Shop our Airplanes Products Drone Products Sales
Thread Tools
Oct 30, 2012, 01:39 PM
Registered User
Joel K. Scholz's Avatar
Discussion

Where would the CG fall on this Dimorphodon?


I estimate the CG at around 2 7/8" from center LE. Grid is 1" squares.I am assuming it will balance like a flying wing. Airfoil klinefolglemanM3. Span 6 feet. Controls ailerons and v-tail
Sign up now
to remove ads between posts
Oct 30, 2012, 02:30 PM
York Electronics
Gary Warner's Avatar
The basic airfoil shape can effect the center of lift and therefore the CG. It is re-flexed? Airfoil number?
Oct 30, 2012, 03:13 PM
Registered User
Joel K. Scholz's Avatar
Gary the reflex is going to have to come from the ailerons.
Oct 30, 2012, 04:13 PM
York Electronics
Gary Warner's Avatar
Hi Joel!! I didn't catch your user name 'till just now. Good to see you and your work - our paths don't cross as much as they used to.

Honestly, you got me with that airfoil.

It is very short-coupled with respect to pitch, suggesting that the CG range is going to to be very narrow. I'd hate to suggest something that's not even close to being flyable. Maybe someone else has experience with that airfoil on either a flying wing design or at least something that's very short-coupled.

Not to leave you without any input, my thoughts for a regular, slightly re-flexed airfoil are that it should be in the 4" range (+/- 1/2")... but I don't want you to use that number without someone's experience saying so.

Have you considered the 'ol trial/error method of a small foam replica to experiment with?

BTW, wild design... It's got Joel written all over it!

Great project!

Looking forward to following your progress on this one.
Oct 30, 2012, 04:50 PM
Registered User
kcaldwel's Avatar
It is going to behave like a plank flying wing. The CG should be at about 22% MAC, and the flyable range will be very small as Gary said: too far forward and you won't have enough elevator power to slow it down to a reasonable trim speed; too far aft and it will be too pitch sensitive.

The airfoil does not make a difference to the CG location or stability (except for second order effects), only the trim.

Kevin

Edit: The second order effects can become important when you are flying with very small static stability margins (22% MAC would be about 3% SM, which is pretty small). Re effects, etc, can cause shifts in the neutral point on the order of a few percent. Normally these small changes don't make much difference, but when you have a very small static margin, they can cause significant changes in pitch stability with speed and AoA.
Last edited by kcaldwel; Oct 30, 2012 at 06:40 PM.
Oct 30, 2012, 06:44 PM
Red Merle ALES
Curtis Suter's Avatar
+1 Kcaldwel!
Oct 30, 2012, 08:12 PM
Registered User
Joel K. Scholz's Avatar
+1 ? can you be more specific?
Oct 31, 2012, 03:40 AM
B for Bruce
BMatthews's Avatar
Your pictures of the model don't seem to match the sketch you showed in your first post.

From the sketch I'd suggest that the 20'ish% chord location would be about at the finger tips of the "hands". But your pictures suggest that the finger tips are far more back than that on the final model.

I suspect that the airfoil you chose is going to require a pretty good dose of aileron reflex since the ailerons are not very wide. And I don't see anything that is going to act like a V tail. At most you've got what appears to be some slight angle in the center section elevator. Or that could be just wishful thinking.
Oct 31, 2012, 07:04 AM
Registered User
I'd make a small chuck glider to test things out before testing the full size ship. Looks cool, it would be a shame to get a dent in it before it's fine tuned.
Oct 31, 2012, 07:35 AM
greg
ciurpita's Avatar
Quote:
Originally Posted by Brandano View Post
I'd make a small chuck glider to test things out before testing the full size ship. Looks cool, it would be a shame to get a dent in it before it's fine tuned.
why not start by lightly throwing it while kneeling on the ground? this way it won't have far to fall, or just glancing impact with the ground. If things are looking right, give it a harder toss and hopefully it will go 40 ft. (from 4ft, 10:1 glide ratio).

greg
Nov 01, 2012, 08:17 AM
Red Merle ALES
Curtis Suter's Avatar
Quote:
Originally Posted by Joel K. Scholz View Post
+1 ? can you be more specific?
Joel,
That just means I agree with Kcaldwel

Curtis
Nov 01, 2012, 10:04 AM
Registered User
richard hanson's Avatar
The KF airfoil is sturdy enough and easy to build but make larger ailerons -if you can.
and if possible make some w shaped bracing - (CF strips CA in place on bottom of aileron ) to stiffen em.
These all foam setups tend to bend easily and positive control can become vague.as speed changes

Most of the reason for the airfoil shape is to gain some rigidity.
airfoils are a compromise - strength first - shape next .
Your setup is a good compromise - but more variable -in undercamber - for slow flying may help.
Nov 01, 2012, 10:16 AM
Registered User
Joel K. Scholz's Avatar
Richard, I used a technigue I found in the foamiies forum to stiffen the foam. You use newspaper/and or tissue. and titebond glue to layer with. It works incredibly well and is vey light. I have no worries about this wing or ailerons warping in flight.
RTF at present without paint is 7.2 oz psf
Nov 01, 2012, 10:26 AM
Registered User
richard hanson's Avatar
Quote:
Originally Posted by Joel K. Scholz View Post
Richard, I used a technigue I found in the foamiies forum to stiffen the foam. You use newspaper/and or tissue. and titebond glue to layer with. It works incredibly well and is vey light. I have no worries about this wing or ailerons warping in flight.
RTF at present without paint is 7.2 oz psf
Good technique -- I did a few models which were covered in silkspan and painted with Titebond- stif and strong - easy to do
I now have a box of carbon bits n pieces - rods /strips etc.- so I just stick those in the surface using a sharp knife to install.
for my teeny tiny models - lighter .
Nov 01, 2012, 02:41 PM
FS One
MSelig's Avatar
Quote:
Originally Posted by Brandano View Post
I'd make a small chuck glider to test things out before testing the full size ship. Looks cool, it would be a shame to get a dent in it before it's fine tuned.
Brandano's got the right idea! So many times I see people skip this simple step w/ novel configurations. It works very well as a test w/o all the math.


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Discussion Fuel's affect on plane's CG; where will the fuel predominantly reside? Ian Davidson Fuel Plane Talk 35 Nov 28, 2014 04:41 AM
Discussion Where does this Camera fall between the #16 and #18 richkphoto Aerial Photography 1 Sep 09, 2012 05:20 PM
Discussion I would think that even this would fall under the 1st Amendment logan5 Life, The Universe, and Politics 15 Nov 04, 2010 02:02 PM
Cool This looks cool, but where would the CG be? Dancy Modeling Science 2 May 24, 2004 07:58 PM
Cool This looks cool, but where would the CG be? Dancy Modeling Science 1 May 23, 2004 11:43 AM