GAIsweetGAI
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Benefits/Penalties Of Fwd-facing Ctrl Surfaces

Mon Apr 16, 2007 3:02 am

What would be the advantages or disadvantages of having forward-facing or forward-mounted control surfaces?
For example: the elevator is placed at the leading edge of the horizontal stabilizer instead of the trailing edge.
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lehpron
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RE: Benefits/Penalties Of Fwd-facing Ctrl Surfaces

Mon Apr 16, 2007 3:22 am

First thing that comes to mind is that the segment (flap or whatever) would introduce a higher angle of attack in order to pitch up compared to the leading edge of the tailplane, increasing the possibility of flow separation.

Quoting GAIsweetGAI (Thread starter):
For example: the elevator is placed at the leading edge of the horizontal stabilizer instead of the trailing edge.

There are wing designs like this, though rarely used (I seen them in an Aerospace Engineer's handbook). I haven't seen too many of these in practice, like a quarter-chord leading edge flaps.

Sorry, I cannot think of any advantages at this moment.
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David L
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RE: Benefits/Penalties Of Fwd-facing Ctrl Surfaces

Mon Apr 16, 2007 3:31 am

Pending the experts' comments, I can think of a few points:

Very little effort would be required to change the aircraft's attitude*, since the oncoming airflow would augment any control surface displacement, e.g. if you raise a forward-mounted aileron, the air airflow would catch it and try to move it further up. On the other hand, the airflow would be trying to augment the slightest displacement so you'd constantly be fighting to keep the control surfaces absolutely centred. IOW, changing the attitude of the aircraft would be easy but maintaining an attitude would require constant effort. One of the advantages of rear-mounted control surfaces is the damping efect - some effort is required to change the attitude but hardly any is required to maintain your desired attitude. Another advantage is that you don't need to protect the control surfaces from being "slammed" into full deflection by the airflow.

I guess it boils down to whether you want to exert some effort occasionally to change your attitude and "relax" most of the time or you want to exert very little effort to change your attitude and spend most of your time fighting to maintain a constant attitude. I know which option I'm backing.

* I'm referring to "attitude" in all three axes. Apologies if that's not The Done Thing.  Smile
 
KELPkid
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RE: Benefits/Penalties Of Fwd-facing Ctrl Surfaces

Mon Apr 16, 2007 6:11 am

I'd think that direct mechanical control would be out of the question, as at high airspeeds, the control might "commit suicide" by getting caught in the relative airflow...similar to one of the reasons that I've heard backwards-opening car doors are called "suicide doors"-if you open them while the car is at speed, the relative airflow will catch the door and blast it fully open, and tear it off of it's hinges  Smile [I cannot verify if this is true or not...] You'll have to ask someone who owns a 190-1968 Lincoln Continental or 1965-1969 4 door Thunderbird  Wink

Could you imagine having your wrists on the yoke when a control surface decides to make a full-deflection move all on it's own?
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vikkyvik
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RE: Benefits/Penalties Of Fwd-facing Ctrl Surfaces

Mon Apr 16, 2007 7:20 am

Quoting David L (Reply 2):
I guess it boils down to whether you want to exert some effort occasionally to change your attitude and "relax" most of the time or you want to exert very little effort to change your attitude and spend most of your time fighting to maintain a constant attitude. I know which option I'm backing.

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Seems that the control surface in question would no longer be dynamically stable. Which, in turn, would make the aircraft less stable as well. You'd likely need FBW or something similar in order to keep it under control.

I think the aerodynamic forces on the control surface would be significantly greater as well, leading to more strengthening needed (= more weight).

It's sorta like balancing a pendulum with the weight up on the top, instead of letting it hang downwards.

Quoting David L (Reply 2):
I'm referring to "attitude" in all three axes. Apologies if that's not The Done Thing.

Don't see anything wrong with that. You could specify by pitch attitude, roll attitude, or yaw attitude  Smile
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David L
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RE: Benefits/Penalties Of Fwd-facing Ctrl Surfaces

Mon Apr 16, 2007 9:01 pm

Quoting Vikkyvik (Reply 4):
It's sorta like balancing a pendulum with the weight up on the top, instead of letting it hang downwards.

Exactly the analogy I thought of after I'd posted but I thought I'd done enough waffling for one day/week/month/...  Smile
 
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Starlionblue
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RE: Benefits/Penalties Of Fwd-facing Ctrl Surfaces

Tue Apr 17, 2007 8:19 am

Quoting KELPkid (Reply 3):
similar to one of the reasons that I've heard backwards-opening car doors are called "suicide doors"-if you open them while the car is at speed, the relative airflow will catch the door and blast it fully open, and tear it off of it's hinges Smile [I cannot verify if this is true or not...] You'll have to ask someone who owns a 190-1968 Lincoln Continental or 1965-1969 4 door Thunderbird

Heck, why not ask the owner of a current RX-8?  Wink Then again, I don't think the rear door can open independently of the front ones.
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KELPkid
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RE: Benefits/Penalties Of Fwd-facing Ctrl Surfaces

Tue Apr 17, 2007 10:08 am

Quoting Starlionblue (Reply 6):
Heck, why not ask the owner of a current RX-8? Wink Then again, I don't think the rear door can open independently of the front ones.

Or a Saturn Ion Coupe or just about any full-sized pickup truck with an extended cab...but I think you're right: the new paradigm in backwards-opening automotive doors is don't let the rear-hinged doors open before the forward-hinged door is open first.
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jetmech
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RE: Benefits/Penalties Of Fwd-facing Ctrl Surfaces

Tue Apr 17, 2007 11:37 am

Quoting GAIsweetGAI (Thread starter):



Quoting David L (Reply 2):



Quoting Vikkyvik (Reply 4):
Seems that the control surface in question would no longer be dynamically stable.

Such a forward mounted control surface could be made stable if the hinge line was mounted forward of the centre of pressure of the control surface. I'm not sure of the benefits or disadvantages. I would say that such a surface may cause unacceptable airflow disturbances over the fixed part of the stabiliser.

Regards, JetMech
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EMBQA
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RE: Benefits/Penalties Of Fwd-facing Ctrl Surfaces

Tue Apr 17, 2007 11:55 am

Quoting GAIsweetGAI (Thread starter):
What would be the advantages or disadvantages of having forward-facing or forward-mounted control surfaces?

Advantages other then it would not work.....??
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2H4
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RE: Benefits/Penalties Of Fwd-facing Ctrl Surfaces

Tue Apr 17, 2007 2:40 pm



Here's what SlamClick had to say about it in this thread:

Quoting SlamClick:
On further reflection:

The whole notion of altering the setting of LEADING EDGE devices at cruise has bothered me since I first opened this thread and I just grokked why.

It is very difficult to push a rope. It is always easier to pull it.

When you have the elevators back on the trailing edge of the horizontal stab, which is, in turn, mounted at the back end of the plane there is a natural tendency for it to fair back to the slipstreamed position. In other words, inherent stability. It is why arrows have the feathers at the back. Weathervaning. If a conventional airplane were to fly backwards I think this tendency would become divergent - that is, a small displacement would be aggravated by airflow.

Happened with sailing ships "taken aback" too. The rudder being pushed backwards through the water would tear off the post unless held pretty much centered by strong hands on the tiller.

Forward-swept wings had this problem to be overcome.

I am just not comfortable with the idea of trying to push an airfoil, hinged at its trailing edge, into a lot of Q.


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