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Sep 01, 2004, 06:55 AM
Thread OP

F5D Design & Building techniques


Hi to you all,

I am starting here a new thread about the design and building techniques for a new 2005 rules F5D airframe.
I started to discuss about this idea here:

https://www.rcgroups.com/forums/showthread.php?t=264565

In any way, I will repeat here under most of things I wrote in that thread.

I am working on this project with some friends of mine, Franko, Steph, Sebastien.
Some of us have been flying at York for the WC, others, like myself have started F5D flying this year.
We also have the help and comments of Ruud Zandvliet who is producing the deutch designed ship, named Gilette.

www.composite-models.nl
https://www.rcgroups.com/forums/showthread.php?t=113414

Our main target is to do like the "big guys" and be able to build our own designed bird and fly with it at local contests and have fun
We have noticed the quite short living period of those tiny things and we are sure we will need several airframes for our own use.
We have crashed and totally destroyed a few planes this year
We also found it very exiting to be able to design and produce our very own model.
I am responsible for the modeling of the plane / plugs / molds.
In fact, we have already designed one plane and I have just finished the modeling of the plugs.

You will find here under a layout of the 2005 rules airplane I am designing, all notes are in french but can of course be translated if you wish.
I am sure there will be lots of things to discuss about the different design options.
This model has been done with the help of a quite well know cad software, I can send a 3D model viewer to anyone of you if you like.

I inspirated myself with different designs I found on the internet and the Tokoloshi is one of them.
It is particulary true for the wing and stab, I did not simply "enlarge" the wing, I lengthened it.
I must say this is primary design and it is subject to changes.
The fuse is a mix between different airframes, P5 and Avionik, we wanted to use a tape to mount the wing because it acts like a fuse on one hand and we have had some issue with the screw fixature type on the other.
I also wanted to have a high point at the tail because my experiences showed me that the fuse and stab can suffer pretty much when rotating on landings.
The tail is designed like a wing and should outperform the small and large designs like the one of my deutch Gilette.
Please understand there is no will to "copy" an existing design.
Last edited by Novy; Sep 01, 2004 at 08:17 AM.
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Sep 01, 2004, 06:57 AM
Thread OP
My co-designer, Franko, told me of some interesting wing shapes being used for F3D flying. Maybe we will consider to do some brainstorming and come out with an derivated shape to fit F5D requirements,...
Sep 01, 2004, 07:44 AM
Thread OP
To start it all with something small and apparently easy, I first modeled the stab.
Franko suggested the use of the MG05 airfoil, wich is far better than the old Naca 007 or 009.
Last edited by Novy; Sep 01, 2004 at 07:46 AM.
Sep 01, 2004, 07:55 AM
Thread OP
You will find here under the layout of half of the stabilizer.
Please do not hesitate if you have any comments on the target surface and chosen aspect ratio.
I am not specialised in aerodynamics, so I try to stay in "proven to work" designs and shapes.
Sep 01, 2004, 07:58 AM
Thread OP
Here is perspective view after modeling of the complete stab.
The black stripes are used to show any imperfections of the geometry - broken tangency.
Sep 01, 2004, 08:15 AM
Thread OP
Well, know comes the plug design.
You surely understood that I like the use of many pictures
Here is one of the stab plug.
The nice thing about the stab is that it is fully symetrical, so only one plug is necessary to obtain two mold halves.
We plan to machine this plug with a milling machine, well as soon as we got the quotes or Franko got his own tooling.
Can't wait for your comments
Sep 01, 2004, 09:07 AM
Registered User
Florrain's Avatar
Before the question arises,

The groove in the master is there to receive a triangular (or square) sectioned strip of plastic which in turn will produce a groove in the female mold.

The groove in the female mold will allow the glasscloth to be trimmed acurately at the trailing edge and to evacuate the excess epoxy that would otherwize be trapped between the 2 mold-halves and produce a much to thick trailing edge.

Someone (Eike, I think) posted on another tread that he was considering a wide root chord in order to get sufficient thickness to allow for the aileron servo to be located inside the wing like in an F5F.

Would a good option be to increase the thickness off the wing section at the root on a very small portion of the wingspan?

Would that really affect the performance?
As far as I know, small increases in thickness does'nt give much penalty in high speed drag compared to small increase in camber but I might be wrong...

