Originally Posted by parkke
Well, the IO-390 is 308lbs dry, the IOX-370 is 255lbs + accessories. I'm expecting very similar dry weights. What other complications were you thinking of? CG location may be corrected through proper ballasting. Iyy can be corrected in a similar manner.
I'm genuinely curious what you think.
I think that adding accessories to an IOX-370 wont add 53 pounds.
I was talking in general terms of using different engines.
If for an example we talk about using a bigger /heavier engine.
Say it requires a longer cowl than the original design. This adds side profile area to the airplane fwd of the C.G. Aerodynamically, this is the same as reducing the size of the vertical stabilizer/rudder. It can have an effect on yaw stability and spin recovery.
Lets say the bigger/heavier engine also requires adding ballast to correct the C.G. position. The logical place to add balance weight is as far aft as possible because then the smallest amount can be used. This corrects the C.G. problem but it increases the polar moment of inertia
In simple terms, adding any amount of weight, a large distance aft from the C.G. make the airplane more resistant to changes in yaw and pitch (changes that we want to be able to induce with the controls), and more difficult to stop pitch and yaw motions once they are in motion (something that we don't want). This can have an impact on pitch and yaw stability, and particularly, spin recovery.
So, I am not saying that using any engine other than what was recommended, means you will have serious problems related to what is described above...
saying that you might
(and not even know it, because how many people flight test to that high of a level of detail... or know how).
Bottom line -
It requires a lot more than a set of scales, to establish that an airplane will fly exactly the same as was intended by the designer, when a different engine is used.