Hello everyone,

I'm new to RCUniverse and I'm delighted with these polite and educated discussions. Thanks for sharing!
I'm looking for information on horizontal stabilizer dihedro selection. I haven't had much success looking for it on available (to me) literature... Thanks!

I have not heard of putting dihedral in the horizontal stabilizer. If you get information on this, could you please share it.

It's really interesting...take a look at the Predator (UAV from Aeronautical Systems) and you'll see that the tail assy is inverted: the rudder and the horiz. stabilizers are mounted in the lower part of the fuselage! and the horiz. stab. is not straight - what's the advantage / trade-off? - I mean, the landing gear must be longer and heavier... In fact, only a few of these UAV's have conventional tail assemblies! The full-size Bonanza had originally a "V-Tail", but it has been replaced by a conventional one in later models... Now, why are these UAV's (some, the same size as our RC creatures) going the other way?
Oh, one last comment: why is it that high performance aircraft like the F-16 and even commercial aircraft, like the B777, have angled horiz. stabs?

When the tail has a dihedral angle in the range of 30 to 45 degrees, it is usually to combine the functions of the vertical and horizontal tails. There are at least two criteria for sizing such a tail. The tail can be sized to have the same pitch and yaw control authority as a conventional tail or it can be sized on the basis of stability considerations. A V-tail or an inverted V-tail can usually be built lighter for the same strength compared to a conventional tail. When a V- tail is used the tail cone of the fuselage has to be stiffer in torsion than for a conventional tail.

See:
http://www.charlesriverrc.org/articles_modeldesign.htm
for several articles on V-tails.

On a similar topic, why did some of the older pattern designs have an anhedral stab (negative dihedral)? I know that anhedral is de-stabilizing, but for a pattern ship why would you want a less stable tail surface?

We seem to talking about 2 things here, V-tails and anhedral stabs, so I'll comment on both from the standpoint of an aerobatic plane.

V-Tails: I have flown several and built a few. In my opinion, they are fun for a while, but not very good from an aerobatic standpoint. You get a lot of unwanted yaw & roll from them. In an acro plane you want to decouple all the axes so you can use rudder, elevator and aileron independently. With a V-tail, this is impossible. When you are in a bank and use elevator, one half of the tail is pulling the wrong direction. V-tail planes also tend to barrel roll. I'm not a pattern flier, but I do like my fun fly planes to fly somewhat correctly.

I once built a Duraplane with reduced incidence (stock, it has a lot), downward tip plates to reduce effective dihedral and a V-tail. Used a Fox .50 I had for power. No gear-hand launched. Talk about a graphic demonstration of rear fuselage torsion from a V-tail. I could snap a couple of times and release and watch the tail rock back and forth. The same with full power spins.

Also tried a V-tail on a PVC fuselage like a SPAD. OS .46 powered. With the lower half of the PVC fuselage cutaway back past the wing to save weight, I also got the wiggling of the tail after snaps. I made another PVC fuselage without the cutaway and screwed the V-tail to it. It reduced the wiggle, but not completely. I finally went to a conventional tail which cured the problem.

The PVC fuselage, Coroplast wing planes are easy to cut-and-try some of these ideas if you are interested in seeing for yourself.

Anhedral Stabs: The first pattern plane with an anhedral stab, as I recall, was Hanno Prettner's Curare. I recall reading that he did it to lower the stab and increase the side area. The F-4 Phantom was a first line fighter in those days (I flew it in Vietnam, by the way) and the anhedral stab on it may have had some influence. I flew a Curare in pattern for a while many years ago so don't put too much credence in the fact that anhedral is supposed ot be destabilizing. The Curare flew great, as did everything else Prettner designed.

One of the reasons for lowering or raising the stab is to correct a knife edge pitch with rudder. Remember, we want pure yaw with rudder. Anhedral would have the effect of lowering the stab.

Joe Bridi had a slight amount of anhedral in his UFO, about an inch, I think, but I feel this was for looks and not for any engineering reason.

