post #: 37

RE: best airfoil for heavy lift airplane? - 11/6/2006 1:03:07 AM   
da Rock



Posts: 7006
Joined: 10/11/2005
From: western, NC, USA
Status: offline 		Yup

(in reply to Tall Paul)
       Post #: 38

RE: best airfoil for heavy lift airplane? - 11/6/2006 1:12:46 AM   
da Rock



Posts: 7006
Joined: 10/11/2005
From: western, NC, USA
Status: offline 		And it's been proven over and over that it's more efficient to carry the weight with the wing, not with the tail.

When loaded down, the larger wing will operate more efficiently than the smaller tail to carry the load. When the larger wing is carrying the weight there is less induced drag from it than there would be produced by the tail were it carrying the same extra weight.

Not only that, but the incidence of the horizontal tail is designed to stabilize the wing, not carry extra weight.

Almost every design that carries the weight with the wing and uses the tail to stabilize, not carry weight, has proven to be the most effective design. Almost every design that tried to move load to the tail wound up being inefficient. There have been multiwing designs that had fore and aft wings and no tail. The idea was to have the rear wing be both the horizontal tail and half the load carrying surface. They didn't work.

Any surface that operates in air that has been disturbed by the wing operates less efficiently and is a poor choice for load carrying.

(in reply to da Rock)
       Post #: 39

RE: best airfoil for heavy lift airplane? - 11/6/2006 1:21:12 AM