The Bungee Elevator Trim System on an EZ
April 1989
This is an area that has generated a lot of questions and this will be an attempt to help answer many of those questions and, hopefully, give everyone a better insight into the EZ bungee pitch trim. First of all, all that follows here assumes you have built your airplane reasonably accurately - that canard incidence is correct and that wing incidence and relative wing incidence is correct. These items can greatly influence elevator's position and will effect the bungee trim system's ability to trim. The elevator shape is critical to the success of this bungee spring-operated pitch trim system. If the elevator is the "perfect" shape, it will float in a faired position relative to the canard at approximately 120 to 130 KIAS, without the springs. This means that at this speed, the aircraft will fly hands off and maintain level flight, even if the springs are disconnected and removed. This is about optimum and not everyone will have this situation.
If you do, it will then be possible to pick a pair of springs that will provide you with enough spring power to trim the plane hands off down to the approach speed (approx. 65 KIAS), as well as to trim hands off up to the maximum level flight speed. This is normal and perfectly acceptable. Now, if you go faster (by descending, for example, you may run out of forward trim and may have to provide this force by maintaining forward pressure on the stick. Again, for an EZ, this is normal and nothing to be worried about. At the same time, you will probably have to "help" the trim system by maintaining back pressure on the stick as you approach a stall or reach full aft stick. This, also, is normal for an EZ and many other planes.
The problem is when your elevator shape causes your elevator to float, no springs, at, say, 80 KIAS or at, say, 160 KIAS. Obviously, if either of these cases applies to your aircraft, your elevator shape is not correct and you will probably not be able to come up with a pair of springs that can provide enough range to cope with as low as 65 KIAS or as high as, say 170 KIAS (max. level speed). This is because the elevator is trying to fly to a different position than the one you need it to be in for the speed you are indicating. If you put a strong enough spring into the system, you may be able to overcome the elevator's lift and force it to a position it does not want to be, however, this is a losing proposition for two reasons. You almost certainly will not be able to trim hands off at the other end of the speed range, and more importantly, your speed stability will be compromised. All EZ's (Vari and Long) have excellent speed stability (as do all Defiants).
That is to say, if you set the power for a given speed and trim for level flight, the airplane will maintain this speed even if you displace the airplane by pushing or pulling the stick. When you release the stick, the plane will quickly return to level flight and be on speed as before provided you did not change power or trim. If you install overly powerful bungee springs in the trim system, to overpower an incorrectly shaped elevator, your airplane will not return to the trim speed. In fact, it will be difficult, maybe impossible, to trim it to fly level at any speed. We have tested this by simply removing the trim springs and flying the airplane. We attempt to fly level at various speeds, increasing speed perhaps 5 Kts at a time, until we find the trim speed at which the EZ flys level, hands off without diving or climbing. This speed should be close to 130 KIAS.
120 KIAS is OK, 135 is OK but much more or much less will require a fixed trim tab on each elevator or a new elevator with the correct shape. A small aluminum tab pop riveted to the bottom trailing edge of each elevator and bent up per sketch (See page 12) can be adjusted to cause the elevator to float exactly at 130 KIAS with no springs. This will allow you to use the weakest possible pair of springs that can provide enough force to hold the plane hands off from approximately 65 KIAS to approximately 170 KIAS. We are not necessarily recommending that everyone go out and fly with no trim springs! On the contrary, while it is not difficult to fly without any springs in the pitch trim system, it is extremely aggravating and tiring because you have to hold the trim force required all the time.
You can never relax or let go of the stick. So keep the flight short (or fly at the elevator's natural trim speed, once you have determined it). Do not attempt to conduct a test flight such as this unless you have plenty of experience in the airplane. We have done this many times and it is not that big a deal. It is just not a good idea for a low "time in type" pilot. With the correct shaped elevator, your bungee trim system should provide you with the capability to trim hands off from around 65 KIAS to around 170 KIAS, no more and probably no less. If you have to push to fly level at 150 or 160 KIAS, your elevator shape is wrong and its lift is stronger than your springs. The only way to fix it is to install the fixed trim tabs (one each side) or to build new, correctly shaped elevator.