I had a high performance fun fly type plane back in 1976-77 time frame when I first saw the Curare. To try anhedral, I sawed the stab on either side and glassed it back on with about 3 inched anhedral. I didn't notice much, if any, difference. Later on I bought the Curare kit and flew a little pattern.

As I mentioned, the 2 1/2 square drail pipe PVC fuselage planes are easy to modify, especially with Coroplast tails. You could use any wing and screw on conventional tail, V-tail, T-tail and anhedral stab very easily. You could also put the stab in the top, bottom and middle of the fuselage. I've had a bunch of those planes, both with Coroplast wings and wings from crashed planes or old junkers. They are fun to play with and they can give you a lot of insight into whether the theory is just a lot of BS or not.

Just a note, on the newer fighter planes, one reason (and probably the biggest) for the tilted tail feathers is to avoid straight surfaces which reflect radar energy.

Many many airplanes have dihedral in the tail. This practice goes back a long way.

You will see it in many twins. From the B26 and Tigercat and many WWII twin aircraft.

You will also find it jet transports . It is in all Boeings less the 727. The DC8 and in Airbuses.

Stab dihedral is in modern twins like King Airs and Saabs, and Cheyenes and most low wing turbine twins without T tails.

If you look at high wing twins like the Rockwell Commander or the Grumnan Tracker you will see the dihedral is very pronounced. And the Mars! Wow. You generally see more on the high wing multi's.

You generally won't find it in single engine aircraft or in multi's with T tails.

I believe the number one reason for having dihedral in the tail is simply to place the tail surfaces in clean air. There are some other benefits.

Some of the weirder tails in fighters mentioned may have been designed to help hide the hot exhaust.

Gentlemen.....Most of the modern era aircraft(Jets and the Like)...have had the advantage of excellent windtunnels and CAD programs for determinig aerodynamic feasability of designs and practicality of buiding them.....alot has changed in a short period of time......F-16, F117 ,FA-18 along with Boeing 777 and A-319,320,321 330,340 are just a few that use computers to do the work that was done by pilots just a few years ago....they use "Stability Augmentation" thru computers to fine "tune" aircraft flying envelopes...so smaller tail surfaces and different designed tails are used because much is handled by computers...theres inherent instability but its offset by those computers.....Dihedral is used for stability and each plane has its own unique characteristics...I.E.B-25 has roughly 5* dihedral to eng. nacelles and 1* O/B to wing tip....with stab having 0*.A-26 on other hand has 4*-5* dihedral on wing and roughly 6* on the stab.........As you mention B-727 its horiz stab is actually slightly neg by 1/2* or so.....so its sometimes trial/error til a good combo between wing and tail is found........thats whats nice about models..If an idea doesn't "Fly"......try a different one til it does......Bill.....

I also find this interesting and would expect tail stabs to reflect the main wing dihedral!

But as far as models are concerned - I think the flat 'building board' has a lot to do with it. So those very rare models ( and I don't mean models of full size replicas!) mainly aerobatic types must have a valid reason other than pure 'looks'.

Hmmm...

Does decalage has anything to do with angled stabilizers?

I spoke with a friend who worked aero on the F-4 series of airplanes. He said the anhedral tail was an attempt to position the tail in a favorable position relative to the wing downwash, with flaps down, when landing on an aircraft carrier.

I remember a series of magazine articles by Don Lowe that discussed the horizontal tail's vertical position and its effects on a pattern model. I think I remember the pitch in knife edge could be controlled with position on the fuselage. In general he recommended a low position for best flying. With the Curare the anhedral achieves the same result while allowing the root of the tail to be higher. It would be just as good with the tail lowered a inch or so. It certainly looks business like with the anhedral tail though.

To add to this, most airliners that use dihedral in their H-Stab also use a high degree of sweep-back, which decreases stability. The dihedral helps correct this. For instance, the DC-10 uses 6* wing dihedral and 10* of stabilizer dihedral, while both have a 35* sweep back.

