Sailplane fuse design

[I'm Rick James]

I'm about to dive into making a fuse/tail mold for my own sailplane design. Specifically it will be 99% DS (dynamic soaring) use and 1% sport flying/racing. Wingspan will be 60" with fuse length being in the neighborhood of 36". Projected "average" speeds will be in the 200mph realm with peaks in the 250 range.

I've been reading about nose, tail and fuse end shape and wanted a few opinions that could solidify my findings.

First the nose. Originally I thought a bullet shape would be the absolute best in efficiency in penetrating the air but it looks like a more rounded shape typical of a DLG pod is better. Does this hold true at higher speeds? Looking at modern jets the shape seems to be a hybrid of both of these. Not bullet nosed and pointy but not as round as a pod either. Any help here would be greatly appreciated.

Next the vertical. I plan on using a X-tail for the strength and trying to get the elevator into some cleaner air. Again looking at modern jets and faster prop aircraft, the vertical is laid back about 30 or so degrees and is angular in profile. Is this the most efficient shape for stability or is there some structural reason for it that I can overcome with composites?

Is there some formula for determining the optimal width of the fuse? I would again guess that I should make it as thin as I can while still maintaining torsional strength and overall rigidity. For a battery I would be running a 4 cell AAA 2x2 pack so that's the absolute widest I would need it to be and once it got past the TE of the wing I could slim it down to about 3/4" I think. How does that sound?

Finally comes the end of the fuse. From what I've read a pointed tail gets the fuse through the air with the least amount of drag. This would be a bit of a hurdle to get over with the connection of the elevator linkage but if it's actually the best shape I could make a seperate glue in tail cone

Thanks for any and all help you can provide. I'll probably have a plug roughed out in the next couple of weeks.

RJ

[Sparky Paul]

Jets fly faster than DS planes.
What works for them isn't what you want, unless you like the looks.
Look at the late 30s fighters for shapes, especially the liquid cooled types.. the Bf-109 series is about as streamlined as it was possible to do.
Slim and smooth makes fast.
And look at what works on the slope.. the bad stuff gets Darwined out.

[HELModels]

Take whatever airfoil is great for the speed you want to fly and remove all the camber. Now revolve that profile around the chord and you have your fuselage shape.

[Ollie]

Study Dr. Drela's fuselage designs. They have the lowest drag design. Just beef up the kevlar and carbon thickness two or three times. It can handle 200MPH air speeds. The fuselages are tough, strong and stiff.

http://www.charlesriverrc.org/articles/supergee/SuperGeeII.htm

[I'm Rick James]

Great link!! Thank you. I printed out the DLG pod and then extended it past the TE of the wing for a little more "meat" in the wing saddle area. If all goes well this weekend I'll be putting together a lathe and spinning one out. I really like the view from the top and will try to simulate that on all sides. I will add a wingsaddle cutout after that. In my drawing I made the vertical fin extend to the bottom sort of like a skeg. It's supposed to be a little bit more stable in DSing plus it'll give the fuse a little bit of uniqueness to it.

I'll post up some pics when I get to where I am ready for a mold.

Thanks again, RJ

[HELModels]

What I said and what you will find on that link do not conflict. Here is a graphic of what I said and it looks very much like a supergee fuselage.

[Ollie]

My advise is to study the design and the materials. Use only unicarbon and fiberglass fabric for the boom! Use only kevlar for the pod. Use the joint between boom and pod and use the bulkhead supporting the forward end of the boom. They are very important structural design details. Don't use molds to form the boom. Use the molds for the pod. Study the building process.

See:
http://www.badger.rchomepage.com/rollboom.html
or
http://www.polecataero.com/products/tailbooms/ Using a Supra tailboom.

[I'm Rick James]

Well I'm about the worst designer when it comes to the computer but here's the basic idea I have in my head. I did this on "paint" in a few minutes so there will be some tweaks going on. I'll shorten the boom, fair the pod into the wing a bit better and possibly sink the wing lower. I just have to watch for pushrod clearance.

Thanks for the other links and pics. I came up with an idea for making fuses and then found out it wasn't original but here it is anyway. For the entire fuse I was thinking of using a braided fiberglass sleeve the proper diameter. Put a balloon or surgical tubing into it and puff it up a bit. Then throw the whole thing in a vac bag so the different pressure in the balloon vs. the bag will cause it to pump all the way up and conform to the mold that i plan on making. The wing fairing and tail will be laid up before the sleeve goes in and vacced at the same time but not receive any benefit from it.......except for popping the bubbles and giving me a nice finish!!!

http://i52.photobucket.com/albums/g40/southern_killafornia/fuse.jpg

Anyway, that's the plan for now. Hopefully it'll all turn out like I imagine.

RJ

[Ollie]

The fuselage boom carries the bending load because the tail's lift going down for high speed. Therefore the boom must be very stiff. That's why the boom needs lots of uni-carbon. At an airspeed of 200MPH and a turning radius of about 50 feet, the wing lifting force might be around 200 pounds and the stab negative lift force might be around 20 to 30 pounds! You need a sturdy aircraft!

See:
http://www.thehelix.com/soaring/articles.htm
"After a trip to Parker Mountain I learned a few great tips from the locals.

1) When a thermal move through it raises the separation layer.

2) If you are standing in the middle of the knife-edge ridge line and feel the wind on your face the separation layer is low. If you feel it on your ankles - don't even bother dropping in.

3) At the expense of roll and aerobatics, an F3B type plane is the ticket for the hard banking technique.

4) With this type of plane you a shooting for a turn radius of 40'."

[I'm Rick James]

Thanks Ollie!

I'll be moving forward from here with the plug and some tutoring by some of the Parker/Vincent locals.

RJ

[nauga]

I have an idea of your skill and experience so I'll try to focus :-)

You obviously have to have room in the fuse for gear. Beyond that, a fineness ratio (ratio of fuse length to max diameter) of somewhere around 3 to 5 or so will minimize (subsonic) drag. Avoid sharp transitions and very blunt or very pointy noses (look at a typical full-scale glider for a near-ideal shape). Swept surfaces on jets are usually for transonic/supersonic reasons - they won't do you any good and may hurt perf/handling. High aspect ratio surfaces, X tail, and stiff as you can get it would be my suggestions.

Nauga,
going one-circle

[nauga]

...a fineness ratio (ratio of fuse length to max diameter) of somewhere around 3 to 5 or so will minimize (subsonic) drag...
Y'know, after writing this and re-reading it a few times it just doesn't seem to have the right 'feel'. I'll get back to this one.

Nauga,
empirically

[I'm Rick James]

I dunno what a fineness ratio is so help a brotha' out!!! When you get the "feel" hit me with a PM so I can call you and talk with you about it Dave.


Thanks, RJ

[nauga]

I dunno what a fineness ratio is so help a brotha' out!!! When you get the "feel" hit me with a PM so I can call you and talk with you about it Dave.
Length divided my max diameter. My previous number has got to be way off for gliders, but I have some digging to do. Gotta get the stink of burning foam out of my clothes first :D

Nauga,
producive for a change

[HELModels]

starting with a "slope" foil, I get a slightly more sailplane looking shape. You get the idea....

[Jeff Charlot]

Lots of good ideas, lots of good advice and input. When all's said and done, the old adage will probably hold true. "If it looks good, it'll fly good." I know you're looking for more specifics, but the above will hold true, even if your calculations are off just a bit.

Looks great!, I'll be back to help with the mold next weekend!

Jeff