The Mad StabEdit
The Mad Stab is a full flying horizontal stab that rotates up to 90° or more in either or both directions. The Mad Stab allows the glider to do several new maneuvers, most notably the flip.
A Full Flying StabEdit
Most lightweight Le Fish gliders use a simple flat plate stab, made from either 5/6 mm sheet Depron or EPP. Depron is stiffer. EPP is more flexible and lighter. The spar is a 3/16" carbon tube located at the 25% of MAC. This spar also serves as the pivot point (axel), often rotating inside a simple tube bushing (which has been glued into the fuse.) The Stab is covered in the lightest laminate material.
You have two basic choices, either a push/pull or pull/pull linkage. Push/pull uses a solid control rod and single sided control horns on the servo and stab. Pull/pull uses some sort of cable and double sided control horns on the servo and stab. To get the maximum possible elevator throws you will need to use a "big" servo horn and corresponding smaller stab horn. With either of these linkages you can get huge elevator throws... enough to do flips. However you will most likely not be able to get a full 90° deflection in both directions.
Controlling the Mad StabEdit
The huge elevator throws that make flips and other Mad maneuvers possible, can make normal aerobatic flying very difficult... where much smaller elevator throws are needed. The obvious solution is to take advantage of most modern digital transmitters and use a lot of EXPO (Exponential). The idea is to "soften" the center of your elevator stick so moderate stick movements give you normal amounts of elevator; but when you push/pull full elevator you get the full Mad throws. You may find that you have to use a negative or a positive number... depending on your transmitter. (See Pulleys for another way to add EXPO to your linkages.)
Even with maximum EXPO you may find that flying at Mad rates all the time can be overwhelming. Most pilots opt use the "dual rates" option to create a low or normal rate for general aerobatic flying.
We have noticed that one of the drawbacks to the full flying Mad stab is that it can be prone to flutter... especially if not built correctly and when you fly too fast. There are a number of things you can do to eliminate or at least minimize flutter in the Mad stab.
- Don't fly too fast! We are flying light ships, often at close proximity to each other. High speeds are not needed to do amazing aerobatics and can be a detriment to control and "safe" crashing.
- Use a high quality servo. This may be the place to use a nice digital servo.
- Make linkages as slop free as possible.
- Put the stab pivot point at or near 25% of MAC.
- Balance the stab at the pivot point. This usually means adding weight forward of the pivot point and/or removing it behind (by "Swissing" the aft part of the stab).