THE FUNDAMENTAL TRIMMING RULES FOR V-TAILS
Lets not beat around the bush about this: V-tails are conceptually complex and difficult to trim. This is because a v-tail is inherently a force mixer of 2 non-coplanar control surfaces moving in unison to produce yaw or pitching moments about the aircraft neutral point.
So what do we need to know and do in order to deal with it? We need to understand, align, and adjust.
Understand
The first thing to understand is that the v-tail is not a fuselage torqueing mechanism. Whatever small torque coupling it might have with the fuselage is inconsequential for our purposes. Its individual surfaces produce forces normal to the plane of the surface that sum to a resultant force that yaws or pitches the fuselage. (If this makes sense to you, either you have an engineering degree or should be granted one.) Because of this summation of forces the v-tail is very sensitive to setup and requires careful and exacting adjustment.
Another thing that we need to understand is that a v-tail is a bit more efficient in producing down force (lifting the nose of the airplane up) than it is at producing up force. This makes it necessary to compensate by giving the ruddervators a little more travel in the down direction than in the up direction. This is referred to as differential. Although I have never encountered a v-tail that 'required' more up than down there may be one out there. (nah!). How much differential is correct for a given airplane depends on a variety of things including the included angle of the tail and where the CG is set. But before we worry about differential we have to get the v-tail properly aligned. This brings us to the first rule.
Align
THE FIRST RULE OF V-TAILS
“Never trust any v-tail to be properly aligned if you haven't done it yourself.” Prebuilt v-tails and molded v-tail mountings may give the appearance of being perfect when they are in fact less than perfect. "So what happens if a v-tail is misaligned?" you may ask. Well if it is tilted it will cause turning just like a tilted stab, if it is cocked it causes both crabbing during level flight and pitch instability when turning. These effects can be difficult to diagnose from the flight of the airplane if you aren't very sensitive to the sticks and realize that the tail can be causing problems that are often attributed to ailerons and wing tips.
The only way I have found to align a v-tail is to do it on the workbench. This was originally described to me by Brian Agnew but it took me a while to recognize its full value. 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. The vertical surface does not have to be perfectly perpendicular to the table surface but it should be close. 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. Trying to adjust an out of alignment v-tail is like a cat chasing its own tail - a waste of time and effort. In the end all you have is a tired cat and perhaps a sore tail.
Adjust
Adjusting the v-tail has some basic requirements that need to be met before flying and some that can only be discerned by some flight testing. Before flight testing, adjust the amount of travel of the ruddervators.
THE SECOND RULE OF V-TAILS
“The amount of ruddervator throw must not be too great.” The amount of throw required is like you would set for elevator - not for rudder. Excessive throw just makes bad things happen. A v-tail is not a case where more (throw) makes it (turn) better. I have always found that about 15 degrees of travel up and down on each ruddervator is a good place to start. Never put more than 20 degrees on.
THE THIRD RULE OF V-TAILS
“Ruddervator travel must be equal by measurement not just by eyeball.” Slight differences in throw are automatically mixes of elevator and rudder and vice versa with every control input. Making sure the throws are exact is important. Additionally they must track together, so check that your linkage geometries match. Once your throws are exactly equal and moderate at no more than 20 degrees each way it is time to flight test. The plane should be quite flyable in this trim.
We flight test for three specific functions: elevator sensitivity, rudder effectiveness and differential setting.
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!
Rick Eckel
raeckel@usa.net
Copyright 2002
Revised 2004
staat wel hoog op mijn verlanglijstje!
