Originally posted by hitech
Casca: The confusion I belive normaly come from a definition of CL on an airplane. Is that CL the wing CL, or the CL of the horizontal stab and all other components of the plane combined.
I belive it also is normaly described that way because it is much easyier to describe in basic flight training. Hence why most people think most tail planes are normaly creating a downward force.
For a plane to be stable the combined CL must be behind the CG, but that is an entirely different statment than the Wings CL must be behind the CG. Of the planes we have found with documentation on tail forces in flight. They have produce up forces when in level flight. The resone is that producing down force requires the primary wing to produce more up force to maintian level flight, hence more drag. When you are designing a fast fighter would that be the normal condition you would wish to set it up in?
HiTech
Apologies in advance for what I anticipate will be a lengthly post. I think I see what you are driving at and if you have information that edifies me I am, of course, eager to learn. I would certainly be interested in seeing a case or two of aircraft that are set up intentionally tail heavy.
To answer the question of the normal condition I would wish to set a fighter up in I would answer, in general terms, that I would want the CG to coincide with the CL at the rearmost allowable loading of the load schedule. This would provide the highest speed, the lowest stall speed, the best fuel economy, probably something approaching neutral static stability (but thats a big enough subject on its own, lets don't go there) and would have a salutory effect on controlability and manuverability (two other big subjects, so let's ignore them for now).
As for considering the Cl of the system instead of the Cl of the foil in isolation you make a valid point. That being said the CG of every weight and balance schedule since Orville and Wilber are referenced with respect to LEMAC (leading edge of the mean aerodynamic chord) of the airfoil. 20% to 30% are average ballpark figures for most plain vanialla airfoils. Now there is every possibility that the aerodynamisicts are dumbing it down so that the mechanic reading the scales can grasp it. What you say may be entirely true, it's just that I have never seen it.
I looked for some weight and balance documents online but they were pretty thin. I was able to come up with a Technical Order W&B schedule for a P-47 which sets the CG limit at 25% to 32% MAC. This does not appear to be a tail heavy aircraft at first glance.
Among the documents I have laying around the office is the Fifth Edition Textbook of the Transportation Safety Institute run by the Federal Aviation Administration. This is the premier aviation safety and accident investigation training organization in the world (apologies to Cranfield in Jolly old England, that's just the way it is). All NTSB investigators go through this school and the majority of investigators for almost any major aviation regulatory agency on the globe find themselves there at one time or another. Many academic luminaries have been and are currently associated with this organization. Plug: My university, CMSU, is offering our Master of Science in Aviation Safety in collaboration with them via distance learning so if anyone has a yen to kick tin, this is your chance.
I'm including a paragraph or two and a couple of illustrations. If I turn out to be wrong about all of this I will cheerfully admit it and notify TSI to change their text.
To give the quote some context, what is being taught in this portion of the text is to use wreckage distribution and failure modes to determine accident causes.
"To be able to analyze an inflight breakup, it first must be clearly understood how an aircraft is aerodynamically loaded in its normal configuration. Reference is made to Fig. CIII-29. It must be remembered that an aircraft rotates around the center of gravity, and the center of gravity is located in close proximity to the quarter chord point of the mean aerodynamic chord (that would be 25% LEMAC mentioned above). The center of gravity in this illustration may be assumed to be in the center of the aircraft and in the area of the spar outline. The arrow under the engine represents the weight of all items ahead of the CG and the arrow under the aft fuselage represents the weight of all items aft of the CG. The sum of the moments of these two forces around the CG,
in addition to the wing pitching moment (my italics, it seems to keep getting lost in the shuffle), if it is a cambered airfoil, will result in a nosedown pitch. This resultant nosedown pitch is prevented by a download on the tail. This download then brings the pitching moment to zero, and my be considered as that balancing force which places the aircraft in a state of equilibrium as far as pitching moments are concerned."
The following illustration is on page 181 of the text and shows the condition that we have been discussing.
No more breakfast forever Part 1
This illustration is on page 182 of the text.
Headed for Page Two on Paul Harvey
This is of course the typical pitch down and breakup sequence (wings failing down if we are going fast enough).
Wish they had used a Piper instead of a Mooney. I used to have a Mooney, great ship. Someone in the game said you had an RV-8. I'm planning on building one at some point (any opinions on the Eggenfellner setup? I'm thinkin we are gonna lose 100LL at some point so the angle valve IO360 might become problematic). Probably belongs in a different thread.
Here at the school we have a V Tail Bonanza in pieces at the airport. We lay it out twice a year for the final in the Accident Investigation class. It belonged to a mortician who was picking up a body with two surviving family members. On the way back he lost it in IMC and wound up in a spiral dive. Upon popping out the the clouds and seeing the ground he hauled back on the yoke hard enough to cause one of the stabilizers and ruddervators to come off of the airplane. With the remaining ruddervator he managed to stay up about ten minutes after which it also left the airplane. It, no surprise here, pitched down and the wings failed in the typical downward direction. This aircraft was right at or out of rear CG limits (the body was in a coffin projecting into the tail cone).
I think the above fairly summarizes how I've arrived at my conclusions although they are not the only sources. If you can show me where I'm off track I'm all ears.
Casca
