Airplane flap control

[sstahovich]

I am working on a project regarding general aviation aircraft. I need to deploy a device, very similar to flaps, on the airplane. For safety reasons the Flaps must be able to return to their original position if any system failure were to occur. In other words the actuators holding power must be low enough for wind resistance to close the flaps as a fail safe if the system were to malfunction or occur an electrical power loss. Right now the best thing I can come up with is a common ball screw linear actuator. Is their any better commercial available options, possibly something with a magnetic clutch?? I will need one for each side of the plane and weight is a significant factor.

 

[rb1957]

intiutively it sounds to me that "the actuators holding power must be low enough for wind resistance to close the flaps as a fail safe" won't work ... how will actuators that that low of a power extend the "flaps" in the first place ?

can't any sort of power assist (electrical, hydraulic) be neutralised (eg with a CB) so that it won't lock down in a failed case ?
then have a manual retract. look into what they do with landing gear, ie manual extension in th eevent of a hydraulic failure.

Quando Omni Flunkus Moritati

 

[drawoh]

sstahovich,

A motor with an arm and a link ought to meet your requirement. Can your ball screw be back driven? Can it be reliably back driven after it gets dirty?

If your actuator must work continuously against the airflow, then it is equivalent to or just plain is a solenoid. It will consume power, all of which will be converted to heat, all the while your device is deployed. Is this acceptable?

--
JHG

 

[MintJulep]

Do you understand that the aerodynamic forces will not be a constant?

You might have a concept of "closed" in your mind, but a dangling thing in the air stream with no positioning forces from an actuator acting on it has no way to read your mind.

It will go to the position where the aerodynamic forces are neutral at the moment. And that position will change with airspeed, angle of attack and probably a few other factors.

 

[rb1957]

sort of like an HP automatic slat ?

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[MikeHalloran]

Ballscrews can in general be back-driven, the difficulty depending on the lead.
Good practice would include fabric gaiters over the exposed portions of the screws to protect them from external contamination, and to limit the spread of internally generated contamination. Gaiters don't weigh a lot. Ordinary ballscrews do. Aircraft may use hollow and/or nonferrous ballscrews, which are not otherwise common.

I recall examining a reactor control rod drive (some decades ago; they may be different now) comprising a roller screw, the nut of which was split on an axial plane, and spring loaded and hinged so the rollers would be pushed out of engagement on power failure, allowing the control rod to drop, scramming the reactor. I think the magnetic field of the drive's motor pulled the nut halves together, so the nut and screw were engaged so long as the motor was powered up. It was relatively simple and sturdy, and definitely not flight-weight, and definitely not affordable by ordinary mortals. I'm not aware of anything similar produced for commercial applications, but it may provide food for thought.

Speaking of which, it should be possible to build a clutched linkage, e.g. using trailer brake (electro)magnets run at reduced power to grip a flat ferrous surface (you need a pulse of higher power to pull them in at startup), interposed between some kind of actuator and the linkage to the flap itself. Startup would also have to include some procedure to align the parts, e.g. by letting the flap drop against a stop (or be driven to a known position by weak springs) and driving the actuator to the corresponding position before engaging the clutch.




Mike Halloran
Pembroke Pines, FL, USA

 

[sstahovich]

I know the aerodynamic forces will be constantly changing, i was going to add a spring to keep constant pressure and act as the closing force in a stalled condition. From what I am gathering, a ball-screw can be pretty easily back-driven but at what cost to the holding power? What i am looking for would be very similar to the function of solenoid as it will always be retracted when power is disconnected. The reactor control rod sounds like an excellent solution budget permitting. drowah.... Heat and constant power requirements are not a major concern at the moment

 

[rb1957]

ok, but tat is very different from your original description. having a spring almost assures retraction ... what if the mechanism jams ? i'm guessing you wouldn't have the "flap" positioned by this means (ie an actuator working against a spring and the external load) as i don't think it'd be very steady (with changing external loads).

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[TheTick]

Hydraulics, perhaps?

 

[sstahovich]

I know hydraulics are definite solution,but i wanted to come up with electric alternative. rb1957.. the flap would definitely need to be positioned by the mechanism. As far as the system jamming i think its going to be a possibility regardless of the system design.

