I would say the answer to this is very simple. The plane is a 60 sized aircraft and generally, you see anything from a 50 to a 75 in them. A 91 is a lot of engine for it and its weight is more than that of a 60. Therefore its nose heavy. I have a Thunder Tiger 61 in mine and it balanced perfect and fly amazingly will on its first flight last week. With the TT 61 it'll do about anything, loads of power for this size of aircraft. I haven't tried to hover yet, but I have a 12x6 on it and wouldn't expect it would be able to. I have a 14x4 coming next week. The plane is light, don't try and overpower an aircraft that doesn't need it.