DHC-4 Caribou rudder control

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Member for

15 years 7 months

Posts: 201

Greeting gents,
I am currently building a radio controlled model of the Caribou complete with main gear shortening system and slotted flaps (it's taking a while!) and have been peering at the various photos available on the web for any clues as to whether that huge high-positioned tail is conventionall powered or whether it might be operated by servo tab, without success so far.
In the hope that someone who is regularly up close and personal with the real beast may be looking in and able to answer my query I post the request here.

For those of you who may wonder why the question is raised at all I will outline the background to it.
In the thirties there was a period when both Boulton and Paul and Short Brothers produced aircraft with servo tabs. Many (if not all) will recall seeing aircraft with an additional tab attached to the trailing edge of the rudder. The Short Scylla was one of these.
The input from the pilot worked directly onto these tabs with the rudder itself free to move until airspeed was sufficient for the servo tab to bite, when the direction change of a tab would (because of its sideways force being exerted at a long moment arm from the rudder's mounting) make the rudder itself move in the opposite direction to control the swing of the aircraft.
This has both advantages and disadvantages which explains why the practice was dropped and today we see only what are normally I believe called balance tabs designed to take some, but not all of the load off the pilot so he can maintain control throughout the speed range without undue tiring.
The main advantage of using a full servo tab system to my mind, having found turning across wind less than positive in some models, is that the rudder would be free to deflect subject to side wind at low speed (on the ground) so that the tail would present less effective side area. This is a matter of more concern for the Caribou with its huge fin and rudder than for other types, but having met the crosswind difficulty even with my model Stirling, which has a far smaller vertical stabiliser and noticing that the Caribou has a larger array of tabs in relation to its rudder area than normal I just wonder if they were in fact designed to overbalance the rudder so as to become servo tabs.

So this is my plea for help... does the Caribou have control direct to the rudder or just to the tabs? I would be glad to hear from anyone who has the definitive answer - or anyone who would like to discuss the general theory come to that.

Robin

Original post

Member for

17 years 7 months

Posts: 1,861

"Handling the DHC-4 Caribou" http://www.flightglobal.com/pdfarchive/view/1959/1959%20-%203033.html

"The control aerodynamics are worth studying, for few of them are readily apparent. Double-slotted, high-lift flaps are lowered to a maximum of 50 deg. They extend out to the wing fences, outboard of which is a fixed leading-edge camber. Double-slotted ailerons are also fitted in two sections each side. The forward section forming the double slot, in conjunction with the inboard flaps, moves purely as a flap. The rear sections form the ailerons, but the inner panels move only slightly when the flaps are in.
When these are lowered all aileron panels move fully to give good lateral control. Servo assistance is provided by geared tabs on the outer panels, a trim-tab being attached to starboard. A further tab on the inner starboard section is directly connected to the rudder to give a righting effect during sustained side-slipping—or, conversely, to produce a co-ordinated turn from rudder alone. The elevator is spring-tab-operated, with a further tab for trimming-out loads from speed and power changes. Trim changes from flap movement are largely taken out by automatic movement of the tailplane. The very large rudder has a spring tab and a combined geared tab and trimming surface. None of this, as I have suggested, is readily apparent; but it adds up to smooth and effective control on all axes right down into the stall, which is what C.A.R.4b requires."

Martin

Member for

15 years 7 months

Posts: 201

Thank you Martin for your response and the link from which you quoted.
I was beginning to suspect that the Caribou's systems were interlinked as I had noticed that the flap deflection was affected by the ailerons' movement, but did not suspect how much.
The direct link between rudder and aileron to produce co-ordinated turns from either is what I had long intended to do via one of only two mixes available on my transmitter, but this is affected by whatever I discovered in the relationship between flap and elevator in pitch change. I have a bit of thinking to do to achieve the automatic trimming out of pitch change due to lowering the flaps. This for other models has normally taken the other mix, but it would be better to do it aerodynamically than by a percentage mix.
I assume that the term 'spring tab operated' means that the elevators are not directly controllled by the pilot's stick input but that this goes to the tab which is sprung to take care of momentary variations in load due to airflow. This is a surprise to me as it was what I had hoped to find in relation to the rudder - which it seems, contrary to my expectation is directly powered but with a balance tab to take some (most?) of the load off the pilot.

As the elevator is a far more vital control than the rudder for a model flyer, once off the ground at least, this is quite a challenge. This proposition is likely to make RC pilots very nervous as we are not used on the whole to using any tabs at all on our control surfaces. I have been using balance tabs on all my main controls for a few years in order to allow for the use of the cheaper servos even on larger models, but the next step is to actually fly a model equipped with a servo tab in order to gain experience - and the confidence to use such a system for such as elevator.

My primary reason for enquiry was the high probability of side gusts while taxying overcoming the nosewheel's adhesion to the ground and weather-cocking the model because of the size of the fin and rudder.
I think that what I may have to do is accept that my elevators will be conventionally powered and the rudder indirectly (by the linking of both tabs into one servo tab) in order to find out how effective this is. Perhaps later, if successful, I might then be comfortable with indirectly powered elevators.
Any further input on this will be gratefully recieved!

