Amphib Expertise required. PoF Question.

[Sqwark2000]

Gidday from Godzone,

Myself and a good friend are trying to sort a PoF question regarding forces on a PBY-5 Catalina @ S&L.

This post is also under Flying instructors but thought that someone in Canada must know this one.

Post reads -

Hi all,

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

[Luftwaffle]

I've just looked at a few pictures of the Catalina, so if no one else is willing to have a go, I will.

I agree that the line of thrust is above the line of drag, producing a nose-down moment.

I agree that the centre of gravity of a tricycle gear must be ahead of the main wheels.

I don't agree that the centre of pressure is behind or nearly co-located with the CofG. To begin with, the main wheels are quite far back, and I believe they are for ground handling only, so the CofG could be only barely forward of them. The mains, from the not-very-good picture I have, appear to be about 2/3 of the way from the leading to trailing edge of the wing. Normally the CofP is about 1/3 of the chord back from the leading edge, but considering the shape of the Catalina, the hull would contribute significantly to the lift, moving the centre of pressure forward from there.

Any portion of the Thrust/Drag moment that the difference in CofG and CofP does not balance must be provided by the tail down force of the horizontal stabilizer.

Does that sound plausible to you?

[pigboat]

Sqwark2000, I've forwarded your post to an acquaintance who used to be a test pilot with Canadair for many years on the CL215. Hope he's able to answer your question.
I flew the Canso (PBY) for a short time in 1966. If it's any help, I remember the aircraft being nose heavy, but not overly so. On water takeoffs, because of the high thrust line, there would be an initial pitch down. You held the yoke full back until the nose rose as high as it would go, then eased forward to place the airplane in the planing attitude. You used elevator trim at any time in this maneuver to help with the stick forces.
If the CofG was outside the forward limits, water takeoffs were supposedly impossible, since the aircraft would not get on the step.
I've not seen this, but only heard of it. The aircraft has ample elevator authority, however. I've seen a picture of a Canso that had landed on a frozen lake. The snow was too deep for the nose wheel, so the pilot landed three point, on the mains and the rear step!

[Sqwark2000]

Thanks for your help. Your comments are appreciated.

Luftwaffle, your post makes sense and my friend and I will go over it when we chat next.

Pigboat - thanks for your help, fwd CoG sounds kinda dodgy in the PBY!