@+
Francois
Last edited by Florrain; Sep 01, 2004 at 09:17 AM.
Sep 01, 2004, 09:33 AM
Thread OP
Well, I am sorry but I don't understand what are the benefits of the aileron servo being mounted inside the wing
Maybe for the room you can spare but this will, I think, not reduce the fuse drag that much.
On the other hand, what is for sure is that it will create a weak point just where enormous forces are concentrated.
Here under, a picture of a section view of the plane.
You will notice a quite large ESC able to manage 200amps.
The motor shown is a Hacker B50 associated to the graupner 38mm spinner.
Sep 01, 2004, 01:13 PM
RCGroups Author
SoarNeck's Avatar
Quote:
Originally Posted by Novy
To start it all with something small and apparently easy, I first modeled the stab.
Franko suggested the use of the MG05 airfoil, wich is far better than the old Naca 007 or 009.
Hi folks,

I'll offer some thoughts, for what they're worth.

I would suggest you not look to the MG05 for your horizontal. It's not quite as efficient as other choices out there, and is probably thicker than you need it to be.

One choice would be the Drela HT14. These were designed as zero-hysterisis (sp?) airfoils, so your stab shouldn't require turbulation. Drag throughout the range of Cl values is lower as well, and the airfoil transitions between lift conditions very smoothly. It's a bit thinner than the MG05, but not by too much. Thinner HT sections would be even better, but you have to decide where your structural limitations come into play. An ideal stab would use an HT14 ->HT12 transition from root to tip, but would be tricky to build (mind you, not with machined molds).

One thing to check into regarding the MG05 is the design hingeline location. The MG06 was designed around a 30% hinge, but I don't know how the 05 behaves. Locating this wrong might cause an additional drag penalty, since the bubble ramps on these airfoils are designed to suit.

Finally, take the elevator hinge to the tip please. That fixed TE portion on the stab will only add drag (and lots of it). It's not likely that you'll flutter your elevator if the linkage is stiff, and proper handling will prevent damage to the servo gears on the ground (by bumping the elevator).

Here's a quick polar:
Sep 01, 2004, 01:18 PM
RCGroups Author
SoarNeck's Avatar
Quote:
Originally Posted by Novy
You will find here under the layout of half of the stabilizer.
Please do not hesitate if you have any comments on the target surface and chosen aspect ratio.
I am not specialised in aerodynamics, so I try to stay in "proven to work" designs and shapes.
Your aspect ratio is moderate here, but it would make for a strong stab. You can get some drag savings by going to a higher AR, but your RE# drops accordingly. Not neccessarily a bad thing, but it means that the airfoil choice becomes important.

Don't neglect the design of the stab or fin...they contribute significantly to the overall drag of the model. In level flight I calculated that the stab of an F5D model contributes about 11% of the induced drag for the model, and the vertical fin about 6%. Induced drag will affect you more in the turns, but profile drag will get you all over the course. Keep the airfoils thinner if you can.
Sep 01, 2004, 01:20 PM
RCGroups Author
SoarNeck's Avatar
Quote:
Originally Posted by Novy
Well, know comes the plug design.
You surely understood that I like the use of many pictures
Here is one of the stab plug.
The nice thing about the stab is that it is fully symetrical, so only one plug is necessary to obtain two mold halves.
We plan to machine this plug with a milling machine, well as soon as we got the quotes or Franko got his own tooling.
Can't wait for your comments
Nice pictures BTW - makes for easy discussion of ideas

I can understand that you want to save machining costs, but I assume you realize that you'll still need two mold halves eventually. There's no way you'll accurately and lightly join stab halves that came out of the same mold.

You'd end up having to pull a composite postive of the machined mold half, and then either make two copies of it, or use one copy along with the master. Sounds like lots of work.
Sep 01, 2004, 01:31 PM
RCGroups Author
SoarNeck's Avatar
Quote:
Originally Posted by Florrain
Someone (Eike, I think) posted on another tread that he was considering a wide root chord in order to get sufficient thickness to allow for the aileron servo to be located inside the wing like in an F5F.
The big advantage would be that you could avoid having to use torque rods if the servos were in the wing, so your linkages would get shorter, lighter and stiffer.

Torque rods like in the D99 are about as bad as it gets from a stress perspective...even in the ideal case, they're putting a small diameter wire under torsion loads, which are much more tricky to handle than bending loads. Since the D99 is skin-hinged, the pivot isn't aligned with the centerline of the rod, and it's actually under a bending load as well. Yuck.

