ideal aerofoil

Homebuilt Aircraft & Kit Plane Forum

Help Support Homebuilt Aircraft & Kit Plane Forum:

Norman

Well-Known Member
HBA Supporter
Joined
Nov 28, 2003
Messages
3,121
Location
Grand Junction, Colorado
Or take profiles from a SparrowHawk.
In my admittedly limited experience that never works out very well (not if you want an exact copy). Even when the guy lifting the coords has decades of experience with precision measuring it still introduces errors that you may not be able to detect but the air can. So you have to smooth the raw coords which may produce a good profile but it's no more similar to the wing you lifted the coords from than the raw numbers from your splash. The lift and moment of your copy will be pretty close to the original wing but the drag is harder to duplicate because imperceptible surface irregularities can have big effects on the boundary layer and sometimes there will be a bump in the velocity gradient that belongs there but whoever (or whatever) smoothed the raw coordinates didn't know that and cut it off.
 

BJC

Well-Known Member
HBA Supporter
Joined
Oct 7, 2013
Messages
13,297
Location
97FL, Florida, USA
I copy profiles by the following process:

Place a 2” wide strip of 3M Scotch packaging tape over the section to be replicated.
Cut a piece of 1/2” plywood to within 1/4” of the profile.
Mark LE and TE vertical lines on the plywood.
Use Bondo to fill the gap between the tape and the plywood.
Remove the plywood with Bondo.
Sand the Bondo flush with the sides of the plywood.
Use the plywood with Bondo as a template to transfer the profile as needed.

BJC
 

WINGITIS

Well-Known Member
Joined
Jun 24, 2020
Messages
435
Location
Wellington, New Zealand
It is probably a fair conclusion to say that an AS-BUILT wing probably never got to be exactly like the profile the designer specified, especially if its an alloy construction, Carbon fiber wings made from accurate mould's could be the exception if professionally done.

So one should probably always attempt to get the original data file or drawings just to be sure.

Norman is correct in that a close copy would probably work within a % error that may be acceptable for some of the airfoil dynamics, for SLOW planes, it WILL however ALWAYS affect the Drag and Lift/Drag adversely.

Which may make the copied airfoil applied to a plane actually UNSAFE.....

Who would want that!?

K
 
Last edited:

Norman

Well-Known Member
HBA Supporter
Joined
Nov 28, 2003
Messages
3,121
Location
Grand Junction, Colorado
I copy profiles by the following process:

Place a 2” wide strip of 3M Scotch packaging tape over the section to be replicated.
Cut a piece of 1/2” plywood to within 1/4” of the profile.
Mark LE and TE vertical lines on the plywood.
Use Bondo to fill the gap between the tape and the plywood.
Remove the plywood with Bondo.
Sand the Bondo flush with the sides of the plywood.
Use the plywood with Bondo as a template to transfer the profile as needed.
That would be fine for NACA 4 and 5 digit profiles and those from any other source before 1935 but NACA 6 and most other profiles designed after 1950 get a bit finicky about the leading edge bump that that method could produce and the waviness that bondo will develop if you keep it for a few months. The method of transferring the profile from your splash to the material you're making the part from can also introduce waves (a penciel line as amazingly thick and of inconsistent thickness when you're trying to work to machinist tolerances). And don't forget the skin offset, that's another chance to screw up the final shape.
 

Kiwi303

Well-Known Member
Joined
Jul 22, 2015
Messages
360
Location
En Zed. Aka The Shire.
But you errors in copying is probably going to be no worse than the average first time builders errors in construction.

There always comes a point of diminishing returns for perfection when people have to step back and say "Thats close enough"

The biggest problem is that one persons "Close Enough" is someone elses "Way Wrong!"
 

Voidhawk9

Well-Known Member
HBA Supporter
Joined
Mar 26, 2012
Messages
584
Location
Timaru, NZ
The biggest problem is that one persons "Close Enough" is someone elses "Way Wrong!"
Indeed, but even relatively small variations in airfoil shape can change it's characteritics quite a bit, depending on what the variation is.
It's even worse with laminar flow types, if you mess those up a bit, you can lose laminar flow much earlier than expected, which can change your lift drag and moment characteristics - everything!
 

WINGITIS

Well-Known Member
Joined
Jun 24, 2020
Messages
435
Location
Wellington, New Zealand
But you errors in copying is probably going to be no worse than the average first time builders errors in construction.

There always comes a point of diminishing returns for perfection when people have to step back and say "Thats close enough"

The biggest problem is that one persons "Close Enough" is someone elses "Way Wrong!"
INDEED! but what if a first time builder built the one you copy off, which was also copied from some manualy typed in co-ordinates from a dubious source previously.

You then spent a LOAD OF HOURS building something that may have issues....

For a Radio Control aircraft that is probably OK.
 

Norman

Well-Known Member
HBA Supporter
Joined
Nov 28, 2003
Messages
3,121
Location
Grand Junction, Colorado
There always comes a point of diminishing returns for perfection when people have to step back and say "Thats close enough"
Why go to all the trouble of lifting a good airfoil from a molded wing if you're not going to even try to build it to the tolerances demanded by such airfoils? You're not going to get the performance that that molded wing can deliver so you might as well pick something out of a catalog that is less difficult to build and will probably out-perform your sloppy copy.
 
