SMALL - espritmodel.com SMALL - Telemetry SMALL - Radio
Reply
Thread Tools
Old Dec 24, 2005, 12:59 AM
Nucking Futs
I'm Rick James's Avatar
Southern Killafornia
Joined Jan 2005
1,626 Posts
Discussion
Most aerodynamically efficient nose shape?

I'm in the process of collaborating ideas and designs for my next project. A discus launched glider.

All the models that I've seen have a pod up front with a carbon fiber boom attached to it and around 2ft. later, the tailfeathers. The wing sits on the boom with some type of mount. I plan on fairing the mount to further reduce drag as well.

All of the nose pods look "wrong" to me. Their bulky, asymetrical and look like lumps of play dough plopped onto the front. I know very little about aerodynamics but the TLAR method isn't working for me. I would imagine a nose shaped like a model rocket would be the ticket. Fairly pointed and symetrical all the way around so no additional lift forces act on the nose. Am I headed in the right direction with this hypothesis, if you can call it that? I have access and the ability to make it from composite materials so the shape shouldn't be an issue.

Thanks all, RJ
I'm Rick James is offline Find More Posts by I'm Rick James
Reply With Quote
Sign up now
to remove ads between posts
Old Dec 24, 2005, 01:55 AM
Beware of the Kiwi
killerkiwi5's Avatar
Chilliwack, BC
Joined Jan 2004
110 Posts
Could be wrong, but I've always thought that rounded noses were better for subsonic speeds. Thats why airliners and other slow jets (A-10) have rounded noses, and the fast jets (F-15/F-16/Concorde) have pointy noses. Dont know the exact reason, maybe the longer noses have more surface area.
killerkiwi5 is offline Find More Posts by killerkiwi5
Reply With Quote
Old Dec 24, 2005, 07:23 AM
Lift is cheap - Drag sucks
Tom Harper's Avatar
Socorro, NM
Joined Jul 2004
3,621 Posts
For slow speeds, low reynolds numbers, I'd use a parabola. The NACA0010 symetrical section is a good standard. You can multiply the coordinates to get any thickness you want.
Tom Harper is online now Find More Posts by Tom Harper
Reply With Quote
Old Dec 27, 2005, 07:54 PM
B for Bruce
BMatthews's Avatar
The 'Wack, BC, Canada
Joined Oct 2002
11,424 Posts
Airfoils work for wings so why not fuselages?

For the top down view a slippery symetrical shape that fits in with your width requirements. For a side view one with proper camber of about 2 to 3%. This matches the idea that the air flows up onto the wing and then downwards off the trailing edge.... and it would look best as well. Here again pick one that is thick enough and has the max thickness point to correspond to your needs for equipment and wing placement.

The so called "laminar flow" types of the NACA 6 digit series are going to look the best and will probably fit in with the design the best. Just do not expect them to deliver on their low skin drag promise but at least they'll be sexy looking.

In truth as long as it's not blunt and square with sharp corners one shape is about as good as the next.
BMatthews is offline Find More Posts by BMatthews
Reply With Quote
Old Dec 27, 2005, 08:02 PM
Registered User
vintage1's Avatar
East Anglia, UK
Joined Sep 2002
29,703 Posts
My understanding is that any design will be a compromise between miinimal cross sectional area, sufficient volume to hold the ballast, all faired in to an approximate airfiol shape.

Trouble is, to get the cross sectional area down, you need more weight or a longer nose...which in itself may intriduce structural problems and extra drag.
vintage1 is offline Find More Posts by vintage1
Reply With Quote
Old Dec 27, 2005, 08:38 PM
Registered User
HELModels's Avatar
Joined Aug 2003
2,636 Posts
A bit more than 1 root chord beyond the 25% of the wing is a good choice for length. Use that as an additional constraint on shape and size.

Afterbody shapes are more important than forebody IMO.
HELModels is online now Find More Posts by HELModels
Reply With Quote
Old Jan 10, 2006, 01:53 AM
Team WC2013 F3K
oakman7004's Avatar
Sweden
Joined Dec 2004
989 Posts
Quote:
Originally Posted by BMatthews
Airfoils work for wings so why not fuselages?

For the top down view a slippery symetrical shape that fits in with your width requirements. For a side view one with proper camber of about 2 to 3%. This matches the idea that the air flows up onto the wing and then downwards off the trailing edge.... and it would look best as well. Here again pick one that is thick enough and has the max thickness point to correspond to your needs for equipment and wing placement.

The so called "laminar flow" types of the NACA 6 digit series are going to look the best and will probably fit in with the design the best. Just do not expect them to deliver on their low skin drag promise but at least they'll be sexy looking.

In truth as long as it's not blunt and square with sharp corners one shape is about as good as the next.

This is what M. Selig did when he designed the OPUS 750 glider...The fuse is clearly "odd" looking but from the side it is like a airfoil and acts like one.
Read more at http://www.nesail.com/detail.php?pro...07cc94960e2ffc

This shape can however be complicated to "convert" to DLG ships with good outcome for weight, stiffness. Thats probably a reson why Dr Drela introduced the "pod" design...

Just my .02

Cheers Jonas Ekman
oakman7004 is online now Find More Posts by oakman7004
Reply With Quote
Old Jan 10, 2006, 06:18 PM
Registered User
Australia, NSW, Sydney
Joined Feb 2003
1,114 Posts
What's interesting about a discus launch glider is that it must fly efficiently at two very wide extremes of airspeed. At launch, there is a dominating requirement for low profle drag. It's the thin, low cambered wing sections and the low cross sectional area fuselages that have brought about the amazingly high launches that modern competition ships achieve.

This requirement for low drag at high speed demands a fuselage optimised for that condition. Here, the induced drag from the wing is low, so the drag of the model is dominated by the profile drag of the fuse, wing, and tails. Profile drag is made up of skin friction drag, form drag, and interference drag, and these factors should all be considered in the fuselage design.

So, some design requirements for a pod and boom fuselage like you are doing would be:
Keep the cross sectional area to a minimum.
Use a streamlined shape that does not taper in too steeply after it's maximum width and cause flow separation.
In Model Aircraft Aerodynamics, Martin Simons suggests using a profile designed for laminar flow, because if well built, laminar flow can be expected over at least the nose portion of a fuselage.
Don't use a pointed nose, because this adds skin surface area.
At launch, the wing angle of attack will be low, so the fuselage should be oriented for minimum drag at this low angle of attack.
Where the fuse meets the wing or any wing pylon or similar, fillet the intersection to minimise interference drag.
Adding camber to the fuselage form to decrease drag at higher thermalling angles of attack is a bad thing at the low launch angle of attack, so think carefully about going this route.
Judging from the current crop of top level DLGs, the thickness to length ratio that is the best compromise between form and skin drag is surprisingly fat, maybe 15 to 20%, so don't make the fuse too long thinking you're improving things.
If in doubt, copy the Supergee 2.

Hope this helps,
Graham.
Salto is offline Find More Posts by Salto
Reply With Quote
Reply


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Question What's the most efficient shape for a wingtip? Hogster Electric Plane Talk 2 May 28, 2005 08:14 AM
most cost efficient 10amp lipo? gomarpyle Batteries and Chargers 10 Nov 30, 2004 04:06 AM