Jets and anhedral wings
 

Hi
Something I have always wondered and never got round to asking the question is about Jets with Anhedral wings. There are many examples of these types of aircraft. eg Harrier, Gnat, Foxbat, Foxhound etc.
Can anyone tell me the advantages and disadvantages of this type of wing and how this would translate into model flying characteristics.
Eg would they fly okay or would you have your hands full?

Mike

RE: Jets and anhedral wings
 

for every 5 degrees of sweep back yoy get the equivilent of 1 degree of aerodynamic dihedral. this would induce stability. i asume the anhedral is to counter this (just a guess but it is logical?)

RE: Jets and anhedral wings
 

These aircraft are too stable and anhedral is added to make them more unstable and maneuverable.
It takes very little aileron movement to make them roll and once they start to roll they want to keep going, on the Harrier anyway. Imagine once one side of the wing is horizontal it is creating the most lift it can make and the other side is
down some 20 degrees or so it is not lifting as much so the roll wants to continue.

RE: Jets and anhedral wings
 

Hi
Okay I have two questions then,
Applying the sanity test.
There are several cargo plane (C17, Galaxy and the dirty bog Antanov etc) that I know of plus a passenger aircraft ( BA something or other four engined city hopper) and a trainer(Gnat) all have anhedral that doesnt make sense to me as I wouldlt think they wouldnt intentionally make these aircraft unstable. A fighter yes I can see your arguement these types of plane I would doubt it.

Second if what you say is true and you built a model with an Anhedral wing would fitting a Gyro on the roll axis help to tame it?

Mike

RE: Jets and anhedral wings
 

The planes you mention also are high wing aircraft which adds more stability, thus need more anhedral. A gyro would help if the plane showed divergent stability.

RE: Jets and anhedral wings
 

mikehannah

I think most designs you have mentioned have high wing mounts. This will give a lot of stability on its own, particulary if you have a heavy load like trucks or tanks carried in the belly. Presumably a small amount of anhedral would partially neutralise the effect and reduce the load on the ailerons in the turn.

RE: Jets and anhedral wings
 

It is the dihedral effect of sweep.

With a straight wing the low wing position requires a fair amount of dihedral for normal stability, midwing just a bit, high wing hardly any or flat. If you then add a bunch of wing sweep the dihedral needs to be reduced the same amount from that base value for all three positions. A high mounted wing will need quite a lot of anhedal to compensate for a 30 or 40 degree sweep which is a common sweep amount for a large high-subsonic airliner.

RE: Jets and anhedral wings
 

The Anhedral takes the edge off of the over-stability - with such long wings roll control is already a problem, and extra dihedral effect is the last thing you want. It's not to make them "unstable" - just get them back to neutral.

Dennis

RE: Jets and anhedral wings
 

Mike
When I say unstable I don't mean to the point that it is uncontrollable, it just takes some of the stability away to increase roll rate. Straight and level the Harrier is still stable and tracks perfect, just in the roll it a lot more sensitive, with reduced aileron throw it handles great. Once I start a turn I find that I hold opposite aileron through the turn, then add even more aileron to straighten out to level flight. Adding a gyro may work but I want to keep it as simple as possible.

RE: Jets and anhedral wings
 

I think Anhedral is rarely used to change stability. There are far more effective and efficient ways to decrease stability than
Bending the whole wing down. Most of the time a full size plane will have anhedral
Because for other reasons. Most common use is with wing tip landing gear. A high wing plane line the
Harrier or B52 would not be very unpractical with very long wing mounted gear. Also I think, with high wing planes, anhedral is
Used to get into ground effect sooner. The C17, witch is primary a heavy lifter, would benefit greatly by keeping its wings
Close to the ground as it can . There Is no point in changing the stability of a C17. There is no question anhedral does change
Stability a small amount, but in most cases anhedral is built into planes for other reasons. FWIW I can’t think of any US fighter that
Have anhedral, except the F104 star fighter.

Eddie Weeks

RE: Jets and anhedral wings
 

On the Harrier is absolutely is!
I highly doubt you would want to make a aircraft more unstable just to accommodate gear.

RE: Jets and anhedral wings
 

Oh gees.

