SMALL - espritmodel.com SMALL - Telemetry SMALL - Radio
Reply
Thread Tools
Old Feb 25, 2012, 04:31 PM
Registered User
Joined Jun 2005
2,462 Posts
Twin Canard Build

The wing bottom was covered with Flame Red which is light weight but thick enough to be easy to manage. The color is bright and should be easy to see. The fuselage, vertical surfaces and nacelles will be a heavier white for lots of contrast. The wing has been pinned down on the board, top down so that the nacelle bottom halves can be added. The wing's center front is supported to prevent the wing from moving.In this position. The center line of the nacelles at front end will measure exactly 1' above the surface of the board as measured on the scale drawing. The rear of the nacelles are solid balsa and are mounted on the flat portion of the ribs. After the nacelle bottom halves are completed, the motors will be mounted, wired and the top halves will be added. This has been tedious and dreaded work but is great fun because it seems to be working.

Charles
canard addict is offline Find More Posts by canard addict
Reply With Quote
Sign up now
to remove ads between posts
Old Feb 26, 2012, 01:28 PM
What could possibly go wrong?
nickchud's Avatar
Market Harborough
Joined Apr 2006
3,695 Posts
Beautiful workmanship Charles! As always!

Cheers

Nick
nickchud is offline Find More Posts by nickchud
Reply With Quote
Old Feb 26, 2012, 09:46 PM
Registered User
Joined Jun 2005
2,462 Posts
Your kind words are appreciated, Nick. It has been a real pain to add simple square nacelles to a swept back tapered wing. I checked a DC-3 wing at the field today and saw that the center section LE of the wing was at 90 degrees to the fuselage center line and appeared non-tapered which greatly simplified the geometry for adding nacelles. It is the K.I.S.S. rule which I should have used. This probably will be my first and last twin. To me it seems to be the natural and best layout for a canard because the motors are near the CG for best balance.
Hopefully, the battery can be a bit forward of the motors to balance the rear wing and vertical surfaces.

Charles
canard addict is offline Find More Posts by canard addict
Reply With Quote
Old Feb 27, 2012, 02:24 AM
What could possibly go wrong?
nickchud's Avatar
Market Harborough
Joined Apr 2006
3,695 Posts
Good to see you're getting your mojo back!

As you know, I like twins. The approach with nacelles that has worked best with me is to put something structural into the spar and the leading edge (or trailing edge) before adding anything like the skin. The geometry is a bit easier that way.

Nick
nickchud is offline Find More Posts by nickchud
Reply With Quote
Old Feb 27, 2012, 09:55 PM
Registered User
Joined Jun 2005
2,462 Posts
Nick, What you are doing there with your Starship would be an impossible task for me. It will be a relief for me just to complete two nacelles. Thanks for the picture.

Charles
canard addict is offline Find More Posts by canard addict
Reply With Quote
Old Feb 28, 2012, 04:11 AM
Registered User
Joined Mar 2011
778 Posts
Hi to all

Does anyone have experience with blunt TE or afterbody aerodynamics?
In their KF airfoil video rcpowers talks about getting a "push" from the vortex formed in the back.
I have noticed motorcyclists get their shirts pushed all the way up their backs by the airflow coming in to fill the low pressure behind them. I also notice leaves and dirt stuck to the backs of SUVs which were sucked forward with enough force to remain attached.
I have also read that it helps the airflow remain attached to the upper surface of the airfoil whereas in turbulent airfoils the airflow starts becoming detached pretty far forward.
Regards to all
Armour
captarmour is offline Find More Posts by captarmour
Reply With Quote
Old Feb 28, 2012, 04:25 AM
What could possibly go wrong?
nickchud's Avatar
Market Harborough
Joined Apr 2006
3,695 Posts
Armour: I don't know the answer, but I'll be watching with interest to see who does.

