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Directional Airflow Ducting Instead Of THS?  
User currently offlinefaro From Egypt, joined Aug 2007, 1556 posts, RR: 0
Posted (1 year 11 months 3 weeks 6 days 21 hours ago) and read 2574 times:

How about replacing the THS by two embeded tailcone ducts, one venting ambient airstream upwards and another venting ambient airstream downwards to provide the necessary degree of longitudinal flight control ? Such ducts would be fed by air intake side-scoops feeding directly into the ducts from the tailcone sides.

Advantages would include lighter weight and simpler actuation mechanism. If the arrangement is insufficent to guarantee longitudinal control authority at very low speeds, a small(er) residual THS may be retained to provide more control authority at T/O and landing. At higher (cruise) speeds, the air side-scoop apertures can even be closed somewhat to reduce drag given the lesser need for longitudinal control forcing.

Why not use the brute force of airstream ducting to effect at least a measure of longitudinal control ? One doesn’t necessarily need airfoils to do this. Surely the engineering trade-offs involved are at least worth looking into ?


Faro


The chalice not my son
15 replies: All unread, jump to last
 
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17068 posts, RR: 66
Reply 1, posted (1 year 11 months 3 weeks 6 days 21 hours ago) and read 2551 times:

First of all, I think you mean the lateral axis. The longitudinal axis goes through the center of the fuselage from nose to tail. Turning about it is controlled with the ailerons. Turning about the lateral axis is controlled with the stabilizer.

Quoting faro (Thread starter):
One doesn’t necessarily need airfoils to do this.

Yes you do. You're redirecting air in a variable fashion. You need airfoils, ducted or not.

Quoting faro (Thread starter):
Advantages would include lighter weight and simpler actuation mechanism.

I don't think this would mean lighter weight since you need structure to enclose the stabilizer. The ability to decrease airflow by reducing the size of the scoops is interesting however.

Also, I don't see how the actuation mechanism would be simpler. You need some way to redirect the air. Ergo some sort of actuator, whether it be pilot muscle strength, hydraulic or electric.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineMir From United States of America, joined Jan 2004, 21677 posts, RR: 55
Reply 2, posted (1 year 11 months 3 weeks 6 days 20 hours ago) and read 2544 times:

Seems like it would be incredibly draggy, more draggy than a horizontal stabilizer. It also seems like it would be incredibly complicated to actuate properly, as opposed to a stabilizer which is mechanically pretty simple to move around.

-Mir



7 billion, one nation, imagination...it's a beautiful day
User currently offlinefaro From Egypt, joined Aug 2007, 1556 posts, RR: 0
Reply 3, posted (1 year 11 months 3 weeks 6 days 20 hours ago) and read 2543 times:

Quoting Starlionblue (Reply 1):
First of all, I think you mean the lateral axis. The longitudinal axis goes through the center of the fuselage from nose to tail. Turning about it is controlled with the ailerons. Turning about the lateral axis is controlled with the stabilizer.

You are indeed correct about the lateral axis. Simply, nowhere do I mention axes, I only talk of longitudinal flight control, ie pitch control, which is effected by the THS.

Quoting Starlionblue (Reply 1):
Yes you do. You're redirecting air in a variable fashion. You need airfoils, ducted or not.

To my mind, you need an efficient ducting profile to minimise curvature drag loss; I fail to see how one would need airfoils within the ducts however.

Quoting Starlionblue (Reply 1):
Also, I don't see how the actuation mechanism would be simpler.

You only need an aperture control system, like a sliding door mechanism. You don't need brute hydraulic pressure to force airfoil surfaces against the ambient airstream.


Faro



The chalice not my son
User currently offlinefaro From Egypt, joined Aug 2007, 1556 posts, RR: 0
Reply 4, posted (1 year 11 months 3 weeks 6 days 20 hours ago) and read 2540 times:

Quoting Mir (Reply 2):
Seems like it would be incredibly draggy, more draggy than a horizontal stabilizer. It also seems like it would be incredibly complicated to actuate properly, as opposed to a stabilizer which is mechanically pretty simple to move around.

It only needs to be less draggy than the THS which is hanging out there whether you need it or not. In effect, if the airframe centre-of-gravity is set in such a way that you need the least ducting control flow in the cruise, you have to a great extent eliminated longitudinal control drag in the cruise. If it is as draggy or more draggy than a THS during longitudinal control deployment, it may not matter as much because those phases of flight are much briefer than the cruise. Making senses here or not?

As to actuation, refer to my previous post, you only need aperture control which needs a lot less exertion/torque.


Faro

[Edited 2012-10-29 04:32:07]


The chalice not my son
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17068 posts, RR: 66
Reply 5, posted (1 year 11 months 3 weeks 6 days 20 hours ago) and read 2523 times:

Quoting faro (Reply 3):

Quoting Starlionblue (Reply 1):
Yes you do. You're redirecting air in a variable fashion. You need airfoils, ducted or not.

