UA752
Topic Author
Posts: 144
Joined: Mon Jan 08, 2001 3:30 pm

Negative Dihedral?

Thu Feb 14, 2002 4:22 pm

What aerodynamical advantage (if any of significance), does the Falcon 50/900/2000 etc. get from the negative dihedral of its horizontil stabs?

MSN
 
lymanm
Posts: 1100
Joined: Tue Jan 23, 2001 2:30 pm

RE: Negative Dihedral?

Thu Feb 14, 2002 4:46 pm

It looks damn cool.
BTW - technically, it's called "anhedral"
buhh bye
 
L-188
Posts: 29881
Joined: Wed Jul 07, 1999 11:27 am

RE: Negative Dihedral?

Thu Feb 14, 2002 5:28 pm

The natural tendency of a wing with Diheidral is that is want to return to a straight and level attitude. This is because if you look at it from the front the trust vetor of the uphill wing will be pointed more or less to the side and the downhill one will be pointed straight up.

This effectivly means that the downhill wing will want to to raise untill the force generated by each wing are equal to each other or counteracting each other.

With Anhedral the uphill wings lift vector goes from pointing to the side in level flight to vertial. This means that the wing has a tendence to continue to roll since it is generating more "real" lift then the downhill side. This is helpful in high wing aircraft especially if you are carrying a heavy load below the center of lift, example being a C-5 Galaxy with a 60 ton tank in it's belly. The anhedral helps the airplane get into a turn.

Most low wings don't need this because their loads are carried about the center of lift in the wing. That is why it is much more common to see those wings with dihedral. The Falcons and some of the Russian airlines are exceptions. I don't know what advantages they would have gained by going with anhedral exept for styline or a more snappy roll response.

OBAMA-WORST PRESIDENT EVER....Even SKOORB would be better.
 
FredT
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RE: Negative Dihedral?

Thu Feb 14, 2002 5:50 pm

L-188,
good explanation although your description of WHY dihedral improves roll stability is a bit flawed. It is a common misconception so you're not by far the first to fall into that trap.  Smile

You have to look at the roll moment created around the center of gravity of the aircraft by the wings. Whether the force of each wing is vertical or tilted to the side does not matter. Unless the wing moves, it is still at the same distance from the Cog. As long as they generate the same lifting force, there won't be a roll moment regardless of bank angle.

However, an unwanted bank is not likely to be a coordinated turn. This means there will be a slip towards the lower wing. The airflow generated by this slip will hit the upwind wing from above due to the dihedral, causing a lower effective angle of attack and less lift. The resulting lift difference between the wings causes a roll moment that will try to roll the aircraft back to level.

Cheers,
/Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
UA752
Topic Author
Posts: 144
Joined: Mon Jan 08, 2001 3:30 pm

RE: Negative Dihedral?

Fri Feb 15, 2002 8:29 am

Thank you all...question answered
 
aaron atp
Posts: 517
Joined: Sun Mar 19, 2000 1:17 pm

RE: Negative Dihedral?

Fri Feb 15, 2002 9:36 am

Even though no one answered the original question with regards to the horizontal stabs?
 
UA752
Topic Author
Posts: 144
Joined: Mon Jan 08, 2001 3:30 pm

RE: Negative Dihedral?

Fri Feb 15, 2002 2:32 pm

Perhaps you can offer some more words aaron?
 
FredT
Posts: 2166
Joined: Thu Feb 07, 2002 9:51 pm

RE: Negative Dihedral?

Fri Feb 15, 2002 5:53 pm

A common reason for angling the tailplanes is to get them out of the propwash, jetwash or downwash from the wing in critical flight attitudes. Oh, and looks might have something to do with it too, aircraft manufacturers DO have a marketing department.  Big grin

Cheers,
/Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
L-188
Posts: 29881
Joined: Wed Jul 07, 1999 11:27 am

RE: Negative Dihedral?

Fri Feb 15, 2002 6:06 pm

Thanks FredT....

It took about three rewrites just to get what I got up there posted.

