Warbird C of G

[John Laverick]

Hi guys, I'm sure this has been done to death somwhere on a thread. However I'll ask here anyway. C of G on a warbird, wheels up or down? I always do it with wheels up. But am I right?

[Don Fry]

You want it right to land mate. A stall a 100 meters is less of a problem than at 2 meters.

[PatMc]

A lot of "warbirds" had fixed U/C or no wheels at all.

Any particular aircraft in mind ?

[Chris Walby]

IMHO, logic would say yes unless you fly around with the wheels down all the time! + I select wheel down at the same time as flaps 1/2 way as the drag of the U/C counteracts the additional of lift of the flaps (if you can't slow the u/c).

Lastly the difference between U/C up to down is a move of C of G forwards (direction of safety) although I am sure there is a model where that is not that way!...I think my DC3 does this very slightly as its multi hinged & tucks up & forwards.

[Andy Blackburn]

Depends what happens to the c.g. when the wheels go up; usually the wheels move backwards when they retract so in that case the c.g. will move back as well.

Personally I always balance warbirds in the middle of the c.g. range with the wheels up and no fuel in the tank, upside-down using a couple of matchsticks taped to the top wing surface to get the fingers in the right place, very slightly nose-heavy or so that the stabiliser is level, whichever is most appropriate.

That way, the c.g. should never be rearwards of that point.

[Denis Watkins]

Always wheels up, the reason was already given as 95% of the flight is wheels up.

IF, forward C of G is introduced at landing speed, even then it is only for a few seconds

And the wheels do hang below the C of G, adding stability in my view

Edited By Denis Watkins on 01/04/2017 16:25:05

[Don Fry]

Oh well, two possible answers and two camps of opinion. As I decline to duel on the matter, vive la difference. But in fairness, on a machine big enough to have retracts, short of a P40 twist turn system, I would not imagine the position shifts so much, that you would notice in flight.

[John Laverick]

Wheels up then!!! Only asked cos I'm building the Tony Niujhus zero and it needs do much lead up front, I thought perhaps someone could offer me a compromise? Loads of lead it is then!!! To be fair, I've just weighed it, and with all that lead it's coming in spot on suggested weight.

[Denis Watkins]

When I was a student John, I made a servo driven counterbalance for a glider which worked brilliantly

The servo was a motor, whose long shaft was threaded, and drove a weight fore and aft over 6 inches

This is taking things too far, but it can be done

Warbirds do require inordinate amounts of weight

[Scott Edwards 2]

Have you actually measured the difference in CG between wheels up/down ? I suspect it's naff all, also any model where such a marginal change in CG makes it unstable has much bigger problems.

Compare the situation to the difference in CG with a full/empty tank. Thats huge in comparison.

[Denis Watkins]

Thank goodness that Scott came along, sound observations

[Josip Vrandecic -Mes]

G-evening John , since I'm flying war birds , I set-up the landing gear down choice to take-off I have no . This is logical because the takeoff / landing is always a delicate operation. For other possible problems, at high altitude, you still have time to correct the situation.
Note: some models (Spitfire) there is a difference in the position (geometry) of down-up, which affects the default CoG.

Cheers!

Edited By Josip Vrandecic -Mes on 01/04/2017 19:29:09

[Jon H]

Wheels up with an empty tank. As most warbirds have wheels that retract aft slightly when you have some fuel in the tank and gear down you are leaning towards nose heavy which is the safer ay to go.

[ChrisB]

As others have said, it depends on the way the wheels fold away. Corsairs, Warhawks etc I'd check CG with the wheels folded away. Spitfires the same. The CG will move slightly forward when wheels down, so check with wheels folded away.

But with P47/P51 Hurricanes etc...i'd do wheels down because they fold away slightly forward so when in flight the CG will be slightly forward when compared to wheels down.

[Martin Harris - Moderator]

Haven't we all missed the point here? The "correct" C of G is specified on the plan by the designer as a result of approximate calculation and experimentation and only he knows whether it's with wheels up or down (if it actually makes any significant difference!)

Full size C of Gs are measured by calculating moments from weights measured at the wheels so by definition, retractable undercarriages must be down - but who knows in the case of a model balanced on a pair of chubby fingertips...

The C of G is not a magical correct point - there is a range of acceptable position giving between sufficient elevator response and not too lively. The plan position (unless a range is specified) will normally be a good compromise location and personal preference can influence where yours ends up.

Edited By Martin Harris on 01/04/2017 23:57:11

[Biggles' Elder Brother - Moderator]

On most prototypes it will make very little difference. Frankly if it does make a negative impact (compromising stability) then you probably have the CoG too marginal anyway!

If you really must have a "rule" then the sensible one would be "balance the aircraft to put the CoG in the design position in the worst case scenario - ie which ever configuration puts the CoG furthest back" That way the other configuation will move the CoG in the "safe" direction - ie forwards. For the vast majority of types that would mean balancing with the U/C retracted because in most cases lowering the U/C brings the CoG forward.

BEB

[Martin McIntosh]

Chaps, the rearward movement on, say, a Spitfire will make a negligible difference to the cg compared with a full/empty tank.

However, if you are talking of a Lancaster, Mosquito or a twist and turn type then the weight of the wheels, oleos etc becomes very significant indeed. Always therefore measure in the retracted mode. Nose heavy on take off/landing may give a nosing over tendency but this must be preferable to a low speed stall.

If i/c I always check it with an empty tank.

[Martin Harris - Moderator]

It still depends on the designer though - unless you've recalculated the C of G yourself you have no idea whether his recommendation is wheels up or down unless it states it on the plan. I would hope that if there was a significant shift on retraction due to undercarriage geometry, the plan or notes would make it clear - in most cases there's only a tiny change - in the case of a full size Mk XIV Spitfire, 0.4" for example. Although, as someone said earlier, 95% of the flight may well be with the gear up, raising the undercarriage is optional, lowering it is compulsory and optimal handling is more important at low speed near the ground.

I agree with BEB - if in doubt go for the safe option - that way you should be sure that you'll get another flight to fine tune the flight characteristics.

[Don Fry]

"and optimal handling is more important at low speed near the ground."

Its the last six inches wot hurts.