Catalina / Flying Boat Thrust Drag Couples

[Sqwark2000]

Hi all,

Question on behalf of a friend. It reads :-

I was asked about the forces for S&L on a flying boat. Easy enough; high thrustline + low drag = nose pitch down. To counter it you have an aft CofG and lift forward = nose pitch up. Same for a pure flying boat (Sunderland)
or for a tail-dragging amphibian (Widgeon), right?

Then came the question 'So how does that work on a Catalina?' The Catalina also has a high thrust line + low line of drag = nose pitch down. Went and had a look - tricylcle
gear means the CofG is forward of the main wheels , very close to where the line of lift must be. It defeats any reasoning the previous answer gave with regards to CofG position. The large wing is directly above the main wheels, I suspect CofG might be high and that the engines may be tilted slightly upwards.

So how does it work? I've tried to work it through, but I've run out of paper trying to solve it. The Kermode and the net haven't helped; they talk about the conventional method we already know. So, if you've got any ideas on this, any help you can give will be much appreciated.

(end question)

So if anyone out there can enlighten myself and my good friend, we'd be mighty appreciative.

Cheers

Sqwark2k

[john_tullamarine]

I know nothing about the specific types.

However, consider that the wing pressure distribution does, of itself, generate significant pitching moments (which is what you are talking about here). Also the total fuselage and, in particular, the tailplane make their various contributions to the balance sums.

The oft sighted simplistic double couple picture contained in many texts is a bit on the conceptual overview side ..... it may be fine to put a lift line forward of the cg if there is a clarifying statement to the effect that this line signifies a net balancing force .. it certainly doesn't signify just the force derived from the wing.

A similar tale can be made for the drag line. That which is represented is a net position - being the aggregated resultant of all the litte bits and pieces of drag contributions.

The cg is not going to be very far aft in any case for reasons of the normal longitudinal stability considerations.

.. a bit like the VERY confusing (to students) stability picture of a ball in a cup .... again without some appropriate explanatory text ..

... a similar story can be made for typical systems diagrams. The actual gadgetry often is nothing like the conceptual layout used to give an explanatory overview ...



[This message has been edited by john_tullamarine (edited 02 June 2001).]

[Kermit 180]

Is it possible that the Catalina may require ballast, and can move water ballast around after takeoff to maintain level flight by using pumps?

[paulc]

Kermit - i am involved in restoring a catalina and can tell you that each section of hull is sealed so it is not possible to pump water fore and aft to alter CofG.

Sqwark2000 - is your friend going to be flying one of these types soon ? or was it just background info. Our gropu owns an airworthy catalina which will be (i hope) flying soon

[Kermit 180]

Hi there again. Had a look at the NZ Warbirds Catalina up close at Ardmore NZ. You're right, it is sealed and there's no pumps. Must be the lift distribution, although it appears the Cat is nose heavy to fly and requires rearward trim for S & L Flight.