I have an Ace Seamaster & in stock form it has a small amount of dihedral. The aircraft has a pylon-mounted engine & the airframe is a high/shoulder wing type. I am toying with the idea of removing the dihedral to enhance inverted aerobatic capability, but I am not quite sure whether this will be beneficial.

With the thrust line well above the center of lift, I am assuming that the effect is somewhat like a conventional low-wing design (ie. needs a small amount of dihedral), However, the center of drag is below the center of lift & the center of mass is close to the center of lift, but slightly below it.

Has anyone actually modified a Seamaster in this manner, or a similar type, & what can I expect from such a change?

I have a Seamaster 40 that I built many years ago. Fantastic on water, snow, wet grass, etc.
I never did bother to install the wheels.

I did not put in dihedral - I don't on most planes. Any loss of float clearance on water or land seems inconsequential.

Build it per the plans, but don't build dihedral in, you'll have a great aircraft. The thing will do every trick in the book, terrific Lomcevak's and all that kind of stuff. Mine uses a .46 engine. I did make the engine pilon longer (I recall there being an option in the book, I made mine the longer version, thinking that I might need a fairly large prop and want the extra clearance). I use a 11*7 prop. If I had it to do over again, I would have the pilon shorter, as I think it would make take-offs on water a bit better.

I use a servo in each wing side, mounted from the top of the wing and a homemade sort of fairing/channel for the pushrod to exit. I found that the coupling in flaperon to elevator made all the difference in takeoffs. No more need to try and yank the best off the water.

There is one more change I would make if I did another Seamaster. I would not use the flat bottom. I think it should have a slight V. Or, as I saw on a larger Seamaster, I might yet try putting a small triangle stock piece across the rear of the step - the fellow said that made it unstick easily from the water.

Good luck!

Thanks Deerslayer. Does yours have the T-tail? I have heard that the early versions had the horizantal stab lower down on the fin. Mine tends to snap on sharp elevator input & I wonder if the stab/elev are being blanked by the wing.

BTW I have a TT .46 pro dragging it around & it fairly leaps off the water.

Mine has the T-tail, i.e., the elevator control runs up the front and over the top of the fin, and is attached to a control horn on the top of the elevator. I haven't really seen many Seamasters recently, so I probably missed whatever evolution may have taken place, but I just did a Google and saw something that must be what you are referring to.

The snap behaviour you describe doesn't sound to me like tail blanking - but I am no expert. Blanking of the tail, one would suppose, should lead to loss of elevator control, initiating either a deep stall or porpoising. In a deep stall, either the CofG is sufficiently forward that the plane eventually drops its nose and unstalls the tail and recovers, or you manage to power out of it - or if the CofG is much too far back it would probably flat spin.

Any planes I have had which are set up to be highly aerobatic and/or control sensitive and/or have an aft CofG are practically guaranteed to snap upon sharp elevator input. That's what I would expect, and desire.

From your description, why don't you simply reduce your elevator travel, or consider putting perhaps -30% or such exponential. The latter will give you less twitchy response to a large range of stick movement, but when you REALLY want it, you will have that full deflection, AND NOW!

Experiment a bit. If you don't have a programmable radio, you can still get some of the exponential effect by using a servo arm that is not the 90 degree setup. Think about it, make a little drawing, and/or try it on the bench with your model. You will be trying to make the one direction (up elevator) less sensitive around neutral, but the other direction will become more sensitive, so think it through!

Another thought is to adjust your ailerons (flaperons, if you have them set up that way) to either droop slightly or to be raised just a bit. That might make a difference.

Thanks again for your suggestions Deerslayer. I'm thoroughly familiar with computer radios & flight dynamics, but I've not had a T-tail model, other than the Seamaster & I have previously reduced its tendency to snap by both shifting the C-of-G forward & reducing elevator throw as in your suggestions. However, this dulls the agility of the model. I've flown T-tailed jets & have experienced the severe kick-up & resulting spin from tail blanking (a decidedly butt-puckering experience). I was wondering if T-tailed models, & the Seamaster in particular, also suffered from this behaviour, despite a huge excess of power & very high control authority. I don't want to make the Seamaster snap-proof, but I certainly prefer to initiate snaps on command, & I also want the ability to pull high G in an emergency situation, if necessary, without an uncommanded snap adding complexity to the emergency.