Anhedral stab--As it applies to aerobatics --
put the model into a hard skid -then hold it there -
each of the flying surfaces - wings stabs -fin etc-- is now approaching the air at an angle which may/ may not , add a lifting- or increased pressure .
That type flying is simply not part of full scale practice --but in an aerobatic model - --here is what it can do -
the stab half which is yawed forward --now has a totally different AOA -and can add down pressure
The stab half on the trailing side , is blanked -in some cases -by the fuselage .
The net gain --was to have the down pressure exerted by the entire horizontal stabilizer, remain constant -even if yawing the model , killed the air flow on the trailing stab half.
OK- any comments as to why this idea is flawed?
Please-- no formulas -

Dick

I think the only flaw in the approach you mention is in trying to guess what the airplane response is going to be. Back when I worked for a living at McDonnell Douglas we could have indeed proposed a cause and effect such as you present. Certainly something similar to that went on in meetings to figure out what was causing the pitch up with the F-4 with the earlier straight horizontal tail. Incidently it doesn't look near as mean with the straight tail. I love the wing tip dihedral with the anhedral tail and teeth on the radome.

Assuming the money was there a wind tunnel model would be built with the capability to vary things such as the anhedral angle, its vertical and longitudinal relationship to the vertical tail and rudder (with and without deflections), fuselage width and depth and with some other generic fuselage and wing (but assuming some useful sizing requirements).

Then in the wind tunnel they would test the matrix of possibilities, look at the results, exclaim what in the heck was wrong with the data, repeat most of the runs, acknowledge they didn't have a clue as to what was happening and then someone would claim that they knew that it would turn out that way all the time! Whoever did it first and often enough usually wound up being boss eventually.

Honestly, someone that has mind experiments so precise that they can give exacting results with respect to something like this is someone that I must stand in awe of but most of the time I tend to think that it is arm waving that usually has left out some very significant variable. It is interesting but not precise.

In Hanno's models the models (I can't remember the name of the model before the Curare but it was published in MAN I believe) that led up to the Curare looked very similar except for the tail anhedral. I also must believe that he was talented enough to determine the difference in flight. He is a great talent at this flying thing.

If I were sufficiently interested in really knowing the difference I would build a typical pattern model with removeable/interchangable horizontal tail mountings. The vertical position and anhedral angles would be the variables. Keeping the mass of the removeable parts the same would minimize the variables and make a good experiment. Then it becomes a matter of flying maneuvers and observing results. It really would be difficult to evaluate with different complete models because of the variables that can change from model to model, much less the expense.

I normally am so happy to get one model to fly more than 10 times without the big oops that I am not the person to do this but it would be interesting wouldn't it.

Ben

Ben - we built a sh-- load of em with various angles and sold hundreds of em in all configurations -flat stabs and anhedral stabs - this goes back to 1976
Our clone of the basic Hanno Curare was the Tiporare- and having been thru many examples and changes of this drooped tail setup -including some special purpose test designs - I can say with some confidance - that the droop did change the skid result.
Having said that - the real differences in skid results were really results obtained by shifting CG fore -or aft.
the download required to maintain level flight -in a skid -is more critical -as the cg moves aft - and this is not a finite thing -
pitch varies from design to design --but the result is always the same - move cg aft - yaw the plane and it pitches down this is in a level -upright flight test.
A lot of anhedral in the tail group minimizes this .
some guys moved the horizontal tailplane down and claimed this is a cure - I do not agree.
The CG for a particular design has to be juggled to get things as neutral as possible - and even on a kit job - the results varied from model to model - any speed change affected it - as did wing loading -- on and on ad
nauseum

Those that say that lowering the horizontal tails will do the same thing as the anhedral perhaps can point to the cause of the effect. Also the effect being enhanced with aft CG also point to one thing. Something that causes a small change in horizontal tail load due to yaw), which you have pointed out. Where does it come from?

Flying straight and level at 1 g with the symmetrical wing does produce a downwash that decreases with distance aft of the wing and varies with the vertical position that is being considered. Smaller as you go up from the wing plane.

The horizontal tail has an upload if the airplane is trimmed (wing lift times distance from 1/4 chord to GC = tail lift times distance from 1/4 of tail to CG). With a high mounted horizontal tail location the downwash is pretty small so the tail angle that is required to make the moments about CG equal to zero has a small component due to downwash.

