Engine/Prop Angle for Amphibs

[Guest]

I am working on designing an amphibian aircraft with an overhead engine (ala Lake, Seastar or Coot), and I was wondering if there is a way to put an angle on the effective thrust vector for the engine/propellor. Most of these aircraft have about 5-10 degrees of upward cant to their engines to offset the fact that the engine is behind and above the COL.Any help would be appreciated.Matt

[Guest Fabio Miguez]

Hey Matt.I don't know the answer to your question, but your message created another question in my head. If the engine is behind and aboce the center of lift, wouldn't that mean that if it were perfectly perpendicular to the airplane's longitudinal axis, whenever you added power, the engine would cause a pitching down moment?If you cant the engine up, wouldn't that worsen this situation?I agree with you they are canted up, as I've seen some, but I was wondering why. Maybe it could be some other aerodynamic factor we are not considering?

[Guest]

In general you are correct. If I raise the engine (or rather the thrust vector) and move it behind the COL, I will have a net moment around the CG when I increase power. However, by canting the engine upwards (or downwards if it is a pusher), I effectively change the thrust vector to reduce the downward pitch moment (by changing the direction of the vector to include a vertical component).I am sure that there is a better answer, as I am not an aero engineer, but as another kind of engineer, that is good enough for me. As an aside, if you look at A/C with the engines mounted above the wings, they all have a slight upward tilt to their engines.Matt

[Guest Fabio Miguez]

Hey Matt.I tried to answer earlier, but unfortunatelly my computer crashed when I hit Preview :-(.Anyways, what I was saying is that as an Aeropscae Engineering student I am far from being knowledgeable, but I have ran into a couple of similar things.I think the answer to our question is in the Center of Gravity, not center of lift. Since the aircraft pivots in the three axis around the CoG, if the engine is above it, when power is added, a pitch down moment will occur.A vertical (upward) component will only counter this if the engine is ahead of the CoG. This, then, must be the case. But I will ask a couple of my professors to make sure. I'll keep you posted.

[Guest]

Fabio,I agree with you. I guess I wasn't all that clear, I was trying to say that the vertical component of the thrust acts as extra lift, and this often counters the rotational moment around the CG, but you are absolutely correct about the location of the engine with regards to the CG and not the CoL.This extra "lift", as one might expect, has varying amounts of effect based on the speed of the aircraft, so a poorly designed amphib can get out of trim very easily.It appears that you and I are on the same page with this one, a strong interest in aeronautics and aircraft, but no practical solution for the .air or .cfg files! :)Matt

[Guest Fabio Miguez]

Hey Matt.Yes, it is too bad Microsoft hasn't done this, but it is really hard to understand, since they have paid attention to much smaller details, and this is rather important!Nonetheless, it was an enlightening discussion, and I look forward to having others with you and our friends of this forum. The more everyone shares, the more the community grows, both physically and in wisdom.