 

[sstahovich]

can't any sort of power assist (electrical, hydraulic) be neutralised (eg with a CB) so that it won't lock down in a failed case ?
then have a manual retract. look into what they do with landing gear, ie manual extension in th eevent of a hydraulic failure.

what is a CB? .. the landing gear solution would be ideal, but I am trying to keep the system as simple as possible and i think it would add too much weight.

 

[MintJulep]

Google for " spring return electric actuator" returns 431,000 hits.

One of those might be exactly what you need, or one of the manufacturers might be able to make a special for you.

 

[tbuelna]

This description seems a bit unusual for a flight control surface actuator. I can't recall seeing a requirement for a control surface actuator that needed to default to a condition where it was decoupled from the driver due to a loss of input power. An unrestrained flight control surface is usually a situation that designers try to avoid. Flight control surface actuators are normally designed to provide one or more levels of operational fault tolerance. For critical aircraft control systems, the actuator would have the ability to function after losing its primary, or even secondary, power supply.

What you need to consider is whether the actuator failure mode where your flap defaults to some neutral position is actually "fail safe". If the flap defaulting to a neutral position due to an actuator failure creates a condition where the aero balance of the aircraft is rapidly upset, it may cause the pilot to lose control. In this regard, it may be safer to have an actuator that can maintain its position in the event it loses power.

 

[DRWeig]

I second MintJulep's suggestion.

Best to you,

Goober Dave

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[rb1957]

"the flap would definitely need to be positioned by the mechanism" ... ok, but as i understand your idea (using an actuator against a spring) won't this respond to the changing airload ? i'd've thought that you'd need some sort of positive positioning (like a lock strut, or some means to lock the actuator at a set position).

"CB" = circuit breaker, so that if your electrical system jammed, you can take control by disabling the electrics.

this is going to be a complicated mechanism, with your assured retraction requirement. airloads, inertia loads, may not be sufficient to retract the "flap" so you need to design in a primary extension/retraction mechanism, including a feature to disable it in the event of a mishap, and a secondary retraction means (and if you're particularly paranoid a third retraction means).

if think MJ's idea is a good primary means, if it includes a locking feature. alternatively a doubling acting hydraulic/electric actuator (with a locking feature). then you need the secondary retraction means, maybe this is where you spring can come into play ?

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[IRstuff]

"if any system failure were to occur"

Sorry, but I think that's an absurd requirement. You cannot possibly compensate for all failures. All designs are compromises amongst competing requirements. For example, you could possibly design some sort of, say, triple failure tolerant mechanism, but it might be ridiculously expensive, weigh too much, and eat up too much volume. Therefore, you must allow for some failure that ought to be as unlikely as possible.

TTFN
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[rb1957]

well at least he doesn't say "any combination of failures" ! i think the OP means a single system failure like loss of electric power or a jam of some component. still it complicates the design mightily; we had one design where a door (covering a sensor window) Had to be closed for landing ... 'cause of the potential of stones and such damaging the window (worth more than the plane !), so we had an electrical primary drive and a manual secondary. sure if you said "any failure" you might say "what about a roller jamming in the track (so that the door couldn't move, period) ?" sure there might be some weasle words about routine maintenance, but ...

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[IRstuff]

Well, "any" failure, to me, encompasses multi-point failures. It would need to say, "any single" failure.

But even assuming only single failures, there are still tons of possible single-point failures that could not all be mitigated. For the desired electrical actuation, the control could potentially be stuck ON, which requires some sort of watchdog, and a secondary disconnect like stacked drivers driven with independent circuits. So, right off the bat, the circuitry complexity is doubled just to handle this one failure.

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[sstahovich]

To keep things simple lets just say I need to demonstrate that the flap will return to its closed position in the event of a power failure. After reviewing some of the ideas posted and the inherent complexity of the mechanical systems I am considering a hybrid hydraulic alternative. My thoughts are this; I could have two double acting hydraulic cylinders. Cylinder A is attached to a electronic linear actuator, cylinder B is attached to the flap. I could remotely control the position of the flap through the linear actuator attached cylinder A. A spring attached to the flap would provide constant back pressure in the system. In the event of a power failure in the open position a relief valve would be manually opened releasing the hydraulic pressure and allowing the spring to close the flap.

 

[rb1957]

i think you're creating failure opportunities ! maybe look into MJ's link ?

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