Charles:
Quote:
Nick, What you are doing there with your Starship would be an impossible task for me
I couldn't do it either if I wasn't home alone. My wife comes back next Monday. On balance, I'd say that I'd sooner have her here and stick to smaller projects. But a chap's got to do something to while away the time.

Cheers

Nick
nickchud is offline Find More Posts by nickchud
Reply With Quote
Old Feb 28, 2012, 07:30 AM
Registered User
Don Stackhouse's Avatar
United States, OH, Bradford
Joined Jun 2005
4,027 Posts
The KF airfoils have been studied by numerous people in numerous places, and the STRONG consensus is that they have a poor L/D. And we're not talking a minor deficit, some of the studies indicate a loss on the order of 30% compared to more conventional sections.

A major part of the problem is that the "steps" in the airfoil do not retain the vortex that is supposed to maintain a smooth flow path above it.

Sharply truncated afterbodies do tend to have less drag than a short but rounded afterbody, but not as low as a longer afterbody that tapers to a point. The Army has done some studies on stepped afterbodies and found that they can be an advantage when it isn't possible to have a tapered-to-a-point afterbody, but still don't have drag as low as a properly tapered afterbody.

There are some who claim the K-F sections have "wonderful stall characteristics" (note the vague, subjective description), but others report no significant difference. I've seen some tuft tests of this, and the behavior of the model in the tests seems to follow the same pattern as conventional airfoils, with the overall behavior being more the result of the "Hershey bar" (i.e.: relatively low aspect ratio, constant chord and airfoil) wing planform, with the stall beginning at the root's trailing edge, then propagating gradually outward in the typical elliptical pattern.

There are cases where a SMALL truncated trailing edge can have advantages. In itself, it imparts a small drag penalty, but it can widen the range of usable lift coefficients if done exactly right, which can then result in a benefit to the required wing area.

Also, the use of a "Gurney flap" (invented by racer Dan Gurney while working with Dr. Robert Liebeck of McDonnell Douglas on improved airfoils for race car wings), also called a "wicker bill", creates a disturbance on the underside of the trailing edge that in itself increases the thickness of the lower surface wake (and increaes the lower surface drag), but in the process helps keep the upper surface flow attached to higher lift coefficients. If done just right, it provides a net benefit.
Don Stackhouse is offline Find More Posts by Don Stackhouse
Last edited by Don Stackhouse; Feb 28, 2012 at 07:37 AM.
Reply With Quote
Old Feb 28, 2012, 08:19 AM
Registered User
Joined Mar 2011
778 Posts
Thanks Don.
I only mentioned KF to quote the "push" that was mentioned by Kline.
So from what u have said my understanding is their benefits are where a long pointed TE or afterbody is not possible, they are more aerodynamic than a rounded one.
I think the Cessna 208 uses a Gurney flap on part of the TE of the flaps.
I see the ATR turbo prop has the truncated TE on the flaps and the dash8 has what looks like twin splitter plates, looking like a "U" on it's side, on the inboard flap TE.
What I would like to know is what is involved in making the blunt TE more aerodynamic than the rounded TE? Could it be connected to the way the motorcyclists T-shirts are pushed way up to their shoulders from the back?
Also I notice some boat props have what looks like a wedge airfoil where the LE is tin and the trailing edge thick looking like a very thin triangular wedge. What r it's advantages and
can something like that work for a wing?
Regards to all.
captarmour is offline Find More Posts by captarmour
Reply With Quote
Old Feb 28, 2012, 08:29 AM
Registered User
Don Stackhouse's Avatar
United States, OH, Bradford
Joined Jun 2005
4,027 Posts
What matters is that the separated flow takes on the shape of a long, tapered afterbody, making the shape "look" to the free-stream flow like it had that shape. I suspect the problem is that the overall flow path involved has a lot more "scrubbing" going on between layers of air, and between the air and the surface.

In any case, the quantitative measurements show it as being worse overall. That "push" you refer to has to come from somewhere, along with the energy to drive it. "There's no free lunch."

