Control harmonisation, a black art?

[TD&H]

Over on the AH&N thread about the Chipmunk's 60th it has been said by a certain well known test and display pilot that the Chipmunk has the nicest control harmonisation of the many types he has flown. In these days of computer designing it would seem to be easy to ensure good, if not excellent, harmonisation. But back in the days when the Chippie was designed would this have been a matter of good luck?

Perhaps, or maybe obviously, it would be easier with an aeroplane having a fairly narrow speed range like the Chipmunk, than for a Hunter or Lightning? Also how much was (is) control harmonisation compromised in aircraft such as the Harrier or Phantom for the benefit of performance? The early powered controls must have given poor feed back, but was this improved with the later and modern systems to give good or even an artificial feel of harmonisation?

[Tarnished]

What a huge question!

I would start by saying that one mans "nicest control harmonisation" could well be another man's "bag of bolts". As you point out achieving good handling qualities across a wide envelope is more difficult than over a narrow one. This is where modern flight control systems really start to pay dividends. By this I'm talking about flight controls where the "gains" are variable depending upon whereabouts in the envelope you are and indeed what task you are undertaking. Phantom/Lightning/Harrier etc had gearing to change control response or control travel based on gear being up or down and to some extent depending on Mach number. But the likes of Typhoon has a variable schedule based on all of the above and in addition mass and cg location (long and lat). Advances in filght simulation now allow a great deal of refinement to be done in advance of flight test, but the proof of the pudding still (thankfully) comes from flying in the real air.


Tarnished

Edited to add more here: http://www.pprune.org/forums/showthread.php?t=188151

[Genghis the Engineer]

And still most pilots won't care so long as there's enough thrust and a good view out of the cockpit (the latter generally being optional!), plus being cheap to operate.

Much as one might wish otherwise, control harmonisation - or just handling qualities in general is a long way down the list of priorities of the majority of operators.

G

(Okay, I'm just envious because I've never managed to get my hands on a Chipmunk, but it is a serious point).

[barit1]

At some point the insurance companies get interested in handling qualities, because if one type ship turns out to have more mishaps (crosswind ldg, tailstrikes etc.) than its competitor, it reflects in premium rates. As a result, customer airlines may find insurance rates a strong bias. :hmm:

[ICT_SLB]

Re crosswind landings - not strictly harmonization more control effectiveness but Transport Canada require any CAT II aircraft to keep within the GS & LOC limits in AT LEAST 15kt Crosswind, 25 Head & 10kt Tail (see Working Note 4).

Postings on other PPRUNE forums refer to control ratcheting with spoiler/aileron interaction and it's my experience that most TPs & AP Engineers on current commercial jets try to get the overall harmonization as smooth as possible but it's very difficult to get, say, the same effect at both HVY/FWD & LT/AFT especially when the weight can nearly double between the two extremes of the envelope.

[TD&H]

Thanks Tarnished for that link. A very interesting read.

BTW do you regard the Chipmunk as a "bag of bolts"?:E I was lucky enough to learn to fly on Chippies, perhaps thats why I agree with Aerbedane's comment.

The computer designs and FBW mean a whole different ability to create a nice aeroplane to fly. But are there basic ways of designing types of controls (eg slotted ailerons), size of control surfaces as percentage of wing/tail/fin area that can lead to good control harmonisation/bag of bolts? Especially for a non-computer designed aeroplane. Say for someone wishing to design a modern Chipmunk? (Which I'm not, BTW)

[Tarnished]

Only ever got one ride in a Chippy, long before I was clever enough to be able to tell the difference between a bag of bolts and a bolt of lightning! Couldn't see from the back seat.

I'm sure there are rules of thumb for "fag packet" design, found this:

http://www.aircraftdesign.com/newcrs.html on a google search under aircraft design.

I don't design them I just try and fly them and pass on my thoughts! Strictly the sizing of the controls relates more to control power in the first case and the harmony bit comes as a by-product.

Tarnished

[John Farley]

The boffins used to say that a good starting point for harmonisation was aileron forces increasing with IAS, elevator forces with IAS squared and rudder with IAS cubed.

[TD&H]

JF would you mind, for the benefit of a Bear of Little Brain, explaining how those criteria are met? From my limited knowledge of aerodynamics I'm having difficulty seeing how each can be different, i.e that all forces would increase at speed squared, as in lift and drag having the V squared laws, assuming being above V min drag.

[barit1]

The boffins used to say that a good starting point for harmonisation was aileron forces increasing with IAS, elevator forces with IAS squared and rudder with IAS cubed.

Kinda wish the A300-600 observed this rule? Might have lead to a different outcome. (http://www.ntsb.gov/ntsb/brief.asp?ev_id=20011130X02321&key=1)

[John Farley]

With manual controls the designer needs a bit of control design expertise to keep the forces on ailerons lighter than v squared.

