RE: anhedral on horizontal stab
 

I built many (really) of the Curare/ Tipos -of many different sizes and sold hundreds of em -
also made a bunch with straight stabs
The various theories make for nice rocking chair musing .
In the years from 1975 to 1995 when I did all of this - I found -thru simple observation that the angled stab worked fine - but no different really than a straight stab --with same area on the same plane weighing the same and flown the same way.
My own idea was that the angled halves, should provide better down force in a skid (the one half anyway). This would then -in level flight help hold the nose up as the other side became blanked out.
If that was really true - I am not sure
the "WHY?" is that the CG setups and the wingloading were far more important in getting a good balanced feeling plane.
a heavy one was harder to get right and a model with the CG out an inch either way needed lots of trim correction.
Also from the eyeball standpoint - the angled stab looked like it should operate in air that -if it were some form of downwash (or hogwash), would see the disturbed air --incrementally .
again no hard proof
the real differences in all of these models ?
weight and recovery thrust -
any real relationship between this setup on the small 700sq in models and a big ol slug of a F4- is whimsy

RE: anhedral on horizontal stab
 

I hit on that in an earlier thread. Or to re-distribute area lower.

RE: anhedral on horizontal stab
 

The tail will have to generate negative lift over at least some part of the airplane's operating range for positive static stability, but not necessarily over the entire operating range. At various angles of attack, as the CP of the wing moves, there can sometimes be conditions where the tail is not lifting, or lifting positively.

Discussion of induced drag penalty as a result of tail-lift (positive or negative), then,
I quote page 351 of Mechanics of Flight:
"...For this reason, a good preliminary design is often achieved by placing the center of gravity such that the lift on the tail is zero when the airplane is trimmed with no elevator deflection at the design cruise airspeed."

If the wing and tail airfoils are symmetrical, this is achieved by placing the CG at the AC of the main wing, as a first approximation, since we are not yet considering the effect of the fuselage, cowl landing gear, etc, which can be significant (several percent of chord difference when accounted for).

This of course results in neutral stability, which we may or may not want depending on mission.

on page 362: "Notice from Example 4.3.1 ... With the center of gravity located about 8.5 inches aft of the wing quarter-chord, at airspeeds above 120 mph the lift on the tail is negative, whereas for airspeeds below 120 mph the lift on the tail is positive..."

Page 385: "This is one of the intrinsic advantages of the canard design. With a conventional aft tail, the lift on the tail is negative over at least some portion of the airplane's operating range...."

Which means that it can sometimes be zero or positive over some of the operating range (low speed/high AOA when wing CP is forward).

RE: anhedral on horizontal stab
 

Argumentative.

And you didn't answer my question.

RE: anhedral on horizontal stab
 

Are you even reading my responses?

As I said, if the a.c. is behind the c.g. which is REQUIRED for longitudinal stability for ALL flight conditions, then a downforce is needed on the tail. If there is any flight condition where a positive force is required on the horizontal tail, then that is an unstable flight condition by definition.

In addition to that, I also said that aircraft can be designed to be longitudinally unstable to take advantage of decreases trim drag, but this causes controllability issues. A good pilot could conrol this, but it would require a lot more effort than a stable aircraft.

In terms of canard aircraft, many people think that canards are better because they always carry an upward force opposed to the downward force of an aft-mounted tail, which reduces trim drag. This is true, but the downwash of the canard negatively affects the wing, which essentially cancels out the decreased trim drag. However, if you design the aircraft such that the canard is well below the wing (See Piaggio P-180 Avanti) then the canard's effect on the wing can be minimized resulting in a more efficient design. Moreover, you can design a 3-surface design (Again see the Avanti) that carries positive load on all surfaces for most of the flight envelope.

I've never done more than glance at Phillip's book, but I suggest you look into some other authors such as Roskam, Raymer, or Torenbeek. In terms of aircraft design, these are THE experts. I would stop considering "Mechanics of Flight" gospel just based on the comment about canards.

RE: anhedral on horizontal stab
 

Also, a really good design could utilize an inverted cambered airfoil in the horizontal tail, so that the aircraft is trimmed in cruise with no control surface deflections

RE: anhedral on horizontal stab
 

Dave Brown used to do something he called a "trim talk" explaining the hows and whys of pattern trimming about the time Dick was building all those Tipos. He explained that a lot of airplanes pitched down when the rudder was applied because half of the stab was blanked out when the airplane yawed and this took away the download coming from that half. The anhedral stab was supposed to generate some compensating download from the "upwind" side.
He said the negative aspect of it was that the plane tended to pitch in crosswind gusts.

I don't know if it really worked, but I suppose modern trimming techniques offer better solutions.

RE: anhedral on horizontal stab
 

Strat, the problem with that type of solution is that it would produce undesirable results in the form of decreased lateral stability. There is a saying in airplane design that "Everything is dependent on everything else." With that in mind, the best solution is the one that solves the problem with the fewest side effects. Adding dihedral in any form should never be used to solve a longitudinal problem, because it primarily affects lateral (rolling) stability.

The reason for the F-4 h. tail anhedral is due to the wing washing out the horizontal tail at large angles of attack. Adding anhedral gets the h. tail below this flow field. This is a common problem on aircraft with cruciform or t-tails. One of the best fixes is to used ventral fins. These are strakes mounted on the underside of the aft fuselage. If they are designed correctly, they will add very little drag in cruise, but help with both horizontal and vertical tail washout problems. Obviously, ventral fins couldn't be used on the F-4 due to the location of the engines. Adding anhedral to the h. tail caused a decrease in lateral stability, so dihedral was added to the wing tips to balance this out.

Check out the Mcdonnell F3H Demon - the predecessor to the F-4 Phantom. Notice, how neither the h. tail nor wing have noticeable dihedral.

RE: anhedral on horizontal stab
 

Hi all
try this - an earlier thread I started at end of 2004 - remarkably similar to the way my thread started........:D

cheers
Rick

RE: anhedral on horizontal stab
 

oops![:'(]
here's the link:
http://www.rcuniverse.com/forum/m_22...tm.htm#2229007

sorry it's late here and been one helluva long week..........[&:]

cheers
Rick