Need another aero-d discussion...

[bozon]

What torque are you talking about?

torque = radius x force (vector product)

Elevators push down against the lever that is the body of the plane from the center of lift to the tail. This is HOW elevators control pitch and has little to do with engines.

Does the wing configuration really make a difference? If the control surface resides aft of CoL the angle change of the main foil will always be the same as the movement of the control surface i.e up - up. ...

Not quite. The contro9lo surface is not a separate part from the whole wing. It only changes the profile. It all boils down to how the pressure is distributed over the wing to produce the net torque to tilt the wing into a new angle of attack. What I don't know is how it magically gets distributed so it easily balances itself in a new steady angle of attack. This is the case of paper planes that can glide quite well and are easily self-balanced. Why do they glide and no just fly like an arrow in an arc into the ground.

[hitech]

torque = radius x force (vector product)

Elevators push down against the lever that is the body of the plane from the center of lift to the tail. This is HOW elevators control pitch and has little to do with engines.

This should read.

Elevators push down against the lever that is the body of the plane from the center of GRAVITY to the tail. This is HOW elevators control pitch and has little to do with engines.

What torque are you talking about?

In the case you are question the term "torque" it is not being used in the engine torque since, but rather in the generic form of a force causing a rotation.


Now back to my original question on planes that have lots of roll with rudder input, I believe the  primary consideration is dihedral.

HiTech

[Mace2004]

quick question - for a properly trimmed aircraft is the thrust line the same as the motion vector? ie. in level flight should the thrust line be parallel to the ground?

It can be but doesn't have to be.  When you say properly trimmed for what speed are your referring?  Properly trimmed for 120kts or 500kts?  You can be properly trimmed for any speed yet your AoA will be different and therefore the relationship between the thrust line and motion vector will change.  Of course, in a very general sense yes the trust line is parallel and you can assume that it's designed to closely match the motion vector at the aircraft's best cruise speed but it would match for only one specific condition.  Also, there are plenty of aircraft in which the thrust line is actually angled down to provide additional lift (generally for takeoff and landing) or in other direction to counteract torque effects and P-factor.

[RufusLeaking]

In the case you are question the term "torque" it is not being used in the engine torque since, but rather in the generic form of a force causing a rotation.

Roger  that.  Even though the physics term, "torque," is correct in the discussion of forces acting on moment arms, I am not used to hearing it with regards to the pitch of an aircraft.  When discussing the x-y-z axis, it is common to use roll (about the x), pitch (about the y) and yaw (about the z).

Torque, in my experience, is used to discuss the engine/prop effect on roll.  For example, I could never get a Cessna 152 to spin opposite the engine's torque.  A T-37, a jet, would enter a spin in either direction.

Now back to my original question on planes that have lots of roll with rudder input, I believe the  primary consideration is dihedral.

Not sure.  In a swept wing aircraft (KC-135), the yaw induced would change the angle of the leading edge of the wing, with one side reducing the sweep angle to the relative wind and the opposite wing increasing the sweep angle.  The decreased sweep increased the lift on that side, and the plane would roll in the direction of the rudder deflection.  And, I would not characterize it as "lots of roll."  It was easily cross controlled with opposite ailerons.

T-38s, with no dihedral, were prone to "rudder roll"' at low speeds, high AOA.  So much so, that cross wind landings were made in a crab, with no rudder input.

[Mace2004]

OK maybe some aero-engineer can enlighten me on how delta wing, elevator-less planes work. I mean planes like the Mirage or F-106.
Normally the torque to keep the wing in an angle against the airflow is provided by a separate elevator and a leverage due to the elevator being far aft of the main wings. In a delta the wing needs to both produce the lift and the counter torque to keep it in a constant angle of attack. How does that work?

(Image removed from quote.)

(Image removed from quote.)

Pretty much the same as a conventional wing and tail aircraft does.  I have a crude picture but can't seem to upload it right now so you'll have to picture it yourself.  

The delta has a Center of Pressure and CoG just as a tailed aircraft but obviously the wing shape is different.  The control surfaces are at the trailing edge of the outer portion of the delta wing.  Control surfaces essentially only affect the portion of the wing directly ahead of them by changing the local AoA.  The affected surfaces (the portion of the delta wing directly in front of the control surfaces) are behind the CoG and CoP just as in a conventional tailed aircraft except they're displaced outboard.  

Use the control surfaces differentially (opposite directions) and they act as ailerons and the airplane rolls but use them together (i.e., deflect them the same direction) and they create a pitching moment to control AoA just like an elevator....hence the term Elevon (as RufusLeaking mentions).  Make sense?  This is also the way a flying wing works except the Elevons can split to create drag for yaw control taking the place of the vertical tail.

The pure delta is an interesting but limited design for a wing.  It's very easy to make them thin but strong and the high sweep generates vortices over the wing's surface that improve lift.  The sweep also keeps the wingtips behind the shock wave created by supersonic flight so they're outstanding for high speed flight but they kind of suck for turning.  Their instantaneous turn rate is very high but the design bleeds like a stuck pig so sustained turning is generally bad and a delta has stability issues at high AoA.  They also have very high takeoff and landing speeds with a very nose-high attitude while landing and cannot use trailing edge flaps.