It is easier to explain this when you have a photo.
OBAMA-WORST PRESIDENT EVER....Even SKOORB would be better.
 
aaron atp
Posts: 517
Joined: Sun Mar 19, 2000 1:17 pm

Perhaps...

Fri Feb 15, 2002 6:11 pm

Horizontal stab anhedral enhances directional stability.

http://www.airliners.net/discussions/tech_ops/read.main/4337/

Ever noticed the ventral fins of some KingAir's (Raisbeck mods later became standard equipment) and Lears? The are sent at an inverted V angle for the same reason some manufacturers build anhedral stabs.

Just think of the Predator drone as the opposite of the V-tail Bonanza, it certainly wouldn't dutch roll, but it could be quite susceptible to spiral instability from an overabundance of directional stability.
aaron
 
FredT
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Joined: Thu Feb 07, 2002 9:51 pm

RE: Negative Dihedral?

Fri Feb 15, 2002 11:38 pm

Aaron,
that explanation doesn't add up. What's your source? Even though the tailplane has negative lift, the effects of dihedral/anhedral on lateral stability will still be the same.

Anhedral or dihedral in the tailplane would indeed add directional stability, just like a vertical tail. Your original post seems to suggest that dihedral would decrease directional stability while anhedral would increase it. Both should in fact increase directional stability.

The most probable reason I can think of, except for the aforementioned ones, is that it is easier to correct a stability problem by altering the stab than by altering the wing late in the design process.

Would you care to elaborate a bit on your theories? I clearly don't know as much as I would like about tailplane anhedral so it would be interesting to discuss the matter for a bit.

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
prebennorholm
Posts: 6447
Joined: Tue Mar 21, 2000 6:25 am

RE: Negative Dihedral?

Sat Feb 16, 2002 7:27 am

Vertical stabilizers with anhedral improves directional stability - you could also say that it counters dutch roll tendency.

The physics behind this is in fact quite simple, it is purely geometrical.

If we assume an A/C heading a little left of its track (sideslips or yaws a little in a gust or so), then on a swept back wing the right wing will present a greater span to the airflow than the left wing. It means more lift on the right wing than the left one. That will initiate a roll to the left which is undesireable since we already had an unwanted yaw to the left.

The horizontal stabilizer with anhedral, however, it will increase its negative angle of attack on its right side, and decrease angle of attack on the left side. That creates a roll force which is opposite the unwanted roll force from the swept back wing. Correctly designed it can counter it 100%.

Then why do so many airliners have a horizontal stabilizer with dihedrad? The answer is structural weight. A few hundred pounds extra weight would mean a few less paying pax, everything else equal.

Take a B737. It saves at lot of weight having the horizontal stab connected directly to the fuselage instead of having it connected to the fin. But the horizontal stab must be out of the engine exhaust, therefore dihedral is definitely needed. It cannot be straight either since it would moke it far too prone to tubolence from the wing, especially with flaps extended.

So the "fix" to solve the dutch roll problem on planes like the B737 is the automatic yaw damper, a gyro operated computer which counters the yaw instantly by actuating the rudder.

Planes with T-tail have already "paid" the weight penalty. So on T-tail planes we most often see horizontals stabs with anhedral.

Back in the 60'es the Germans created a biz jet, the Hansa Jet, with a swept forward wing (it never was successful). One of the reason for that was that dutch roll tendensy was actually killed at the root, as opposed to swept back wings (another reason was to have the cabin totally forward and unobstructed by the wing main spar).

Forward sweep has, however, a lot of structural disadvantages which can only be countered by accepting weight penalties. In addition, on large airliners forward sweep would mean that the wing main spar and the main landing gear would compete for the same room, meaning that the landing gear most likely would have to be podded on the wing like on a Tu-134 = increased drag.

It's all compromises, compromises and even more compromises.

Regards, Preben Norholm
Always keep your number of landings equal to your number of take-offs
 
prebennorholm
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RE: Negative Dihedral?

Sat Feb 16, 2002 8:00 am

Just one more thing: Have a look at all modern fighter interceptor and fighter bomber planes with swept back wing and ordinary tail. They all have an anhedral tailplane.