If one wanted to save weight in a torque rod, you could substitue tube of the same diamter as wire and lose no appreciable stiffness. Harder to bend, but lighter by quite a bit.

RDS linkages would work just as well as torque rods. That way, you could keep the servos inboard (lower roll inertia), keep the servos inside the wing (more space in the fuse), and thicken only the center of the wing to accept the servo. Your building workload would go up by about 10 hours per model, however. It's something I'm considering if I decide to do my own design.

Links for RDS:
http://www.charlesriverrc.org/articl...0PDFs/rdsf.pdf
http://www.charlesriverrc.org/articl...0PDFs/rdsa.pdf

Normal buried wing servos:
http://www.charlesriverrc.org/articl...s/althorns.pdf

Quote:
Originally Posted by Florrain
Would a good option be to increase the thickness off the wing section at the root on a very small portion of the wingspan?

Would that really affect the performance?
If you did it at the center of the wing, you'd probably be better off than further outboard. I've recently learned that the propwash means that the center of the wing only effectively sees fairly low angles of attack, so you can run slightly thicker airfoils with less drag penalty than if you forced them to accept high AoA behavior as well.

You'd have to do the calcs to see how much you're prepared to accept.

Quote:
Originally Posted by Florrain
As far as I know, small increases in thickness does'nt give much penalty in high speed drag compared to small increase in camber but I might be wrong...
Generally true.
Sep 01, 2004, 01:39 PM
RCGroups Author
SoarNeck's Avatar
Quote:
Originally Posted by Novy
On the other hand, what is for sure is that it will create a weak point just where enormous forces are concentrated.
Not really a big deal. Ideally, the spar on these models should be designed to take the full aeroload and the rest of the wing provides stiffness and torsional strength. Even then, the spars on these models aredesigned for min acceptable stiffness, not minimum strength. If designed to only take the bending loads, the wing spar would be too flexible to be useful.

If you added even 1/32" balsa facing between the wing skins all around the servo, you shouldn't lose much wing stiffness (like a shearweb).

Some people at the WC cut their servos into the D99 wing, I'm told, so obviously Sergei agrees with the above comments. Don't know if they used the facing I mentioned.

Quote:
Originally Posted by Novy
Here under, a picture of a section view of the plane.
You will notice a quite large ESC able to manage 200amps.
The motor shown is a Hacker B50 associated to the graupner 38mm spinner.
Throwing you hat in with the B50 crowd, eh? What happens if the long B40 becomes the motor of choice? Your nose might end up too long, and at the minimum the profile becomes too big. You'd have more volume of air to suck up extra heat (or evacuate hot air/pass cooling air through...probably will have to happen at 200A currents).

Might be wise to do a removable nosecone. Then you can mold a B40 version and a B50 version.

Hope all this helped. Good luck with your project!
Adam
Sep 01, 2004, 01:40 PM
RCGroups Author
SoarNeck's Avatar
Last thought - why dihedral in the wing? No reason for it...annhedral would even be better.
Sep 01, 2004, 03:01 PM
Registered User
Florrain's Avatar
Hi Soarneck,

Thank you for your input.

About the single stabilizer master, i think you got us wrong.
The intent is to mill positive masters from wich we will pull composite negative molds.

I know the picture might be confusing because the section is so thin you can't see very well that it actually protrude from the parting plate on that snapshot.

This brings me to the selection of stabilizer section.

Thank you for suggesting the Drela airfoil. It's always good to have some positive feedback and i will take a look at it.

You give us a profili CL/CD graph as a mean to compare the performance of both airfoils but I'm more interrested in the CL/AOA graph of your airfoil because that's were you can see all the interrest of the MG05.
I know it's not a spectacular airfoil for low drag but it's a damn good one for response to small changes of AOA an thus a fantastic improvement from a NACA 00x.

Then we must address the structural issue of having a very thin airfoil on the stabilizer of a very fast moving plane.
I must admit we might have been conservative with a 9% thickness but I'm really concerned about torsional strength.

About the position of the elevator, I have had models with MG05 stabilizers with the elevator at 25% chord and those stabilizers performed above average.
I agree with you that a control surface going to the tip is very effective and I like this very much on fast gliders ailerons but I see one drawback to this on an F5D racer: it will be very very very difficult to assess the correct position of the trim and control deflection because a few 10th of mm make such a huge difference at high speed.

@+
Francois


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