- Mar 5, 2017
- Reaction score
YES! THANK YOU @Mugs914!!The clue as to why these designs use a lifting tail plane (and are designed to) is in the CG location that you noted on the plans.
Imagine it this way:
In a conventional situation (downforce on the tail plane), the wing center of lift is the fulcrum of a beam (the fusalage), like the pivot point on a see saw. The CG is slightly to one side of the fulcrum, causing that side of the beam to drop. Now we add a downward force (tailplane) to the high end of the beam that brings the beam back to level. This of course is massively simplified, but you get the idea.
So, in this case our objects are arranged, front to rear, like this:
(Where v and ^ indicate force/load direction, CG = Center of gravity, CL = Center of lift (fulcrum), BF = Balancing force.)
In a "lifting tail" arrangement the wing center of lift is still the fulcrum for our beam, but the GC is placed on the opposite side of the fulcrum, again causing that end of the beam to drop. This time we add lift to the low end of the beam to bring it back to level.
In this case our objects are arranged like this:
Both achieve equilibrium by balancing masses and dynamic forces, but in different ways.
If you look at the first (bad) drawing, you'll notice that the wing not only has to lift the entire mass of the glider, but overcome the downforce of the tail as well.
In the second bad drawing the mass of the glider is divided between two lifting surfaces, and there is no extra downforce working counter to the wing's lift. Imagine it like a reverse canard arrangement.
Both are trimmed by varying relative incidence between the wing and tail. In the case of the conventional tail, raising the trailing edge increases downforce. In a lifting tail raising the trailing edge decreases lift. Net effect is the same...
The lifting tail set-up has the potential to be more efficient, but can be more difficult to trim. Like O1d Dude said, the free flight guys have had it figured out for quite a while, but it still tough to get right over two very different flight regimes.
Sorry to ramble so. I hope this made some kind sense...
Finally a definitive answer and it makes total sense! You can ramble anytime you want, this was eye-opening for me, and clear and logical. Thanks for putting it in layman's terms that I could digest.