challenger rudder control and authority

The following information is provided by our friends at National Ultralight, Inc. and is displayed here with their permission.

The Challenger does indeed need more rudder activity than planes such as the Cessna 172. This is due to intentional design differences in the ailerons and the vertical tail. Over the years it has been our experience that new Challenger pilots typically take anywhere from 15 minutes to several hours to become proficient on the pedals.

The Challenger's need for proper rudder inputs is common to many ultralights, classic taildraggers, and aerobatic planes as well as most if not all gliders. In fact pilots of such aircraft generally graduate from the conversion course in less time than Cessna and Airbus drivers with more hours in their logs since the latter are accustomed to flying with their feet flat on the floor!

The short answer to your question on the Challenger rudder/tail is that with a proper checkout you should adapt quite quickly since it is simply a matter of teaching your feet how to use rudder pedals.

For the long answer to your question you need to recall the concept of adverse yaw and the influence of weathervaning tendencies. Adverse yaw is a secondary effect of the ailerons and causes the nose to move in
the opposite direction of the roll. It exists because the aileron going down sticks into the slipstream and creates more drag whereas the aileron going up hides behind the wing and creates less drag.

Also, the downgoing aileron creates more lift on that wing thus creating more induced drag on that side, while the upgoing aileron creates less lift and less induced drag on the other side. The combination of these two effects results in the upgoing wing being "held back" which moves the nose in the opposite direction to the roll.

Straight-and-level cruisers such as the Cessna 172 compensate for this in two ways. First, they use differential ailerons, in effect restricting the downward travel of the ailerons. For example the maximum upward deflection in some airplanes is 21 degrees while the downward deflection is only 14 degrees. Second, they use a very large vertical stabilizer to make the plane strongly inclined to point straight into the relative airflow and less inclined to slip or skid.

The Challenger's huge ailerons are targeted to be highly effective even at very, very low speeds so their downward travel is not restricted. As a consequence, when aileron is applied a proportional amount of rudder
is needed simultaneously for coordination.

When pilots new to adverse yaw apply aileron in the Challenger they almost invariably apply rudder after the aileron rather than at the same time. To compound this they also usually apply too much rudder because the Challenger's rudder is very powerful and only needs a light touch. The GA guys are used to fairly weak rudders (due to the proportionally large fixed vertical tail surface) and high pedal pressures (due to the various springs and nose wheel shimmy dampers in GA rudder circuits). Of course after the initial booboo they then overcorrect and the nose nose heads of in the in the opposite direction!

Further, the Challenger has relatively low weathervaning tendencies, especially when doors and/or floats are installed. This is a reflection on the proportion of side area at the front versus the back. As a consequence, errors in rudder coordination are not automatically corrected and may even be amplified.

The absolute best way to detect rudder errors early is to install a slip indicator or yaw string. (In case you're not familiar with a yaw string, it is a concept borrowed from gliders and consists of a piece of wool 3 or 4 inches long taped to the center of the
windscreen at the bottom. The yaw string serves the same purpose as the ball in the turn and bank indicator except that it is much more sensitive and you don't have to look in at the panel to see it.)

The end result is that pilots used to flying with their feet on the floor usually start their Challenger careers by motoring around with the nose wagging from side to side! After a while, usually a handful of hours, they learn to stop putting in too much rudder too late and learn to add just a bit of pedal pressure as the ailerons are fed in. Then the plane magically straightens out and flies exactly the way they want.

The benefits of having low weathervaning tendencies are numerous. Compared to conventional aircraft, the Challenger is much more maneuverable in yaw and has vastly more side-slipping capability. Rates and angles
of descent can be easily increased from their normal gentle levels to create glide slopes approaching that of a helicopter!

As well, the Challenger has higher crosswind limits than many General Aviation aircraft, the 172 being a case in point. This is a benefit not just of the low weathervaning but also of the fact that all the controls are highly effective
even during low speed rollouts. It is worth noting that, compared to Cessna's and such, the Challenger's rudder is unusually large relative to the fixed portion of the vertical tail. This further evidences the reduced weathervaning
exposure and the presence of sufficient rudder authority to overpower undesirable swings into wind.

On floats, that low weathervaning tendency and powerful rudder make the Challenger much easier to taxi and dock since the nose points the way you intend, rather than into the wind like a giant metal rooster on top of a barn. The same applies on skis while taxiing in the wind on ice or hard, crusted snow.

The rudder/tail 'issue' arises from the feet-on-the-floor brigade who want the Challenger to be just like their 172 or whatever they're used to driving. They've generally never flown anything really different and have only ever
droned around at five thousand feet in straight and level cruise.

The loudest voices in this brigade are the people who've never bothered to go through a proper type conversion from an experienced Challenger pilot - either they bought a used plane and don't have dealer support or they've flown 'real' airplanes and think they are above needing instruction to fly a 'toy' ultralight.

The people most pleased with the Challenger's standard handling understand the whole new world waiting for those who take advantage of the Challenger's very unique capabilities for short field operations, floats, skis, soaring, and meandering around on low and slow sightseeing missions. These people would be outraged if anyone tried to add more tail to their Challenger!

What we suggest to people is that they get a proper checkout in the Challenger with the standard tail configuration and then spend some time getting acclimatized to the airplane. The best way to do this is to fly a bunch of hours
over a few days.

If at that time they still want to increase the directional stability of their Challenger, albeit at the expense of all the other capabilities referenced above, then it is quite easy to increase the vertical surface area at the tail by adding "tail-lets". Tail-lets are vertical plates mounted at the ends of the horizontal stabilizers and are readily constructed from plans provided by the factory.

Importantly, since the tail-lets are approved by the factory they don't impact your Challenger's eligibility to register in the Advanced Ultralight category (in Canada).

There is a mod for a bigger tail offered on the aftermarket but it is not approved by the factory so you won't be able to register in the Advanced Ultralight category. (In Canada only). (Two such modified planes slipped through Transport Canada's net in the past but since last fall inspections are required for the issuance of a Statement Of Conformity.)

Bryan Quickmire, National Ultralight, Inc.