Such planes don't want a yaw damper to keep the dutch roll tendency within "acceptable limits". They want to be able to aim the canon sight or the bomb sight rock stable on a target.

Therefore they always have an anhedral tailplane which has been designed to exactly counter 100% the dutch roll tendency created by the main wing. The F-4 Phantom is the most prominent example. It needs that extra tailplane anhedral because of the dihedral main wing tips which further increase the dutch roll tendency compared to a straight wing.

Regards, Preben Norholm
Always keep your number of landings equal to your number of take-offs
 
EssentialPowr
Posts: 1646
Joined: Wed Sep 06, 2000 10:30 pm

RE: Negative Dihedral?

Sat Feb 16, 2002 2:10 pm

"The horizontal stabilizer with anhedral, however, it will increase its negative angle of attack on its right side, and decrease angle of attack on the left side. That creates a roll force which is opposite the unwanted roll force from the swept back wing. Correctly designed it can counter it 100%."

"So the "fix" to solve the dutch roll problem on planes like the B737 is the automatic yaw damper, a gyro operated computer which counters the yaw instantly by actuating the rudder."

"Planes with T-tail have already "paid" the weight penalty. So on T-tail planes we most often see horizontals stabs with anhedral..."


So from the above 3 paragraphs, and assuming it was "correctly designed" are you saying the 727, for example, or the F4 are immune from Dutch Roll tendencies?

Ventral fins on contemporary Lears ensure sufficient pitch response at high alpha; ie they guarantee specific stall behavior. The ventral fins have negligible loading in the normal flight envelope.
 
aaron atp
Posts: 517
Joined: Sun Mar 19, 2000 1:17 pm

RE: Negative Dihedral?

Sat Feb 16, 2002 5:46 pm

Raisbeck Engineering

Quotes from the page for anhedral strakes:
  • Eliminated (200, F90) or raised (300, 350) yaw damper limits, increasing dispatch reliability

  • Directional stability is increased

  •  
    Metwrench
    Posts: 715
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    RE: Negative Dihedral?

    Sat Feb 16, 2002 6:02 pm

    So why the exaggerated anhedral wing on the BAe 146/AVRO RJ?

    I was told it enhanced STOL. I can see the benefit of ground effect, what else?
     
    prebennorholm
    Posts: 6447
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    RE: Negative Dihedral?

    Sat Feb 16, 2002 8:13 pm

    EssentialPowr: So from the above 3 paragraphs, and assuming it was "correctly designed" are you saying the 727, for example, or the F4 are immune from Dutch Roll tendencies?

    No, not all way. We are talking about counter acting forces which may not cancel out completely in all flight regimes.

    I would assume that on for instance the F-4 priority has been given to stability in the weapon aiming regime. It may work very differently in for instance landing configuration.

    In the perfect world the designers would make the counter acting forces cancel out 100% in all flight regimes, but I doubt that it is possible. My best guess is that designers and test pilots work together on creating the best overall compromises.

    Regards, Preben Norholm
    Always keep your number of landings equal to your number of take-offs
     
    prebennorholm
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    Joined: Tue Mar 21, 2000 6:25 am

    RE: Negative Dihedral?

    Sat Feb 16, 2002 8:53 pm

    Metwrench: So why the exaggerated anhedral wing on the BAe 146/AVRO RJ?

    Well, the 146/ARJ has no ground clearance problem on the wing. Many other planes with swept back wing enjoy the same comfort: Tu-134, C-5, B-52, Il-76, C-141, C-17 etc.

    They all have a wing with some anhedral. It does geometrically counter dutch roll to some degree, but of course to various degree depending on the wing bending due to load.

    So let's instead look at the "other" planes, for instance all Airbus airliners and Boeing planes with wing mounted engines. I would assume that their wing dihedral is a compromise in order to improve ground clearance. It could be countered by longer landing gears. But weight of landing gears increase expenentally related to length. You see especially on the 737 the outmost have been done to keep landing gear legs short in order to save weight.

    It is very practical indeed that on ordinary airliners the wing main spar can pass through the fuselage at a place which is not wanted for something else.