Typically you need to put a tab on the aileron itself which moves in the opposite direction to the surface. So aileron up tab down. This lightens the force to displace the aileron. Commonly known as a ‘balance tab’ or‘geared tab’ .There are also ‘spring tabs’ where the linkage driving the geared tab is ‘spongy’ (incorporates a spring) and can be used to further fine tune the forces left to the pilot.

The advantages of the spring tabs (which made the previously heavy ailerons on the Meteor a joy after they were fitted) is that the spring blows off a bit as speed increases so the designer can make the ailerons super light at low speed (where the spring strut acts as if it is rigid and so gives max tab deflection). At high speed the spring looses the battle and does not move the tab so far thus increasing the stick forces.

If you want to make a control heavier than v squared you again fit a tab to the trailing edge but make it work in the same direction as the surface so making it artificially heavy. Known as an ‘anti-balance’ tab.

If you have ordinary power controls (without fly by wire) you just have a ‘q’ feel device which knows the IAS and increases the artificial spring centering forces by fiddling with the position of the fixed end of the spring and hence changing the effective spring rate that the pilot is opposing.

Regards
JF

[TD&H]

JF, thank you for that, and I can certainly appreciate how that works for aeroplanes with a large speed range.

Thinking back to the Chipmunk, which has no tabs, except for the fixed, ground adjustable one on the rudder, then it does seem to be a touch of the black art or maybe devine intervention that gives the delightful 'Oh! I have slipped the surly bonds' feeling. Oh for a bit of devine intervention to get my hands back on one.

[enicalyth]

I flew the Chippie at Edinburgh UAS in the 1960's. It was a pile of poo. Nothing to do with me but the fool of a Polack designer who went on to disgrace himself with significant parts of the Concorde. He knew next to sod all. I wonder if after all these years it would turn out to be a delight after all? Is there, do you think, an equivalent of a tooth fairy that swops young men for old but exacts revenge by applying retroactive fond memories to absolute crates? As a boy in the early fifties the Vampire exercised a fascination but when I got my hands on a Venom it had become sedate. The JP was for wusses, the Gnat had a delightful bite and everything went downhill from there. But let me assure you the Chippie was a bag of bolts. Unless I was a very bad pupil you understand.:p

Oh to be young again. Anyone got Dorian Gray's phone number?

[TD&H]

I don't know about Dorian Gray's phone number, but I've just copied your reply over onto the AH&N thread about the Chipmunk. I trust none of the enthusiasts of the Chipmunk have your phone number or you may have to don battle bowler and retire below the parapet!:E

[Shaggy Sheep Driver]

Here's what I posted over on the 'Chippy' thread:

He could not be more wrong! Of course, it won't have the whizz bang handling of a Gnat - it's only a 145hp light aeroplane. But among all the very many light aircraft I've flown, none come close in handling terms.

Some (like the Acrosport or Yak52) have much higher roll rates etc, but none are as well harmonised, or give that 'strap-on-a-pair-of-wings' feeling quite like a Chippy does.

Those who've flown the Spitfire and Chippy say the former is just like a bigger, infinately more powerful, Chippy.

Be intersting to know just what enicalyth does consider to be a nice-handling light aeroplane?

SSD

['India-Mike]

I'm a Chippy owner, with about 150 hours on type. After 600 hours of tricycle SEP flying, I have to confess I found it a handful up and away (never mind the tailwheel bit). It's of another era - marginal stability in pitch (I think it's a lightly damped phugoid), and very light control forces. The latter are so light they make a nonsense of trying to assess harmonisation let alone quantify it. Having got to grips with the aeroplane, an IMC renewal in it (VDF let downs, SRA's) brought the instability in pitch home to roost. Control forces so light in such an aeroplane in my limited, non-tp, view, emphasise the nonsense of the concept of harmonisation. Now Pup vs. Bulldog on the other hand, allow one to interpret the concept of harmonisation - the latter's elevator is just too heavy.
I believe there is a Darrol Stinton preview assessment of the Chippy, but even with my current ETPS contact have not been able to access it. Perhaps that might shed more light on the aeroplane. It's got in my opinion a couple of (in the contemporary context) deficiencies that are actually assets!
Remember Genghis there's an open invite up here - challenge you to a formal preview!

[Shaggy Sheep Driver]

Interesting. I'd really like to read that Darrol Stinton assessment if anyone knows where a copy can be had.

I've been a part owner of Chippy G-BCSL since 1979 (except for a couple of years when I left the group to join a Yak52 group). The Chippy's control forces are light and friction-free, which is one thing that (in my book) makes it such a delight to fly. But they have loads of feedback for the pilot - unlike the Acrosport, which probably has as light (and certainly more powerful) controls with very little 'feel'. That's quite a dangerous combination, in my opinion.

Chippy contol forces do get heavier with increased IAS, as you'd expect. Makes gentle aeros a delight! I've never flown any other aeroplane that 'talks' to its pilot through the stick in quite the way a Chippy does.