To my mind, you need an efficient ducting profile to minimise curvature drag loss; I fail to see how one would need airfoils within the ducts however.

You would need some way to change the geometry of the ducts. This implies a movable airfoil. This airfoil can be the duct wall itself, the intake door, or whatever. But I am pretty sure that by definition you need an airfoil.

Quoting faro (Reply 3):
Quoting Starlionblue (Reply 1):
Also, I don't see how the actuation mechanism would be simpler.

You only need an aperture control system, like a sliding door mechanism. You don't need brute hydraulic pressure to force airfoil surfaces against the ambient airstream.

Ah, you mean at the duct intake, right? Well, if you want to deflect enough air into the duct, you'd need a pretty strong sliding door mechanism. And when the door is closed you need to deflect that air. Either you close the duct inlet flush (think 777 APU intake), meaning a beefy mechanism to move the duct inlet in and out of the airstream, or you keep the inlet in the airstream, meaning lots of drag when it is closed.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineMir From United States of America, joined Jan 2004, 21677 posts, RR: 55
Reply 6, posted (1 year 11 months 3 weeks 6 days 20 hours ago) and read 2521 times:

Quoting faro (Reply 4):
It only needs to be less draggy than the THS which is hanging out there whether you need it or not.

It won't be. A duct that has a 90 degree turn in it is incredibly draggy. It's also more difficult to control in the event of a malfunction - if you lose your horizontal stabilizer trim, it's still relatively easy from a mechanical standpoint to use elevator control to keep the aircraft stabilized. If your air doors start jamming, that's it - the plane is not going to be controllable.

Quoting faro (Reply 4):
If it is as draggy or more draggy than a THS during longitudinal control deployment, it may not matter as much because those phases of flight are much briefer than the cruise.

Unless your center of gravity is directly above your center of lift, you're always having control deployment.

-Mir



7 billion, one nation, imagination...it's a beautiful day
User currently offlinefaro From Egypt, joined Aug 2007, 1556 posts, RR: 0
Reply 7, posted (1 year 11 months 3 weeks 6 days 19 hours ago) and read 2510 times:

Quoting Starlionblue (Reply 5):
You would need some way to change the geometry of the ducts. This implies a movable airfoil. This airfoil can be the duct wall itself, the intake door, or whatever. But I am pretty sure that by definition you need an airfoil.

So a question of semantix really; if any surface over which one experiences airflow is an airfoil then I agree, yes.

Quoting Starlionblue (Reply 5):
Either you close the duct inlet flush (think 777 APU intake), meaning a beefy mechanism to move the duct inlet in and out of the airstream, or you keep the inlet in the airstream, meaning lots of drag when it is closed.

In the open (ie deployed) position, the duct will generate drag, of course. No-one is contesting that. The point is i) how much compared to a THS and ii) for how long (ie, optimise the set-up for minimal flow requirement in the cruise so that periods of greater control deployment are much briefer time-wise.

In movement, I really don't believe that it would require all that much actuation force. Perhaps a specialist can illuminate us on this point: what physical parameters would one need to determine the force required to move a sliding, aperture-like door near the tailcone with surface area X at various airspeeds?

In the closed position, you essentially have no drag and no actuation force required.


Faro



The chalice not my son
User currently offlinefaro From Egypt, joined Aug 2007, 1556 posts, RR: 0
Reply 8, posted (1 year 11 months 3 weeks 6 days 19 hours ago) and read 2510 times:

Quoting Mir (Reply 6):
A duct that has a 90 degree turn in it is incredibly draggy.

No-one says it has to be a 90° deflection; it would in fact be quite a bit less.

Quoting Mir (Reply 6):
t's also more difficult to control in the event of a malfunction - if you lose your horizontal stabilizer trim, it's still relatively easy from a mechanical standpoint to use elevator control to keep the aircraft stabilized. If your air doors start jamming, that's it - the plane is not going to be controllable.

Yes, that is a issue.

Need more thought on that one although, IIRC, some THS loss situations (extreme deployments?) may lead to inability to retain control via elevators alone. Perhaps that's a type-specific thing unless there's a regulatory requirement re THS loss that mandates residual longitudinal controlability via elevators.


Faro



The chalice not my son
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17068 posts, RR: 66
Reply 9, posted (1 year 11 months 3 weeks 6 days 19 hours ago) and read 2500 times:

The purpose here is to decrease the drag of the stabilizer in cruise, when it could be way smaller, right? Wouldn't a much simpler solution be a variable geometry stabilizer that retracts part way in cruise?

