Nose length?

[irish_lord99]

I'm wondering if there is a rule of thumb for the ratio of fuselage you want ahead of the COG to what's aft?

I'm working on a design for a 2.5m traditional balsa-type sail plane and as it stands now I've got 35cm of fuselage ahead of the COG and 88 aft. It looks a little tail heavy, but it's hard to tell. I am going to place the battery (150 grams) in the very front, and I think I can fit all the radio gear ahead of the COG. I plan on using pull-pull for the rudder, so that should cut down on weight in the tail.

What do you guys think? Do I need to extend the nose?

Thanks,
~Jake

[fnev]

You ask for a rule of thumb:

- Nose length around 1.5 root chord
- Tail length around 2.5/3 root chord between wing trail edge at root and stabilizer leading edge at root.

BTW: yes, nose length may have an influence on aircraft stability and maneuverability.

[irish_lord99]

Wow, thanks for the quick response! One question though, is the length of the nose as you said measured from the LE of the root chord or from the COG? I'm assuming LE?

Thanks,
Jake

[StevenatorLTFO]

Leading edge of the wing

[thermalsurfer]

Andy Lennon's model aircraft design book goes into this subject. If I remember correctly he recommended laying out your radio equipment as it would sit in the fuselage on a simple see-saw type beam with the pivot at the proposed CG (35 % or so MAC) and seeing how far forward/aft the components must be shifted on the beam to get it to balance. This will give you a nose length that will be darn close to what is needed without resorting to massive amounts of lead. He also provides weights/area for different construction techniques to estimate the weight of a structure when it's finished so you can get the weight of the tail / aft fuselage structure on the beam.

[irish_lord99]

Cool, thanks! Looks like I need to draw a longer nose.

lata'

Jake

[tkallev]

Leading edge of the wing
My thumb-rule book says from the CG :cool:
tk

[FrogChief]

Personally, I find that most sailplanes would benefit from a bit longer nose. Helps keep noseweight at a minimum.

[StevenatorLTFO]

My thumb-rule book says from the CG :cool:
tk

:p :p :p

[irish_lord99]

Okay, well I've changed it up a bit and this is what I've got so far. It all just barely fit on an A0 page, and I'll have to draw the rudder separately.

I won't be able to work on it again for a long time since we're making a trip to the States this summer and then moving from our flat almost as soon as we get back.

Thanks for the tips and input.

~Jake

[fnev]

IT IS from the leading edge…!!! LOL

Just had a quick look at your drawing: everything is right.

Just one comment: You seem to have used the root chord as the datum for you whole glider. Nothing drastically wrong about this choice, but you might end up with a low tail attitude in flight. Personally I prefer a slightly tail up attitude.

Rather draw your fuselage with its datum horizontal (nothing wrong by choosing the top of the fuselage as the reference) and put the proper wing incidence (between 2 and 3 degrees is usually OK). Since you have a full flying stabilizer you can leave it at zero (as it is currently); just be prepared to put around 1 to 2 degrees of down if your CoG is not to far forward…

[atjurhs]

Jake,

I too am a scratch builder, and one of the VERY best & free tools that you can find to check-out your designs is from Curtis (CloudIFR) called SailplaneCalcs. It's an excel file in which you type in all your design's measurements, and it shows lots of results with acceptable range of values for each calculation. So you'll know not only if you're in the ballpark but also within the infield. I've used it with all my scratch builds (both X-tail and V-tail) and it's always been right on the button. Here's a link to where you can download it:

http://www.tailwindgliders.com/Files.html

[irish_lord99]

fnev,

You're right about the incidence. I plan on building the fuselage sides and then cutting the sing saddle out after I've finished the center wing panel. To draft that is a bit difficult, partially because I'm not an expert with Solidworks. To get the airfoil I import it from profili as a separate sheet and then just place it over the fuse where I want it. I'm not sure how to angle the second sheet to show the desired incidence. Anyways, these aren't really true "plans" per say, more like guidelines for how to build the sides and join them together. Sometime I draw what I consider "true" plans, but I'm usually too lazy and I just get the basics on paper and make the rest up as I go along. :D


Thanks atjurhs,

I used sailplanecalc for this glider design to determine tail size, dihedral, etc. But I have a bad habit of making planes with either too long a nose or too short and I didn't see a function in sailplanecalc for determining nose length.

I do love that spreadsheet though and use it whenever I'm working on a design.

~Jake

[atjurhs]

Jake, you're right. Ohhhhhh, Curtis, here's a new function to add to Sailplane Calcs :rolleyes: