Advice for pod-and-boom Mirage?

[chlee]

Hi all,

I'm building a 112" span RES glider with a Mirage-type wing and a Bubble Dancer 4-layer carbon boom. Can somebody with experience with pod and boom fuselages of this size give me pointers on building a sturdy pod-boom connection?

I'm tentatively thinking of two 1/8" birch ply, 1.4 oz fiberglass-reinforced bulkheads with holes for the boom. They would be in the rear of the pod, separated by 4". Is this enough to take a 20 lb hi-start launch or a hard landing? How does this compare to, say, the Legionair or Laserarts Majestic?


Also, I'm replacing the old Mirage airfoil with an AG-34 thickened to 12%, no sheeting. I dunno how well the fat AG-34 will do with turbulator spars, but it did look a lot better than the original Mirage airfoil when I checked XFoil. Also, the flat surface on bottom will make building easier. Any suggestions/warnings/words of wisdom?

Thanks in advance,
- Chung

[Ollie]

If you are not already a member of CRRC you should join. Then you should ask Dr. Drela because he can give you better answers than I can.

My opinion is that thickening the AG 34 by keeping the bottom flat will increase the mean line camber which will decrease the airfoils useful speed range and lift to drag ratio. The facets on the upper surface will intersect at higher angles which may cause premature seperation and greatly increase drag. Turbulator spars ahead of the main spar will also increase drag especially at the high end of the speed range. It would be far better to stick with the true AG34 and build a spar that is stiff and strong enough to allow for the smaller spar depth.

The boom interface you propose is overkill. With 4 inch bulkhead spacing 1/16 birch ply or 1/8 lite ply bulkheads are strong enough. No reasonable structure will withstand a very hard landing on a hard surface like concrete. Other things will break before the suggested pod-boom interface.

[Tony D.]

What Ollie said and IMO if you are going to thicken the AG-34 to 12% with no sheeting and turbulators you might as well use this:

http://www.nasg.com/afdb/show-airfoil-e.phtml?id=62

section instead.

If you have X-Foil do a comparison of the thickened AG-34 and the Clark Y at the link I have posted.



Tony D.

[chlee]

Thanks for the replies, Ollie and Tony.

I wish I could somehow post the Xfoil polars here... The 12% AG-34 seems to do quite a bit better than either the Mirage airfoil or Clark Y. It's also better than 12% E-205. By 'better,' I mean it has lower Cd for all Cl, particularly at Re<90K. However, from what you gentlemen are telling me, I now wonder how meaningful these polars are if there are so many unintended effects of modifying/turbulating the airfoil.

The problem is, I'm trying to build an inexpensive, wooden wing with very low yaw inertia--hence the thick section and the aversion to sheeting. But perhaps I could have sheeting if I kept the outer panel sheeting very light, say, 1/32" balsa... Sorry, just thinking aloud.

Actually, in a slightly different topic last year, Prof. Drela did say the stock Mirage foil was very good for these purposes. But then I got ahold of Xfoil and some coordinates and proceeded to get myself in trouble. (I hope he's trolling this forum..)

Ollie, thanks for the boom advice. Also, I did join CRRC last year, partly due to an earlier recommendation from you.

- Chung

[Ollie]

Believe the polars. Especially if the polars are based on the faceted leading edge that exists between the ribs. If not, redo X Foil accordingly.

In an unsheated open bay wing with an unfaceted airfoil at the ribs and facets between the ribs, the shape of the airfoil at the rib is far less important than the shape of the airfoil between the ribs. In this situation the lift varies along the span from rib to rib. The resultant ups and downs in the spanwise lift distribution introduces additional induced drag due to the airfoil change between ribs. The airfoil at the rib can have good performance and the airfoil between the ribs can have good performance but the performance of the wing will suffer because the airfoils don't match smoothly along the span.

[Mark Miller]

I have had some folks buy Mirage rib kits form me to make a pod and boom version. The Mirage in stock form can be a great performer for what it is if you add some ballast for windy days. My OFB Augie McKibben flies his all the time in good wind with 20 oz. in it. His comes out in stock form with spoilers at 37 oz.

I have laser cut Mirage rib sets, Laser cut fuselage sets and a combo short kit including canopy available currently. Short kits are $60.00, Wing kit is $35.00 and fuselage kit is $35.00. Shipping is additional. The wing kit contains all ribs and ply joiners and ply joiner box parts. The fuselage has the fuselage sides, doublers, formers, canopy tray and vacuum formed canopy.

Mark Miller
Isthmus Model Co.
http://www.isthmusmodels.com