Last edited:

BJC

Well-Known Member
HBA Supporter
Joined
Oct 7, 2013
Messages
13,297
Location
97FL, Florida, USA
Let say, an airfoil (a set) for a microlift 12 metre sailplane of 145 and 345 empty and gross weights, with a rectangulare shape and a 30 inches chord. If that make sense, what would be a suggestion?
I would copy the Greg Cole airfoil on the Windward Performance SparrowHawk.
That would be fine for NACA 4 and 5 digit profiles and those from any other source before 1935 but NACA 6 and most other profiles designed after 1950 get a bit finicky about the leading edge bump that that method could produce and the waviness that bondo will develop if you keep it for a few months.
So what airfoil(s) would you recommend?


BJC
 

WINGITIS

Well-Known Member
Joined
Jun 24, 2020
Messages
435
Location
Wellington, New Zealand
NONE, you have to do a lot more work on airfoil key inputs first.

You would need the Reynolds Numbers for the glide speed, max speed and climb speed, the air density and temps you wanted to fly in the most, sort the assumption that you do or do not want performance in the rain or ice.

How much stability(CM) you want from the airfoil, ETC ETC.

Then with your Chord and Span you can do a bit of analysis work with KNOWN airfoils already chosen by other GOOD DESIGNERS in your category.

There is no simple way around doing that work and the associated calculations, if you want a decent airfoil for your application.

For a new design arguably it is the most important choice you will be making!

If you WANT TO LEARN about the process...

Perhaps start here:


By the time you have read through all that you will/should have a grasp on the terminology and there is a starter spreadsheet there for RN.

Cheers
K
 
Last edited:

Kiwi303

Well-Known Member
Joined
Jul 22, 2015
Messages
360
Location
En Zed. Aka The Shire.
For example KIWI303 the KIWI itself ended up copying the same airfoil over and over again, WITH ERRORS, then in the end it could no longer fly!

The Supersonic Nosecone is still intact though:D

optimised for rocket propulsion, but forgot beans weren't available until after european colonisation!
 

WINGITIS

Well-Known Member
Joined
Jun 24, 2020
Messages
435
Location
Wellington, New Zealand
optimised for rocket propulsion, but forgot beans weren't available until after european colonisation!
YEAH THE BEANS I FORGOT THEM! Sixty Bean Power to the tin....

Also I believe the KIWI undercarriage is strengthened for NAVAL CARRIER LANDINGS! From the hopping off the banks it can do......

And because it usually only comes out at night it must be STEALTHY.

Never mind the KIWI-TNT nuclear test, SHHHH the Greenies do not know about that one. It was to be tested at Auckland as well.....

Do not tell anyone about New Zealand's Uranium deposits either, remember the Pike River coverup/s....:cool:

Perhaps the Uranium affected the copying of the KIWI's airfoils process?
 

BJC

Well-Known Member
HBA Supporter
Joined
Oct 7, 2013
Messages
13,297
Location
97FL, Florida, USA
Time to post this PDF by Orion again?
Thank you for posting that, Hot. I looked for it earlier today but did not find it.

I will add another data point. A Boeing CFD run to explore the deleterious effect of a flat spot, 1/8” deep over the spar, in the GA(W) 2 Mod on the Glasair III, predicted a 3 knot reduction in the 275 MPH top speed.


BJC
 

Norman

Well-Known Member
HBA Supporter
Joined
Nov 28, 2003
Messages
3,121
Location
Grand Junction, Colorado
So what airfoil(s) would you recommend?
Selecting an airfoil or set of airfoils is up to the designer. First he has to outline the mission. Then round up candidate airfoils and their polars. To me the most important airfoil characteristics are CLmax, CL at minimum drag, and lift to drag ratio but I don't do sailplanes so you may have different priorities (for instance you may want the lift to equal the weight of your glider at your circling speed and best power factor).
 
Last edited:

REVAN

Well-Known Member
Joined
Dec 6, 2016
Messages
279
Location
Tucson, Arizona USA
what is the ideal aerofoil ... laminar flow ... equal pressure at the entry as the exit ... forward camber and rear thickness ... and what realy is praticle to achieve and how critical ... or just a general foil thats good enough across the board ...
The ideal airfoil is one that works optimally with the wing plan and use. One negative of the original NACA airfoil studies and publications is that many designers have subsequently been sub-optimizing wing design to airfoil selection. Much attention is paid to selecting "the best airfoil" based on 2-D performance data, and then that airfoil is used to design an aero-mechanically sub-optimal 3-D wing.

I believe that we could have much better aircraft if we designers paid more attention to to wing taper, twist and sweep, to come up with a wing that will behave well from a stability and control perspective, with the airfoil selection being a later consideration. The best wings often have several different airfoils employed along the wing's span, these airfoils selected based on what's happening in the 3-D flow field in which the 2-D airfoil operates.
 

cirrus232

New Member
Joined
Jun 4, 2021
Messages
4
Of course, the original question is ill-posed. Need requirements, and then the task is to find the 'best fit', i.e. the best airfoil for the defined objective(s). Others here have posted the same.
 
Top