There have been many attempts to eliminate the delta's limits but you don't see many pure deltas around any more. Even so, almost all modern fighters include some aspect of the delta.  Tailed deltas like the MiG021, A-4, and F16 became very common in the 60's.  Modern tailess deltas like the Typhoon, Gripon, and Rapale all use canards in front of the wing.  Modern non-delta winged fighters like the F18 still use aspects of a delta design by inducing the delta's vortices with leading edge extensions.

[bozon]

The delta has a Center of Pressure and CoG just as a tailed aircraft but obviously the wing shape is different.  The control surfaces are at the trailing edge of the outer portion of the delta wing.  Control surfaces essentially only affect the portion of the wing directly ahead of them by changing the local AoA.  The affected surfaces (the portion of the delta wing directly in front of the control surfaces) are behind the CoG and CoP just as in a conventional tailed aircraft except they're displaced outboard.  

Use the control surfaces differentially (opposite directions) and they act as ailerons and the airplane rolls but use them together (i.e., deflect them the same direction) and they create a pitching moment to control AoA just like an elevator....hence the term Elevon (as RufusLeaking mentions).  Make sense?  This is also the way a flying wing works except the Elevons can split to create drag for yaw control taking the place of the vertical tail.

And here is my problem with this description: to increase AoA, the elevons deflect UP. On a normal wing, deflecting the ailerons up lowers the AoA, the lift and this is the wing that drops in the roll. So why does deflecting the elevons up, which according the this description should lower the angle on attack of that portion of the wing ends up increasing the pitch of the plane and increasing the lift.

This should read.

Elevators push down against the lever that is the body of the plane from the center of GRAVITY to the tail. This is HOW elevators control pitch and has little to do with engines.

Of course. Misplaced terms.

[Mace2004]

And here is my problem with this description: to increase AoA, the elevons deflect UP. On a normal wing, deflecting the ailerons up lowers the AoA, the lift and this is the wing that drops in the roll. So why does deflecting the elevons up, which according the this description should lower the angle on attack of that portion of the wing ends up increasing the pitch of the plane and increasing the lift.

Think of the wingtips (the portion of the wings ahead of the elevons) as separate airfoils.  Remember that the elevon only affects the AoA of the wing in front of it (the "local AoA"), the rest of the wing can have a different AoA.  When both elevons deflect upward they do create a lower (or negative) AoA and downward force but only on the wingtips.  Since the wingtips are behind the CG that creates a nose-up pitching moment exactly as does a conventional horizontal tail. This pitching moment will increase the AoA of the rest of the wing and therefore create additional lift.

For rolling, the elevons act exactly as do conventional ailerons.  You want to go left, the left elevon comes up creating a downward force, the right elevon goes down creating an upward force and the airplane rolls left.  Again, this is the same as a conventional aircraft. 

The fact that the elevons operate differently for pitch and roll means that controlling them is more complex that conventional controls.  In mechanical systems, the pilot's input (pitch or roll) goes into a box called a mixer that sorts them out to move the elevons in the right direction, either together or in opposite directions.  With fly-by-wire systems, it's easy as it's all controlled by software.

[Kenne]

LOL!  I'm too busy photoshopping .  How about "what generates lift?"   

this is a no brianer...differences in air pressure generate lift!

[Stoney]

this is a no brianer...differences in air pressure generate lift!

Oh boy...

[dtango]

this is a no brianer...differences in air pressure generate lift!

Absolutely, but what creates the differences in pressure?     

[Charge]

http://www.grc.nasa.gov/WWW/K-12/airplane/wrong3.html

 

-C+

[dtango]

I have a very brief moment to post and wanted to add to the discussion on delta wings.  Rufusleaking and Mace have discussed in general how delta wing aircraft deal with longitudinal stability when there isn't a horizontal tail.  Essentially the elevons are trimmed to counteract pitchup or down moments as needed.  Bozon pointed out the F-102 as the example aircraft in question to discuss the topic. That's an instructive aircraft actually because delta wing's have other longitudinal stability issues that can affect them that require more than just elevon control surfaces to address.  Here is an image of the F-102:



Notice the 4 "mystery thingies" I've annotated on the wings.  Those little "mystery thingies" I've pointed out are really important in dealing with uncontrolled pitchup of the F-102 in certain flight regimes.

[dtango]

http://www.grc.nasa.gov/WWW/K-12/airplane/wrong3.html

 

-C+

   

[Baumer]

And here I was thinking this would deteriorate into a discussion about hip to waist ratio, i.e. the "Coke Bottle" needed to get the 102 supersonic.

As they say good fences make good neighbors.   

[dtango]

As they say good fences make good neighbors.   

But of course what the heck do wing fences have to do with longitudinal stability?