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Alignment Don't trust alignment to the manufacturer. Set up a jig on a long table. The jig holds the fuselage, with the wings level, on a centerline that is squared up to a perpendicular surface. The jig can be foam blocks or anything else handy that you can use to stabilize the fuselage along a centerline drawn on the table. The perpendicular surface can be a wall or a board clamped onto the table. Use a carpenter’s square to make sure that the board and the line are perpendicular in the horizontal plane. Set the fuse on the centerline and slowly push the v-tail back against the vertical surface. See where it touches the vertical surface. It should touch both sides of the v-tail at the same time and at the same elevation (height) above the table (assuming that the wings are jigged level with the table). If your v-tail fails the alignment test you need to correct it before you even think about trying to go to the adjustment stage. Adjust Ruddervators Before flight testing, adjust the amount of travel of the ruddervators to about 15 degrees of travel up and down on each ruddervator making sure the throws of both control surfaces move exactly together. The plane should now be ready for flight testing. The first adjustment is to get the elevator sensitivity in the ballpark. I find that I usually need to reduce the elevator sensitivity. I use the elevator dual rate to reduce it since on my radio (JR388) it only affects the ruddervator throws for elevator inputs. The rudder throws are not affected and maintains rudder effectiveness. Rudder effectiveness can be increased (up to a point) by using a dual rate setting to add throw to the rudder function.
THE FOURTH RULE OF V-TAILS
“Rudder effectiveness is adversely affected by incorrect differential settings.” So the next thing to evaluate is the differential in the v-tail. Since I set the v-tail up for all equal throws I know that initially I have no differential. You can increase the down throw of the ruddervators either by changing the endpoint (travel adjustment) for each surface or you can use your transmitter to mix elevator to rudder such that all rudder inputs have a small component of down elevator mixed in. I have found that 2% to 8% is the range that I end up using. It can be quite subtle particularly if you fly a very aft CG. "But how do you know the right amount?" you ask. Ahhh, the really tricky part!
The differential requirement is determined by flight testing. Adjustments are made so that there is no interaction (coupling) between rudder inputs and elevator. To find out if your current setup has coupling you fly the plane straight and level and then hold the rudder stick to one side to cause the plane to yaw. If you have an aileron ship you use the ailerons to keep the wings level. If you are testing a bent wing its a little trickier to read the response but usually a few iterations will tell you what you need to know. Basically the plane should neither drop its nose and pick up speed nor raise its nose and slow down when you hold the rudder. If your differential is way off the plane can pitch pretty dramatically. As you get closer to the right amount the response will be harder to read. But when you get it right-on the rudder response will come alive and carving into turns will become a complete joy!
Once your differential is good you can use dual rate for rudder to increase or decrease your rudder response to the left stick. The mix that puts rudder on the right stick can be adjusted to your preference.
The last thing to adjust is the incidence of the v-tail. Once you have flown the plane, set your preferred CG and adjusted your elevator trim to your preferred position for buoyant level flight examine the elevators. If they are uniformly up or down from neutral with the fixed portion of the ruddervators you can shim the whole assembly to get the elevators neutral. Trim the front up to get rid of down elevator or trim the back up to get rid of up elevator. You must fly and trim again after every shim change. A few tries should get you on the money. You may need to revisit your differential settings if you have to shim very much.
This description of the v-tail is does not address many theoretical aspects of the operation of the v-tail nor does it get into the design of v-tails. These discussions are left to others and there have been a number of good guides published on the web and elsewhere. I just wanted to describe how the RC pilot can go about extracting the maximum potential from a v-tailed airplane. Hope this makes your v-tail flying as enjoyable as it has made mine!
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