I'm interested that India Mike thinks the pitch stability to be marginal. I haven't found that; once she's trimmed, she holds attitude and if disturbed will regain trimmed attitude - though not as quickly as, say, a C172.

I suppose it's quite possible that 'by the book', assessed by a professional test pilot (which I'm ceratinly not!) the Chippy might actually not score well. I don't fly instruments, for instance (I don't know any Chippy pilots that do, at least in that aeroplane), and for that, a more stable, less manouverable (chuckable??) aeroplane might be better. But I don't think that's what Chippys are about. ;)

Genghis, you are welcome to the back seat in SL any time its free (quite often!). It's an aeroplane every pilot should sample - but beware! It will spoil you for anything else!

SSD

[enicalyth]

Hi Shaggy, how's it goin'

As you can tell I made a right a**e of the Chippy. Just did not get on with it at all. I think about 3hrs behind average before I went solo. I did not like her and she sure reciprocated big time. I was indeed a very bad pupil but got over it, a late improver. I wonder indeed what I would make of it now?

The nearest I got to strapping on a fabled warbird was a P-40 when in the States but that is no Spit. I don't know if you know but the old Spit gate guardian at RAF Turnhouse, home to my UAS, was one of the lucky ones. Some couple of years ago I visited RAF Cosford to see the reserve collection and it was being restored to display standard and there was talk at the time of its perhaps becoming airworthy.

Apart from a joyride in an old Auster at age fourteen or fifteen which barely counts the Chipmunk was the first aircraft I was in that I can positively remember. Nobody has that much fun these days that the first real aircraft one gets in is the one that you learn in. It just does not happen in today's jet age. Prior to that flight I was two but no one is able to recall what the passenger aircraft was as Dad has passed on. As a boy I came to England on a Union Castle liner to receive my secondary schooling. Nowadays kids think nothing at all about flying and a ship to them is boring. Furthermore I learned to fly before I learned to drive a car though I had had motorbikes. Again these days kids are driving just as soon as they can. Really really amazing changes in such a short space of time.

You did spot the irony however? Chippie's designer was I believe largely responsible for the aerodynamic design of Concorde. Quite a feat in a lifetime to have both these aircraft on one's drawing board. Another such example is Hurricane to Harrier and another still Merlin to Olympus in the world of engines. Can it be possible for such design leaps to happen in a youngster's lifetime, another Teddy Petter?

As I said "Oh to be young again" it was fun. I just hope the greater opportunity that youth has now does not take the edge off the excitement.

Nice handling light aircraft? I've not really found one that has had me ecstatic so that says everything about my abilities. I seem to gravitate towards dull Pipers. My family is too large to permit owning but I do enjoy sailing and it is affordable.

My wife and I will shortly be passing through London as we are bound to see my old homeland, a remote rock island, last of the Empire. If I have not offended thee overmuch I'd be delighted if you could give me a ride in your lovely old bag of bolts! It might work the Dorian Gray magic on these staid old bones.

Have fun. Gosh. I've just remembered my first cross country! I think that beat any experience before or since, even overtopping the sight of my instructors back retreating for a cup of tea leaving me at last to get on with it.

the "E"

[Shaggy Sheep Driver]

E

If you, me, and the chippy are in the same place at the same time, and W&B permit, you are welcome to ride the back seat. She's Liverpool based, so unless we meet at a fly-in or suchlike it would have to be 'oop north'.

Didn't know Jakimiuk was on the Concorde design team. That's another aeroplane he got right, then. :ok:

Merlin to Olympus? The latter was a Bristol engine, wan't it? It only later became a RR powerplant.

But regarding design leaps, we don't seem to do them these days. Wrights flew early last century. By 1949 we had a 4-engined pressurised jet transport flying (dH Comet). By the '60s we had SSTs (Concorde & TU144) and we'd been to the Moon and back. What has aviation acheived since the '60s? We have, err, 4 engine subsonic pressurised jet transports! OK, they are bigger, quieter, safer, and far more efficient. But still, basically, grown up versions of that dH Comet!

So the lovely Chippy is a child of the heyday of aviation. :)

Cheers

SSD

[Genghis the Engineer]

As of-course is what might be regarded as the American equivalent - the wonderful supercub.

G

[john_tullamarine]

.. ah, SuperCub .. now we're talking real aeroplanes .... a few hundred hours misbehaving myself in those a long time ago ..

[Shaggy Sheep Driver]

That's interesting. The Supercub is an aeroplane that distincly underwhelmed me. It is a fun machine, but the handling compared to a Chippy is awful. Terrible ailerons (if you want it to roll, you send it a telegram 3 weeks in advance!). And use of rudder to keep the beast in balance has to be most un-natural.

Actually, I preferred the little L4 (J3). Probably even worse handling than the Supercub, but such fun to fly on a summers evening, from the back seat, with the whole side of the aeroplane open.

With a slight crossind from the left, the view through those big open doors, as the ground rushes towards you at, perhaps, 500 feet AGL, is just superb. As long as you are not in a hurry. :)

Vince