How do you measure the nose and tail moments?

[Howard Rush]

Tim's airplane, as seen in Salem

[Ted Fancher]

Tim's airplane, as seen in Salem

Hmmm?  Thanks, Howard.

With those small flaps and the light weight (I'm assuming it is at least 500 square inches of wing area or more) Tim might want to try the CG at around 20% to start with.  The half span flaps will produce less negative pitching moment (due to camber) and a very light wing loading might prefer a little further forward CG.  I think I'd start at 20% and play with it for a while to see if there is anything to be gained by a further aft CG.

Airplanes with very light wingloadings are less affected by a moment between the CG and the AC (both ends of the equation are reduced, less weight from G increases and, thus, less lift required) and the smaller flaps will reduce control force feel at the handle which can be regained by a little distance between the two.  A CG somwhere between a "standard" stunter and a flapless one might well be in order.

See, Tim.  I told you it might be necessary to massage the message a bit!

Ted

[W.D. Roland]

So, Did we ever nail down where to measure Tail Moments from ?

Having a 75% chord measurement to add to a hinge to hinge measure works?

Why don't we just use the real measurement?

David

[Ted Fancher]

So, Did we ever nail down where to measure Tail Moments from ?

Having a 75% chord measurement to add to a hinge to hinge measure works?

Why don't we just use the real measurement?

David

David,

The correct way would be to measure from the design CG (for designing your own airplane) to 25% MAC of the tailplane or, for an existing airplane from the actual CG to the 25%MAC.  Unfortunately, nobody in the stunt world has ever done it that way so we've got an untenable position where everybody just measures from hingeline to hingeline which, technically, doesn't mean much but when you're comparing airplane types as consistent in planform as historically normal stunt ships it is better than nothing.  The basic mold for stunt design is so cast in concrete that any full flapped design of more or less average proportions is going to fall into the noise range.

I know that's not what you want but that's the way it is! 

If you're seriously interested in developing a unique concept (or simply a conventional design of your own to perfect) I'd encourage you to use the proper CG to MAC.  Plan your CG as a primary integral part of the design itself, adapt the planform to support that position and then your measurements based on tail MAC will be consistent.  If nothing else you'll be unique!

Ted

[RC Storick]

Everyone would agree that Billy W is the Man. If you have ever watched the movie the man and the museum he states for some of his aircraft "they just didn't work out the way they were suppose to". If there was a magic formula don't you think he would use it?

This was brought to my attention by my coach.

[W.D. Roland]

Hi Ted
CG to hing line is what I have always done.
Well ok, used high point until Dad got hold of me.
Being back into C/L has been some what of a ? ? ? with the hing line to hinge line measure,  for me this is a
meaningless number unless the discussion/communication is on airplanes whose wings the same chord and CG.
Trying to clear up confusion.

We need a SSD (Society of Stunt Designers) to establish a set of standards!

Hey Robert, I have some of those "they just didn't work out the way they were suppose to" airplanes

Seriously guys 

David
Tries not to be an ass, rarely successful.

[Serge_Krauss]

Everyone would agree that Billy W is the Man. If you have ever watched the movie the man and the museum he states for some of his aircraft "they just didn't work out the way they were suppose to". If there was a magic formula don't you think he would use it?

That's about my favorite CD from Bob Hunt. I love Bill's planes and his descriptions. That CD is a treasure trove. In answer to your question, no, I don't think he would. Choice of nose length and stab size are one reason the "Ares", for example, must be built light. Another is of course the Fox .35. While I love the long nose, that shaft extension did not help matters. The "Ares" is my favorite stunter of all time, by the way.

"Magic formulas" are not anything with which I'm familiar. The expressions I've posted from time to time are simply shorthand for common-sense physical laws and their applications.  For instance, the tail moment arm is just a wrench or lever. It extends from from the a.c. of the horizontal tail, the point where the force is exerted, to the c.g. (balance point), the lever's fulcrom or place where the jaws of the wrench act. Using the hinge lines for these two points in a comparison is akin to holding your wrench near the jaws or not attaching it where it can directly rotate anything. It's not "magic", but simply describes the placement of your wrench (fuselage tail arm) and hand (tail lift) to twist/rotate the plane. The hinge-hinge thing does not correspond to nor describe the torque arm you are using. That's all.

Incidentally these expressions actually come from the category of physics/engineering also known as "Mechanics"; it is "the science that treats the motion of bodies" and encompasses much of our knowledge of universal laws (some engineering texts on my shelves have that word in their titles). It concerns the actions of forces and consequent changes in motion.

SK