"A side effect of dihedral can be roll-coupling, a tendency for an aircraft to "corkscrew" through the air under certain conditions. This rolling motion, called a dutch roll, is unpleasant to experience, and can lead to loss of control or can overstress an aircraft. A certain amount of anhedral can combat this effect. Pronounced anhedral is also often seen on aircraft with a high mounted wing, such as the BAe 146, Lockheed Galaxy and others. In such designs, the high mounted wing itself confers roll stability (due to the pendulum effect of the fuselage, engines, etc), so additional dihedral is not required. In fact, such designs can be excessively stable, so the anhedral is added to cancel out some of the roll stability to ensure that the aircraft can be easily manoeuvred.

Sweptback wing also increases roll stability. This is another reason for anhedral configuration on military aircraft with high sweep angle, as well as on some airliners, even on low-wing aircraft such as Tu-134 and Tu-154."

From:

http://en.wikipedia.org/wiki/Anhedral

RE: Jets and anhedral wings
 

I stand corrected.. Learn something new every day..

Eddie Weeks

RE: Jets and anhedral wings
 

Hi
Gents many thanks for the info. You have answered my questions.
Now if anyone has any info on a Mig 31 undercarriage and how it retracts that is my next problem.

Mike

RE: Jets and anhedral wings
 

There are far many more aircraft with anhedral that don't have gear on the wingtips. A-6 Intruder, A-7, F-8 Crusader, F-11, - F-104, F-105, and to a lesser degree the F-14, F-15, F-16 - all of these Mid or Top Wing Aircraft. Mid or top wing position increased the Dihedral "effect", thus to keep it in parameter, the Anhederal is added

Anhedral has the opposite effect as Dihedral - one of the more popular texts is 'Aircraft Performance, Stability and Control' (by Perkins / Hage), covers Diheral effect and it's control.

Roll coupling is complicated by Dihedral effect and Wing sweep, (researched after Melburn Apt was killed in the X-2) is also controlled by several other features, such as Dorsal Fins, etc. Roll coupling is a concern is high speed manuevers.

In a large Transport (C-17) you already have roll control problems due to the long wingspan, so exceccive dihederal effect is not wanted. Check these links also

[link]http://www.aerospaceweb.org/question/dynamics/q0055.shtml[/link]

and the NASA link: [link]http://www.grc.nasa.gov/WWW/K-12/airplane/geom.html[/link]

Anhederal on the stabilizer gives directional stability - as on the F-4 anhederal was added to the tail, and dihederal to the wing tips after the test aircraft experienced directional stability problems.

When they get all of the wind tunnel stuff done, then they bring in Mechanical Engineers to design the gear, and they will get gear to the ground, no matter where the wing is.

Dennis

RE: Jets and anhedral wings
 

Hi,

Don't shoot me if I'm way off, but doesn't the anhedral also relieve some of the g-load on the wing?

RE: Jets and anhedral wings
 

No, well - infantesimally I guess since it slightly reduces the projected area of the wing but basically - no.

RE: Jets and anhedral wings
 

No, your G-loading would be the same - the anhedral won't have any effect on the lift. You would notice a different dihedral / anhedral angle in flight though, as the wings will flex. Like a B-52 on the ground, the wheels on the wing tips are due to the "droop" - simply from weight the wing 'bends' or flexs down, and in flight up to some degree.

Dennis

RE: Jets and anhedral wings
 

Doesnt the F-18 have a small amount of anhedral as well?

RE: Jets and anhedral wings
 

Anhedral counters Dutch Roll. So when the aircraft starts to yaw and roll in the Dutch Roll the relative wind will strike the top of the wing forcing it down and back thus "fighting" the Dutch Roll tendencies.

RE: Jets and anhedral wings
 

Though it doesn't reduce the G load it does slightly decrease the tension loads during positive Gs.

RE: Jets and anhedral wings
 

"Though it doesn't reduce the G load it does slightly decrease the tension loads during positive Gs."

Huh? How so?

RE: Jets and anhedral wings
 

Put simply......
Which would support more weight, A or B?

RE: Jets and anhedral wings
 

B could support more weight with all things being equal, however that is not the case. B still needs to lift the same amount of aircraft, but has less planform area due to the anhedral. There fore its wingspan (calculated as in the plane of each wing half) must be wider to make up for the loss of horizontal wing area. Therefor B must weigh more and have a bit greater bending moment capability at the root.

RE: Jets and anhedral wings
 

B only needs more airspeed to generate the same lift as A just as any other "shorter" wing.