Slab tailplane balnce points.

[RetiredBA/BY]

Can any TP or engineeer direct me to a publication which deals with mass balance points (C of G) on all flying stabs. (such as the Hawk or F4) or give me a short explanation of the basic theory of balncing thsee components.

Regards, David Gladwin.

[WeekendFlyer]

Can't direct you to a publication, but the key issues are as follows:

1. The hinge line should be ahead of the centre of pressure CP otherwise the whole surface will want to flip 180. Having said that, with irreversible power actuated controls, the hinge line could be behind the CP for low speed flight, given that the CP moves aft as the critical Mach No increases. This photo of a Tornado is interesting in that respect: http://www.aerophot.co.uk/images/VAL06Valley%20GR4%2008.jpg

2. The length of the moment arm between the CP and hinge line depends on what characteristics the designer wants in terms of b1/b2 ratio, which essentially sets how "heavy" the controls feel (if they are reversible, e.g. PA28, Robin DR400, etc). For aircraft with irreversible, power actuated controls (e.g. F4, Hawk, etc), the moment arm is fairly small to minimise the loads on the actuator. Next time you have a chance to look at an aircraft with an all-moving horizontal tailplane you will see the hinge line is normally some way back from the leading edge.

3. Not sure about CG position, but would suggest that most of the mass of the control surface would be near the hinge line.

Hope this helps!

[Notsonew]

A search on google using 'stabilators on aircraft' comes up with masses of info. As usual wikipedia has a concise explanation. I was tempted to put my email address in my profile so you could email me. A long shot but did you fly at the pointed end of BA/BY? and did you once suffer an erroneous fire warning in a JP?

Interestingly the feel in PA28/PA34 is maintained by an anti - servo tab which also acts as a trim tab. Some think anti -(over?) balance tab would be a more appropriate name as it helps to stop the CP (AC) moving across the hinge line and giving you more movement of the stabilator than you demanded.

I will now see if you have an email address on your profile.

[Notsonew]

Retired BA/BY (David Gladwin) I have activated my email for you -click on my user name and navigate from there.
Regards

[RetiredBA/BY]

Gentlemen, Many thanks for taking the time to reply, much appreciated.

As an ex CFS QFI I am very familiar with the RAF explanation of mass balancing normal controls to prevent torsional and flexural flutter but cant remember anything about slab tailplane mass balancing and flutter.

The machine in question is a large scale MODEL of the BAe Hawk so IAS limit is around 175 k so C Of P movement with high Mach no. is not an issue. I have a mass of servo power to resist flutter (I guess at high a of A with forward C of P the servos are resisting movement of the tail rather than driving it). I have partly balanced the stab to minimise servo loads but the C of G is still well aft of the pivot axis. Question is, what is the best position of the Cof G to minimise possibilty of FLUTTER , should it be ON or AFT of the pivot axis ?

Yes I'm afraid I am the guy who wrote off a JP after ejecting after a false fire warning, only doing what my instructor told me to do though !

Regards, DG.

[Notsonew]

Thanks David,

We shared the same instructor and you and I visited him in Sick Quarters where he was prostrate with a very sore back (single stage cartridge or 2 stage - I cannot remember).

I do remember you were very keen on flying aircraft models even in those days.

Given my 'basket weavers' degree hit me with this :- how do you apply Reynolds Number on your big project.

I will probably lose you in the first sentence but I may re read the CFS notes on the subject.

Sorry I came up with a post nowhere near your question - I usually do. Hope somebody has an answer for you. Try the various aircraft manufacturers' design departments and give your credentials - they may well reply.

Regards and best of luck

[411A]

The only large civil jet to use an all-moving tailplane is the Lockheed TriStar, and it works very well.
Initial comments from new pilots to the type were....'this can't be good, it ain't a jet fighter' and 'combined with DLC it is a disaster waiting to happen, hope it's not me'.

How wrong they were...TriStar, it works to perfection.
And, why not...it's a Lockheed.:):)

[HarryMann]

Etkin is a std text on S&C (Dynamics of Flight - Stability And Control), Bernard Etkin
It worth referring to, but doesn't give a specific solution for all moving tails

This book on the web has most pages readable and does touch on the subject

http://books.google.com/books?id=D-ctX2Q-CSIC&printsec=frontcover

The mass-balance and flutter solutions are distinct but very close to each other... pretty close the hinge point I'd say, and the hinge-point close to the a.c .
c.g. slightly ahead if anything IMHO