Guys,

The elevator setup on my Skygate Hawk is as in the pics below. Has 2 or 4 servos driving a giant carbon pushrod via the pivoting arm thingy that I can't think of the correct term for whilst I sit typing this [:@]

Now, I was planning on using 2 8711 servos but have just realised that if one servo fails I will have no elevator movement as the one servo left moving will just be spinning the arm without pushing the pushrod???

If I swapped to 4 servos and 1 failed do you think there would still be elevator authority??

Awaiting comments!

Cheers,
Mark

Bellcrank.

However, even a model of this calibur, there are a thousand things that could go wrong ending in loss of the airplane. Is it really necessary to "plan" on the event that a servo fails? You are certainly more than adequate on power with the 8711, in fact, the servos will hardly have any force on them percentage to the capabilities as you have it setup.

Just a side note, the calcs I did on the original Isobar at 200 MPH and full deflection of the elevators, the force was 220 OZ. At the time, the only servo available was 8411 at 155 oz. By the way, testing of the servo showed 1.5 on the servo at 155 oz torque.

Even if you doubled the size of the control surface (which the hawk certainly is at least double), you are more than double the torque with the 8711, and the odds full deflection at 200 mph??

With these higher end digital servos, none of us are flying them to their "limits", in fact even the most abrasive maneuvers are not likely to push these servos enough to even make them warm.

with that setup if one servo fails you'll just loose half the deflection on the surface, not the whole control FWIW.

that being said, if it happened to fail with that one servo at full deflection (either up or down) you'd only be able to bring it back to center with full opposite stick. if you were lucky to have it fail in neutral then you'd likely be able to save the jet but have allot less (half to be exact) elevator authority.

I know Mark, I was discussing this item with Ali when he first assembled his Red Arrows one. I could'nt fathom it then and can't now.

I do have to say if that was my model I would not fly it with that setup!

m

The Airworld system is much better but that too has it's shortcummings.

KC I cannot agree with that, if one servo fails to idle you loose all the throw.

m

How about twin servos inline with oneanother?

Surely you would get half movement if one servo stopped working? The servo with fixed position would act as the pivot point and the servo moving would swing the long arm back and forth so you would get some movement.

How often do servos stop working anyway, not very often these days. I'm guessing all the Hawks flying in Germany use the kit set up and I've seen a lot of footage of them flying.
I'd leave it alone.
I was told Skygate built Ali's model anyway so it would have the stock set up for elevator etc

Matt

only if you strip one servo and it flops, in most failures they lock and with the servo locked the fulcrum point is just half with one servo operating, if you don't believe it just unplug one servo from the RX and give it a go, it'll cut the deflection by half. for every 1" of servo movement with one locked in a fixed position, the push rod will only move 1/2" but that servo will have twice the torque, it's called a 'Simple Machine' or in laymen terms a leaver

Guys,

Doh. Bellcrank - thank you!!

There are cutouts for 4 servos so you could have 2 ganged in line on each side.

I was fairly OK with this setup until I heard of one crash in Germany after a suspected elevator servo failed.

Sean, I have pretty much all bases covered redundancy/backup wise except this. I agree you can't plan for everything but giving it some thought and putting it out to you guys may come up with a simple answer.

As for movement, to my mind it requires both servos pushing equally in order to drive the pushrod fore and aft. If one servo fails I think the other servo will just be rotating the bellcrank around the 'pivot' of the bellcrank and not 'pushing' the pushrod fore and aft??

This model is so close to hitting 20kg (and the associated hassle in the UK and international comps with this) that Skygate need every oz shaved off where they can hence the reason most fly with this setup.

Jury still out

Rgds,
Mark

with the one failed servo being fixed, you've then changed the pivot point to that servo, that moves the push rod 1/2 the travel of the other servo at twice the torque, when you get the oppertunity, just unplug the one servo and you'll see it first hand.

the only way you'd get no deflection on the push rod is if you set the servos to travel opposite each other (one go to the front as one goes back), then you'd make this a bell crank, in the configuration it's setup as and intended it's a leaver, not a bell crank.

Mark

If one servo lost power or disconnected then you are right, if it locked up you would have some control

Dave w

How about using 2 direct pushrods (one on each servo) instead of the single ganged point? If you strip a gear or something then you would be fine as the dead servo would freewheel. But I dont know if it would help you if one servo "locks up". I am talking about like on Skymasters F-4.

Too late for that layout change now.

Guess my question now is, would having 2 servos ganged per side help or if one locked up would it more than likely burn out the other one trying to move it!!

Rgds,
Mark

with the 8711 i doubt seriously you'd ever strip the gears of it in flight and if it failed it would most likely be in a locked position. all things considered IF that happened in flight, I'd take my chances with being able to safely get her on the ground with just the 1/2 deflection, or you could set up a extra 'dual rate' with extreme servo throw for just such an occasion so you'd still have ample elevator authority, you could call it 'Crash Mode'

Now there's a flight condition name it would hurt you to switch to

I think I will stick to the stock setup with twin 8711 and MAYBE think about 'crash mode' although I seriously doubt I'd remember I had it once i realized this thing was in trouble! Besides I'd be too busy filling my underpants up [X(]

One side observation...
I noticed the use of stock nylon servo arms on this application. That stab is huge & those splines will probably strip before any servo gears do. I'm surpised you're not using aluminum arms here.

I think you're doing the right thing considering possible failure points. That's one large, awesome jet with a lot of time & money tied up. It's not worth losing over a minor detail.

That's how the large scale aerobatic dudes set up their huge models. Sure they don't fly as fast as jets but, boy, don't they wring them out.

Perhaps check out some of the suppliers for that sort of gear.

It looks a long way from the 'pivoting arm thingy' to the pushrod support, adequate? - John.

Guys,

Those are the pics from the Skygate CD. I am using 2" SWB double loc metals arms.

I have ganged servos in other models such as my 3m Extra 260 and these work well however the fact remains that all solutions are fine if the servo fails but is still movable. I think that all applications are doomed if a servo fails in a seized state and that, I guess, is that!

If one servo fails in the seized state your ok because the other will still power the tail. What I would be inclined to do is mechanically limit the rotation of the bell crank to say 5degrees, that way you maintain a degree of control whatever happens. Nice model Mark.

m

Hi Mick,

Thanks. Will rig it all up and do some real world testing to see how much movement is available with a servo seized in different positions and also how much movement is lost in excess 'slop' created by the pivot.

You bringing your Hawk to Bickley?

Rgds,
Mark

How about this setup. This would perhaps give you little extra insurance only if the forward pull is
setup so that the elevator goes in the up position.
Just a thought