Robin

Member for

17 years 5 months

Posts: 8,980

I think you will find the elevator on the BAe 146 is a servo tab system

Member for

19 years 7 months

Posts: 108

The Convair piston/turbo prop series (240, 340, 440, 580 etc) have both trim and servo tabs

Member for

19 years 1 month

Posts: 6,043

A spring tab is not the same as a servo tab !
What most people call a servo tab is actually more akin to a balance tab.
A true servo tab is one that directly operates the control surface from the control wheel (a la Brittania - and the [say] ailerons will 'droop' when there is no airflow over the control surface)!

Member for

15 years 7 months

Posts: 201

So long as we are all talking of servo tabs as being directly controlled by the pilot and the only means of controlling the surface to which it is attached (apart from trim tabs) then I am learning that the servo tab didn't die out with the disappearance of the added on surface but became incorporated in the main area of the surface itself in a good number of aircraft of different manufacture.
It will help a lot to know what some of them are because I can then go to three-view drawings (on the dubious assumption that they are accurate!) and work out what size these tabs are so as to gain confidence that what I build into my models has a good chance of working.

When you say 'spring tabs are not the same as servo tabs' basv, my feeling is that they COULD be, but with the spring connection to the surface they are operating offering damping effect. Is that generally believed so?

If so then it is still possible that the Caribou's rudder is completely tab controlled - which will be the outcome I am still hoping to find!

Robin

Member for

19 years 1 month

Posts: 6,043

Sorry to disappoint but a spring tab is a variant of a Balance Tab - a balance tab moves in the opposite direction to the control surface and therefore assists the pilot in moving the controls (ie lightens up the control force),so the main control surface is moved directly by the pilot and the tab is operated by the surface moving !

rgds baz

edit ...not many aircraft ever had Servo tabs - but they were funny to watch taxying downwind - as the control surfaces could be in any position :D

Member for

15 years 7 months

Posts: 201

Oh well, one can't have everything work out in life! Maybe God wants to keep us on our toes?!

I am already faced with changing the foam nosewheels for soft rubber (if I can find them) to stand any chance of keeping the nose on the ground and not being swung into wind by every gust of wind and I am expecting to have to add a fair lump of extra nose weight to balance the model for flight anyway.

Interesting that TonyT asserts that the BAa146 has servo tab operated elevators and scrooge that those Convairs also used the system (for unspecified surface) and Baz that the Brittania had servo tab on the ailerons and that he has seen a plane with servo tab equipped rudder taxying downwind. I assume this was on film/video. All of this is food for further study so thank you gents.
I have also just noticed that wieesso's post says that trim changes from operating the flaps of the Caribou were 'largely taken out by automatic movement of the tailplane'... not the elevator as I might have expected. This would, I assume, be the raising of the leading edge to give the tailplane assembly the inverted camber more appropriate for low-speed flight as well as the intended change in angle of attack.

All interesting stuff!
Robin

Member for

14 years 2 months

Posts: 585

Here are a couple of diagrams of the Caribou rudder system and spring tab mechanism:

[ATTACH=CONFIG]234038[/ATTACH]
[ATTACH=CONFIG]234039[/ATTACH]

Attachments

Member for

14 years 2 months

Posts: 585

The aileron system is a bit of a nightmare - it is interconnected to the flaps and rudder...

[ATTACH=CONFIG]234040[/ATTACH]

Attachments

Member for

14 years 2 months

Posts: 585

The horizontal stabilizer is interconnected to the flaps and moves through a range of four degrees between flaps up and flaps down...

[ATTACH=CONFIG]234041[/ATTACH]

The elevator system also has a spring tab mechanism...

[ATTACH=CONFIG]234042[/ATTACH]
[ATTACH=CONFIG]234043[/ATTACH]

Attachments

Member for

19 years 1 month

Posts: 6,043

Interesting that TonyT asserts that the BAa146 has servo tab operated elevators and scrooge that those Convairs also used the system (for unspecified surface) and Baz that the Brittania had servo tab on the ailerons and that he has seen a plane with servo tab equipped rudder taxying downwind. I assume this was on film/video. All of this is food for further study so thank you gents.

Robin

Trouble is Robin - there are 'Servo Tabs' and Servo Tabs...

Tony T's 146 'Servo Tabs' will be some sort of spring/geared/balance tab - you will not normally find true Servo Tabs on a Jet ! ; )

I am old enough to have seen Brits and Belslows etc taxying with drooping ailerons LOL - so not from a film !

rgds baz

Member for

15 years 7 months

Posts: 201

Thankyou AA. That is about as definitive as information gets!

My systems at model scale will be hugely simplified, but there is a wealth of information in there that will be most valuable - even the actual section of the tailplane which I can now lift off and incorporate. It may not be drawn exactly but it will be a lot closer than my guesses based on a tiny 3-view drawing!

Baz, I bow to your age and experience! When Belfasts and Britannias were operating I was in no state to know what I was looking at (beyond recognising the type).

I would have expected the ailerons to be mass balanced anyway as to my mind ailerons hanging down would offer air resistance and slow the rate of acceleration in the early stages of the take-off run. It can't have been thought a serious consideration.

Thanks to all responders - I consider myself sated!

Robin