Quoting faro (Reply 8):
Need more thought on that one although, IIRC, some THS loss situations (extreme deployments?) may lead to inability to retain control via elevators alone. Perhaps that's a type-specific thing unless there's a regulatory requirement re THS loss that mandates residual longitudinal controlability via elevators.

Lateral controllability. We are talking about the lateral axis, not the longitudinal.

If you're going to have elevators, you might as well have a stabilizer. And yes, airplanes must by regulation be controllable with the elevator in case of stuck trim (or runaway trim leading to disconnecting the trim mechanism).



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineMir From United States of America, joined Jan 2004, 21677 posts, RR: 55
Reply 10, posted (1 year 11 months 3 weeks 6 days 19 hours ago) and read 2500 times:

Quoting faro (Reply 8):
No-one says it has to be a 90° deflection; it would in fact be quite a bit less.

Then you're going to have to have more airflow going through in order to get the force that you need, which means larger doors and more drag.

You're going to get more drag than a horizontal stabilizer somewhere, be it at the forward doors, be it in the duct, be it in the the outgoing airflow.

-Mir



7 billion, one nation, imagination...it's a beautiful day
User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 11, posted (1 year 11 months 3 weeks 6 days 17 hours ago) and read 2477 times:

Quoting faro (Thread starter):
If the arrangement is insufficent to guarantee longitudinal control authority at very low speeds, a small(er) residual THS may be retained to provide more control authority at T/O and landing.

You can't use a smaller THS...the sizing criteria for the THS *is* low speed. With your duct system, you have to be able to handle a failure of the duct actuation system in any position. When a THS fails it just sits there (hence stab trim cutouts) and it still provides stabilizing force; the elevators are then sized to provide adequate pitch authority with a THS stuck at the worst CG location. If your ducts fail closed and aren't helping you any, you're still going to end up with a full sized THS to handle the worst case scenario.

Quoting faro (Thread starter):
Why not use the brute force of airstream ducting to effect at least a measure of longitudinal control ? One doesn’t necessarily need airfoils to do this. Surely the engineering trade-offs involved are at least worth looking into ?

You want to use dynamic pressure (aka kinetic energy of the airstream) to exert a force on the airframe. That's what airfoils are designed to do. The reason we use airfoils is that they're, by far, the most efficient way to do it that we know about. Put another way, for a given force an airfoil is the least draggy way to obtain that force from the "brute force of airstream". If that weren't the case we'd be using some kind of funny symmetric body with ducts in it rather than a wing to generate primary lift.

Put another way; if this were a good engineering trade-off you'd be better off applying it to the main wing than to the THS. So yes, such engineering trade-offs are absolutely worth looking at, but you don't have to look very far before you discover that an airfoil is the best way to "extract" force from a moving airstream.

Tom.


User currently offlineMir From United States of America, joined Jan 2004, 21677 posts, RR: 55
Reply 12, posted (1 year 11 months 3 weeks 6 days 17 hours ago) and read 2467 times:

Quoting tdscanuck (Reply 11):
If your ducts fail closed and aren't helping you any, you're still going to end up with a full sized THS to handle the worst case scenario.

And if they fail open, you're not going to have enough control authority at low speeds to do what you need to do.

-Mir



7 billion, one nation, imagination...it's a beautiful day
User currently offlinewingscrubber From UK - England, joined Sep 2001, 852 posts, RR: 0
Reply 13, posted (1 year 11 months 1 week 2 days 3 hours ago) and read 2086 times:

Faro, sounds like you're describing a 'puffer' reaction jet system, this has already been implemented on VTOL aircraft, but generally has no useful applications otherwise. Notable exceptions though, include the NOTAR ducted tailbooms, used in place of the tail rotor, and the maneuvering thrusters used on the space shuttle.

http://ftp.rta.nato.int/public//PubF.../MP/RTO-MP-051///MP-051-PSF-26.pdf



Resident TechOps Troll
User currently offlineflipdewaf From United Kingdom, joined Jul 2006, 1577 posts, RR: 0
Reply 14, posted (1 year 11 months 6 days 19 hours ago) and read 2086 times:
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Quoting wingscrubber (Reply 13):
include the NOTAR ducted tailbooms

Aren't they using the coander effect?

Fred


User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 15, posted (1 year 11 months 6 days 15 hours ago) and read 2086 times:

Quoting flipdewaf (Reply 14):
Quoting wingscrubber (Reply 13):
include the NOTAR ducted tailbooms

Aren't they using the coander effect?

Sort of. NOTAR uses coanda to cause the exhausted air to follow the boom profile. However, that's not where most of the control force is coming from. The big boost is the Magnus effect that has on the downwash from the main rotor; Magnus effect is usually considered on a spinning body but the exhaust air from the NOTAR system makes it look like the boom is spinning from the perspective of the main downwash.

Some NOTAR systems also have a direct jet at the back to provide finer control.

Tom.


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