Trimming a warbird

OK will have to be more specific…AFAIK all spit trainers have Merlin engines…ergo ballasting requirements will be a little different from Griffon engined marks…watch the video and you will see what James and I are referring to !

rgds baz

Hi Tony
I was really referring specifically to 2 seat spitfires re the elevator cruise angle,having babbling ballast in the back would put the CofG quite a long way aft therefore you would need a reasonable amount of (a/c) nose down trim to maintain level flight – a single seat spit would have only needed large amounts of fwd trim if it had a full rear fuselage fuel tank.

The aircraft is ballasted with a lot of lead in the rear, having no one in the rear seat would be the one that would effect trim more one, would have thought………… do you know how much lead the likes of a 19 carries for example?

Hi Tony
I was really referring specifically to 2 seat spitfires re the elevator cruise angle,having babbling ballast in the back would put the CofG quite a long way aft therefore you would need a reasonable amount of (a/c) nose down trim to maintain level flight – a single seat spit would have only needed large amounts of fwd trim if it had a full rear fuselage fuel tank.

Lots of rogue a/c stories around…:D

Edit …a little aileron droop was fairly normal on light a/c,during flight the ailerons would ‘float ‘ up a little due to aerodynamic loads and slight ‘give’ in the control cables !

The fact the elevators are not level could simply be due to build standards and simply need to be a little off to get it straight, go look at airliners etc, the VC10 ailerons can be defected upwards together to reduce wing bending…. various airbuses they droop to increase efficence and fuel burn……… even the lowly cessna 152 on later models have a 1/2 a degees droop built in on the ailerons at set up.

Bazy i know of a Cessna in the midlands had a similar problem in the cruise it would yaw badly, the Engineer I know was pulling his hair out trying to find a cause, took them ages, but it was one of the later ones with the taxi landing lights in the wing….. these are covered bay a leading edge shaped perspex cover……

when in the cruise vibration would make it move sideways in effect opening up a 4 inch hole in the leading edge, on landing as it was throttled back reduced vibration used to put it back to where it should be, it was only after several test flights and various adjustments that it stuck and they were able to find and fix the cause..

Similar there was a wellington during the wwar that did funny things, a frustrated engineering officer ordered the fabric removing from the horizontal tailplane in an effort to find the cause and they found a huge section of geodetic structure had never been fitted!

More recently I noticed the exact same thing in a recent bit of Spitfire footage – it was a two seater though, with a happy, babbling Scot in the back at the time, so I wondered if that was normal for the two-seat example with a excess babbling load aft of the c of g, or not, and if the single seat examples flew like that in cruise?

I would say that a single seater elevators would be much closer to neutral – I also have noticed the elevator position whilst watching babbling presenters in 2 seaters … me – I would be speachless with enjoyment :).

A little off topic but when I worked on Chipmunks many years ago one of our pilots told me that his a/c kept going out of trim – ie rolling out of a turn but at the same airspeed – the a/c would then be slightly out of trim…unusual !
After having a look round the a/c…couldnt see anything obvious but I eventually clamped one horn at neutral to check elevator symmetry – looked ok but when i waggled the unclamped horn (as you do with a/c) – it moved LOL
The fix…tighten the ring of bolts which clamped the 2 elevator torque shaft flanges together (they were all loose LOL :D)

Wedge the term trimming came from the original use of a bit of cord doped on the back of the surface to weigh it down to get it to balance, it was adjusted by “trimming” a bit off with a pair of scissors etc, hence the usage of th word trimming.

JDK would not suprise me, I have some aircraft here that even vary from side to side on the elevators!

Interesting question that relates to something I’d noticed and wondered about.

While riding in the back of a colleague’s Birddog, the elevator horn balances were notably pointed above the tailplane as we were in cruise, indicating the degree of either nose down control input or trim. I thought I’m not that fat, nor carrying that much camera gear…

More recently I noticed the exact same thing in a recent bit of Spitfire footage – it was a two seater though, with a happy, babbling Scot in the back at the time, so I wondered if that was normal for the two-seat example with a excess babbling load aft of the c of g, or not, and if the single seat examples flew like that in cruise?

Far be it from me to argue with Mr Brothers, but the aerodynamic effects of what I said are indisputable.

… , it would be b***dy uncomfortable to fly an aircraft on a patrol with it so out of trim to be visible, just on the off chance your attacker saw you first, then before he shot you down you would have to retrim properly to have some chance in fighting back.

The above sounds sensible to me

Moggy

you obviously switched off after your last post on here :diablo:

http://forum.keypublishing.com/showthread.php?t=20822

http://forum.keypublishing.com/showthread.php?t=94764

Peter Brothers always did it 😉

Thought the reasoning for that was if you slotted yourself on the tail of an aircraft it would not be flying straight ahead, but off on a tangent

Not really. The pilot would correct for S&L flight, keeping the ball in the centre, using rudder pressure.

Flying out of balance as you describe would impact on both speed and range, neither desirable.

It would also make your aircraft a slightly larger target for the opponent directly behind you on your track.

Moggy

I have a few questions regarding elevator trim in WWII warbirds.

1.) In general, if a pilot trims his elevator to a neutral setting and takes his hands of the stick, is the trim tab flush with the elevator causing zero deflection.

Thank you in advance.

The zero trim position of the tab is effected by many variable factors such as speed ,altitute, fuel load, crew weight, ammunition load and power setting which means the hands off position could be in any number of places during different stages of a flight.

Richard

In fact in combat some pilots deliberately flew with aircraft out of trim, particularly with rudder trim. That way if they were caught by surprise, the aircraft would yaw or pitch more erratically.

Thought the reasoning for that was if you slotted yourself on the tail of an aircraft it would not be flying straight ahead, but off on a tangent as the nose would be offset from the direction of travel, thereby making it harder to get behind it.

Trim settings vary with speed, power and other factors such as loading etc.

Most aircraft are rigged to fly with the trim at a fairly neutral setting at crusing speed and cruise power.

A more powerful warbird may have quite marked trim changes with power setting, but equally, that power easily negates the relatively small deflection of a trim tab and elevator.

In fact in combat some pilots deliberately flew with aircraft out of trim, particularly with rudder trim. That way if they were caught by surprise, the aircraft would yaw or pitch more erratically.

Some lower powered aircraft, which might be more affected by drag, use other trim systems. The Tiger Moth uses a spring loading on the base of the control column and the Piper Cub a screw jack which alters the tailplane incidence.

Hope this helps.

Steve

1.) In general, if a pilot trims his elevator to a neutral setting and takes his hands of the stick, is the trim tab flush with the elevator causing zero deflection.

In theory, neutral trim should be at 0 degrees. What really affects whether the pilot can perform straight and level flight with the elevator trim at neutral is the tailplane incidence. Generally you’d want to have a greater range of ‘up’ trim than ‘down’, therefore, neutral could correspond to +2 or 3 degrees

2.) From an aerodynamic point of view, how much drag would be caused by trimming to a position where the elevators were deflected.

No easy answer – it depends on the size and position of the trim tab. Generally the benefits of having trim will outweigh the small amount of drag induced.

3.) Is it correct to assume that an aircraft designer would design the tail configuration so that at a cruise altitude and power setting the elevator trim would be in the neutral position to minimise the effect drag whilst cruising and to save on fuel economy.

Not neccessarily. Neutral trim in the cockpit could correspond with an elevator trim of, say, 0.5-1 degree. Depending on tailplane configuration. There will always be a tolerance on the rigging of trim systems too………….

In short, find the maintenance manual for the particular type and find the chapter on rigging. Every aircraft is different