Originally posted by Citabria
it would seem ah has a flaw in the cg of its airplanes, when one looses its tail its engine heavy nose goes up and its nonexistant tailpoints downward.
Hi Citabria,
I think I posted this explanation in another thread, but I'll post it again here to save folk searching for it.
The behaviour you have described appears to be perfectly correct to me, and is exactly what I would expect from the excellent flight model in AcesHigh. I think the confusion in this discussion has been caused by some misunderstanding of the system of forces known to exist during flight, and in the way they are used in trying to explain what happens when the tail has gone away.
Just bear with me while I build an image of what happens… There you are flying along, minding your own business, in level flight at some constant speed. At this point the stability diagram you find in most text books is doing us proud! All the forces on your aircraft are balanced. The prop’ thrust is balanced with drag, the lift with weight and so on. All the moments are balanced too, the pitching yawing and rolling moments are all in equilibrium. The nose down pitching moment caused by the weight and lift couple is balanced at the tail, and so on.
Now, since the question only concerns what direction the nose points when the tail goes away, let’s just think about those pitching moments. Most folk think like this… During flight the weight and lift couple was trying to rotate the nose downwards, and the tail was preventing this from happening. So, if we remove the tail, there will be nothing to prevent that rotation, and the nose will drop… Not so fast! That’s not what happens, you might not see this right away, so bear with me, I’m going to go slowly… But first, the flaw in that reasoning is that it overlooks the fact that the nose down pitching moment that existed during controlled flight, also goes away with the tail. You see, the tail wasn’t only responsible for the balancing moment, it was also indirectly responsible for the lift that was produced the nose down pitching moment in the first place. Once the tail has gone, the couple rotating the nose down, no longer exists… So what does happen?
Firstly, the wings are only producing lift when they are forced to do so by the control surfaces at the tail. Those surfaces (using a small force but long lever arm) rotate the wings against the airflow, forcing the wings to fly at an angle to the free air stream, thereby rotating the airstream, causing downwash, and thus lift. When the elevators go away with the tail, the wings will begin to move upwards, due to the lift already there, but they won’t go far because as they move the lift decays rapidly until the wings weather vane, and no longer produce any lift. That all happens in just a few degrees, so when the tail goes away, the nose might move down slightly, but only momentarily, because now that only leaves an engine, with the wings and forward/mid fuselage acting as little more than dead weight that simply wants to fall downwards, with a propeller attached to it that is still producing thrust.
Now, all you really need to consider at this point is how a heavy lump of metal with a propeller attached to it would fall. I think most people can see intuitively, that the heavy lump would fall first, dragging the propeller behind it. An admittedly weak analogy would be the stable condition that arises with a man hanging beneath a parachute. The aircraft falls, dragging the prop behind it, and falls more slowly because the prop is producing thrust and slowing it down.
That's exactly what happens in AcesHigh... Kudos HT!
Hope that helps.
Badboy