    I am 6'2". And I always ride in the back of airliners. I have lost count of how many times I have banged my forehead against the wing main spar on a 146 or ARJ when bording - and deboarding - I will never learn. And the overhead bins under the main spar are so narrow that they hardly accept my laptop computer.

    That's not the perfect compromise for passenger comfort. But it does have advantages concerning dutch roll.

    The 146/ARJ paid the disadvantage of narrow landing gear track. But it certainly gained the advantage of short and lightweight landing gears. A very different sort of compromise.

    Anyway I am pretty convinced that in the future we will see mostly "traditional" airliners which count on yaw dampers for yaw stability. When intergrated in the FBW control the yaw damper software can also vary the gains of the yaw damper to cancel out dutch roll tendencies perfectly well in all flight regimes.

    Regards, Preben Norholm
    Always keep your number of landings equal to your number of take-offs
     
    EssentialPowr
    Posts: 1646
    Joined: Wed Sep 06, 2000 10:30 pm

    RE: Negative Dihedral?

    Sun Feb 17, 2002 2:00 am

    That's my point. Dutch roll will not be prevented by tail anhedral, even if it's "Correctly designed it can counter it 100%". Not gonna happen.

    In fact, lateral stability L = CqSb
    Where C = the rolling moment coefficient
    q = dynamic pressure
    S = wing area
    b = wing span.

    From this equation we see that directional stability is a largely a function of WING design, not tail design. C, the rolling coefficient, takes into account other factors such as engine location, wing position relative to the fuselage, vertical fin area, flap size and location, etc.

    The point is that tail anhedral/dihedral has a very slight effect on directional stability; obviously the largest component made by the tail are the aspects of the vertical fin. If tail anhedral were the "100% correctly designed fix" for Dutch roll, then swept wing a/c with the "sufficient" amount of tail anhedral wouldn't need a yaw damper. This is not the case. The F4, by the way, had a stability augmentation system on it...a yaw damper...in order to make it a stable weapons platform.

    Landing gear length has remained constant on the 737 not b/c of increasing gear weight. As an original design with the relatively narrow diameter JT8ds, a low ground profile made the a/c easier to load and service.

    As the 737 was lengthened, in order to preserve the same sight picture and landing visuals for the same TYPE rating, the landing gear length HAD to remain constant from the short -500 to the -900. It would have been better for most operators of the -900 to have lengthened the landing gear about 18", and eliminate the tail stike package that weighs at least as much as the longer gear would have. But that would have nullified the common type rating as the resulting a/c would have been taller.
     
    EssentialPowr
    Posts: 1646
    Joined: Wed Sep 06, 2000 10:30 pm

    RE: Negative Dihedral?

    Sun Feb 17, 2002 2:03 am

    So to answer the original question-

    It's just slight enough anhedral to be distinctive; there are no supersonic exhaust interference concerns...

    Marketing does it; it's a Falcon trademark.
     
    EssentialPowr
    Posts: 1646
    Joined: Wed Sep 06, 2000 10:30 pm

    RE: Negative Dihedral?

    Sun Feb 17, 2002 7:16 am

    AaronATP: My discussion has been on lateral stability, and the relatively minor impact horizontal tail configurations have on dynamic lateral stability as given by the Lateral Stability equation.

    I have not addressed ventral fins on the the King Air Series; as a straight wing design it is certainly not as prone to yaw and roll coupling as a swept wing. Further, as in the case of the new Lears, the King Air 90 and 200 (and maybe the 300?) are not required to have a stick pusher in the stall regime, unlike jets, unless it can be shown the design has sufficient downward pitching moment to provide "a clean (I forget the FEd verbage)" break at the stall.

    In short, the primary reason for the ventral fins on the Lears is for High alpha pitch moment. For a straight wing design like the King Air (particularly the "blender bottom" 1900D), ventral strakes mounted in an "anhedral" fashion are done so as that configuration minimizes drag in the a/c's normal flight attitude; the greater the vertical fin component, the greater the contribution to lateral stability.

    -But Dynamic Directional stability is still largely a function of the wing for any a/c design.

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