Hi Roger.

I'm not following you around. I'm not......Really.... I'm not!

OK I've got one of these, but freely admit to not being a four-stroke afficionado. Mine's fitted with a .61, as the suggested .46 flies them (having seen one initially powered by same) with all the aerobatic 'performance' of a Tiger Moth. The .61 is about right IMO, allowing decent RC like performance or scale like performance. I'm not looking for pattern like performance. If I was, it needs more because of all that bloat.

A nice enough design in every other respect, the Seagull Decathlon's singular failing is that weighs in like Pavrotti. I know..I know it has a generous wing area and decent section, but 3.63kg (8lb) and I presume that's your dry aka zero fuel weight spec (?), is a LOT of weight on a 70 4 stroke to expect any vertical other than courtesty of kinetic, let alone ½ decent vertical we expect in the RC sky.

Once run-in, propped appropriately and tuned to peak, your 70 should fly it scale like. But that's about all. The real McCoys either Standard or Super Decathlon aren't exactly performance powerhouses. If you can squeeze a single vertical roll out of them and exit into a hammerhead or stall turn before inadvertantly tailsliding (non-approved maneouvre 'cos you can't hold the large empennage surfaces rigid with cables) you're doing well. And they have constant speed props which we RCers don't.

Just about anything will fly it on that wing, but the moment you point the nose skyward altering the balance of forces to rely predominently or wholly upon thrust vs weight instead of lift vs weight, well just do the "thrust required" arithmetic.

To have any hope at all with that 70 in this model, tuned to deliver its peak power potential is the byword. A gurgling rich run-in setting isn't it. And IME&KO you'd want a climb prop. ie: Pitch it down, 5" is good, and go for a diameter with that pitch which will allow it to achieve circa its peak power curve RPM. If it won't achieve what you want it to do on a 5" pitch of appropriate diameter, then you need a more powerful engine. With the unavoidable characteristics and limitations of fixed pitch, the flight envelope simply narrows too much below 5" unless 3D is where you want to 'play' or you want to putter at low power settings around like a Cub.

Someone like Minnflyer who has fitted his with a 70 Surpass will offer you a more favourable pro 4 stroke bias perspective towards propping your Magnum 70. His flies nicely, and rather scale like.

Me - I just reckon you need a pumped FS-90 Surpass II! [8D]

Otherwise, your key to vertical is pre manoeuvre dive entry for speed is your friend given that KE=½mv².

sigrun
"Castor oil is cheaper than synthetic so if someone is bragging about castor, then you know what they are selling."
- DJ