Yawing the airplane disturbs the otherwise reasonably smooth flowfield around the fuselage probably causing a slight decrease in dynamic pressure near the fuselage creating a decrease in lift around that general area of the wing and a decrease in downwash. So with yaw at the location of the higher horizontal tail you have a small local angle of attack change (due to downwash change) and a lower dynamic pressure. The lower dynamic pressure (which decreases the upload on the tail) and the decreased downwash (which increases the upload on the tail) gives a net airplane nose down moment.

Again, as you said an aft CG will make the airplane more responsive to tail load changes due to a decrease in aircraft stability.

If the above is true and if the guys saying that a lower horizontal tail is acting the same as an anhedral tail are right then moving the horizontal tail down or introducing anhedral (effectively lowering the horizontal tail) has apparently moved the tail to an area where the net result of change in tail lift occuring due to yaw is small.

Since the loss in dynamic pressure at the tail is close to the same with either horizontal tail position (for the same yaw angle) the nose down pitching effect, or lack of it, must come from the change in downwash at the new lower horizontal position.

Since at the lowered horizontal tail position the local angle of attack due to downwash is greater (more down) the airplane would require a retrim of the horizontal slightly leading edge up to keep the overall airplane pitching moment the same as before the lowered/anhedraled tail was enabled.

If an assumption is made that the change in downwash due to yaw is close to the same with either high or low horizontal tail position and if the total downwash is greater at the low horizontal tail position then the percentage change in pitching moment due to downwash will be less for the case where the horizontal tail is in the low position.

This gives you an airplane that will respond less in pitch when yawed in level upright flight.

I have probably missed something because it is late and I am getting sleepy but it seems like it should be something similiar to the above.

So long as Hanson and Lanterman are discussing the esoterics of anhedral, thought I'd throw two cents in.

Decalage is totally independent of stabilizers, angled ot not. Decalage applies only to airplanes with two or more main lifting surfaces, biplanes, triplanes, etc. It is the difference in incidence of the upper and lower wings. Stab incidence is NOT referenced by the word "Decalage."

My twins have no decalage, but they inspire much badinage.
(Now run for your dictionary.) (Haw.)(Look up decalage too.)

Bill.

William,

How the hell do you keep coming up whit this flight line poetry???

Some of 'em are quite nice actually.

Rudeboy:

Thanks for the kind words.

The couplets are the product of a fertile imagination coupled with a warped mind.

I started them as a way to push the thought of twin engined planes into everyone's thoughts, hopng to get people to choose a twin when the "One or Two" question came up.

One of my favorite twins is a "Bashed" Hobbistar 60, it's pictured in the Feb 2003 issue of Model Airplane News on page 96. Blowing my own horn there, a little bit. And the check from MAN was a nice Christmas present, too. Thank you, Air Age Publications.

And I do sometimes have to stretch things to make an appropriate couplet. Truly, I try to pick a word that fits into the post, somehow, then think of one that rhymes, and see what I can do with them. Like the "Decalage/Badinage" pair before.

Like the twin, the rhymes
. should go with the times.

(Bad enough?)

Bill.

Finding couplets is generally luck
Using rhyming ,timing and pluck
Some time I give up
and go walk my pup
Or simply just say
what the heck.

Maybe a good idea to pass to the RC Universe management: "NEW FORUM : AIRPLANE POETRY"

Hi all

William, you mentioned Dacalage, I am building the French Stampe and believe the upper wings should have more incidence than the lower ones?
I find this odd, as I would expect more stability the other way?

Would you know??

StarSkipZ:

The Stampe is an oddity, but from France that's not unusual. (Slam)

Generally a bipane will have the lower wing around two degrees more positive than the upper, and as I understand it that's to make the lower wing stall first, leading to a more gentle stall and recovery.

And in spite of its reversal of the usual decalage (there's that word again) the Stampe was a gentle flier.

Dick, one for you:

A fellow named Dick Hanson,
was visited by Charles Manson.
. He said a man who writes such doggerel
. should be sent quite straight to 'ell
Along with that bloody Bill Robison!

Bill.

I see you owned a French car then!!! Will

The're even spelt unusually....