As far as the wedge-shaped airfoils on boat props, that has to do with managing cavitation effects. We don't operate in a liquid that can locally transition into vapor, so it does not apply in our case.

Truncated trailing edges could be there for aerodynamic effects (I've personally used them to great advantage, but their effects need to be integrated into the rest of the design to do so), or they could be there for things like manufacturing ease, or durability in service.
Don Stackhouse is offline Find More Posts by Don Stackhouse
Last edited by Don Stackhouse; Feb 28, 2012 at 08:39 AM.
Reply With Quote
Old Feb 28, 2012, 08:38 AM
Registered User
Joined Mar 2011
778 Posts
Agreed, a blunt TE should be a lot stronger than a sharp one
captarmour is offline Find More Posts by captarmour
Reply With Quote
Old Feb 28, 2012, 12:19 PM
Registered User
Joined Apr 2004
10 Posts
For the Carnard Addict

Charles,
You did a fine job showing your nacelle build out on you twin engine bird. The precisness of you building gives me a goal to shot for. Thanks for taking the time to show us your work.
Hank
hank ramm is offline Find More Posts by hank ramm
Reply With Quote
Old Feb 28, 2012, 02:49 PM
Visitor from Reality
United States, VA, Arlington
Joined Dec 1996
12,788 Posts
Okay, one day I will dig out my canard CAD file and build the durn thing.

If I'm really brave, I'll show it off here first After a few years of designing models and being especially fond of ones not having tailplanes, really must try sticking the thing on the front and see what that does.

On the subject of trailing edges! The above makes for fascinating reading. On a practical level, my designs have alternated between a flat, constant thickness aileron - if it works that good on Bruce Tharpe's Four Star 40 wing, it'll do for me - and some with a tapered TE.

The latter have always been aimed at around a 1/16" thickness, allied to a glue joint that runs to the extreme trailing edge to give a hardened edge at where 'hangar rash' strikes often.

The Big Question?

Would a model aircraft (regardless of where the horizontal stab is ) be noticeably better going from a Four Star style 1/4" thick rectangular section aileron to having its ailerons blended in to the wing section with as close to a knife edge', or whatever the correct term is, TE?

regards

Dereck
(Who has a constant drive to build a Despretz Flash for some unknown reason...)
Dereck is offline Find More Posts by Dereck
Reply With Quote
Old Feb 29, 2012, 06:49 AM
Registered User
Don Stackhouse's Avatar
United States, OH, Bradford
Joined Jun 2005
4,027 Posts
That depends. A tapered, sharp control surface will have less drag. However, there can be a wake behind the wing (especially if the airfoil is too thick) that blankets a tapered control surface. You will get weak control authority until the control surface deflects far enough to start getting out of that wake. It's a little like having some exponential dialed in.

If that's what you want, then it's a plus. However, if you want strong control response everywhere including in the middle, without that "soft" spot in the center, then a thick control surface, ideally with a squared-off trailing edge, will get out of the wake sooner, and will have more immediate response, although with a drag penalty. Note, once the trailing edge is thick enough to eliminate the wake effect , making it any thicker will just add more drag, it will not make the control response any better. Just like most "medicines", the "dosage" is important.
Don Stackhouse is offline Find More Posts by Don Stackhouse
Reply With Quote
Old Feb 29, 2012, 08:30 AM
Registered User
Joined Jun 2005
2,462 Posts
Hank ramm, Thanks for the encouraging comments! You did not mention the baby Goose. If you are like me and just enjoy having it around in one piece for awhile then that is great. If you can possibly find a smooth place for an ROG, it would be super because it probably will become airborne so gently you will hardly notice it at first.Hand launching a canard is a delicate matter because the canard must be flying at the proper angle for the main wing to fly.Please be sure that DOWN elevator means nose UP. Best of luck with it and let us know your results.

Charles
canard addict is offline Find More Posts by canard addict
Reply With Quote
Reply


Thread Tools