Challenger Ultralight Flying Safety Tutorial

or How Not to Kill Yourself in an Ultralight Aircraft

Ultralight Challengers are one of the safest and most forgiving aircraft flying. Yet any aircraft that is flown by an untrained pilot will prove to be very unforgiving.

Type training on the Challenger by a qualified instructor is a must. Even an experienced GA pilot must take adequate cross training when converting to flying ultralights.

Not convinced? Take note of this direct quote from the Transport Canada web site regarding ultralight aircraft accidents:

“The Challenger ultralight … is a unique aircraft designed specifically for very slow flight operation and, as such, it has unique flying and control characteristics. For your safety, obtain dual training and experience prior to attempting solo flight in this aircraft type.”

This article will concentrate on the differences between flying General Aviation (GA) aircraft, such as Cessnas and Pipers, and Ultralight Aviation aircraft such as the Quad City Challenger.

There are 5 ways that the Challenger and other similarly configured ultralights differ from most GA aircraft. These differences are not disadvantages or weaknesses in design. They are merely differences. Ultralights have characteristics that make them behave differently than their big, fast, sleek GA cousins.

These differences in characteristics require that a pilot initially trained on GA aircraft and now converting to fly ultralights must change flying habits and procedures to suit ultralight flying. This is a most critical point and bears repeating for emphasis. Ultralight flying demands a change in flying procedures from GA flying.

The 5 characteristics of the Challenger Ultralight that require adjustment in procedures are as follows:

1. Low Kinetic Energy
2. High Propeller Thrust Line
3. Required Rudder Input
4. Gust Response
5. Use of Flaperons

Low Kinetic Energy

Compared to many GA aircraft, ultralights are slow and very draggy. They have a big, high lift, high drag wing. That big wing combined with their very low weight enables them to be powered by low horse power, low cost engines like the Rotax 503 found on most Challengers. This design keeps the aircraft cost within reach of many pilot owners.

The big high lift wing also gives these planes remarkable short take off and landing performance. They can thus fit into small grassy fields that most GA aircraft could never touch.

This design can also prove disastrous in inexperienced hands. The reason why is physics. Good, old fashioned, Newtonian physics.

Kinetic energy is proportional to weight x the square of velocity.
The actual equation is:
Ek=½MV²
where
Ek = Kinetic Energy
M = Mass
V = Velocity

This means a GA airplane weighing 1800 lbs has twice the kinetic energy of a 900 lb ultralight if they are flying at the same airspeed.

If that same GA airplane is flying twice as fast as the ultralight, it will have 8 times the kinetic energy.

In the event of an engine failure, the sleek GA aircraft will be able to translate all that kinetic energy into some extra altitude while achieving best glide speed. The ultralight will not.

Low speed combined with very light weight equals low kinetic energy. In the event of an engine failure, an ultralight has no kinetic energy to convert aircraft speed to altitude. Instead, upon engine stoppage an ultralight pilot must immediately, without delay, push the nose down and achieve and maintain best glide speed.

The cruise speed on a Challenger is around 65 to 75 mph. Best glide speed is 55 mph. That means a change of only 10-20 mph. When the engine suddenly stops on a light and draggy ultralight, those 10 – 20 mph bleed off very quickly. Too quickly to be converted to any altitude.

The worst case scenario is on the climb out after take off. The aircraft is in a nose up attitude. If the engine quits, the loss of thrust will result in an immediate loss of speed and the high thrust line means that the plane will actually pitch up further. These two factors will quickly result in a stall and crash if the pilot does not act quickly to push the control stick forward and get the aircraft nose down to achieve best glide speed.

At the same time, low kinetic energy also can mean safety. The heavy GA plane in a forced off-field landing has a higher weight and a higher landing speed. That means lot of impact energy. An ultralight has very low weight combined with a very low landing speed. That translates to a comparably small amount of impact energy. Therefore, ultralight crashes are often more survivable than GA crashes.

High Propeller Thrust Line

The Challenger has the engine mounted in the middle of the aircraft. The engine turns the “pusher” prop via a belt driven “re-drive” with the propeller shaft located above and behind the high wing. The result is an aircraft configuration that is quite foreign to most GA pilots. Most GA jockeys are used to flying conventional airplanes with front mounted engines with thrust lines close to the aircraft center line pulling the plane through the air.

These two aircraft configurations behave quite differently. The difference can prove fatal to the untrained.

When you advance the throttle on most GA aircraft with center line tractor engines, the aircraft responds by accelerating and climbing. When you retard the throttle, the aircraft decelerates and begins to descend. There is very little effect on the pitch axis.

When you advance the throttle on an aircraft with a high thrust line like the Challenger, the result is that the aircraft nose actually pushes down. The high mid-engine pusher configuration causes this characteristic.

Of course, there are a few GA aircraft with similar configurations and characteristics such as the Lake Buccaneer. Pilots learn these flight attributes during their check rides. Proficiency must be attained before they are given sole command.

Quad City Challengers should be treated with the same respect.

This high thrust line characteristic affects pilot procedures. Combined with the low kinetic energy as discussed above, the Challenger pitches up and loses airspeed quickly when the throttle is retarded or the engine stops. The pilot must be “quick on the stick” and get the nose down fast, especially in the worst case scenario during the take off climb out.

The high thrust line also means that the Challenger has to be pulled off the runway during the take off run. The high thrust line tends to keep the aircraft nose pushed down on the ground even as rotation speed is passed. Firm back pressure on the control stick is required to rotate the nose up and achieve take off.

Likewise, during landing, throttle adjustments affect the pitch and trim. This can be dangerous as one closes in on terra firma on final approach. A well trained mind combined with a gentle hand on the stick is needed. Otherwise your landing may resemble the antics of a playful porpoise, but less graceful and more costly. Any sudden throttle advance, such as when aborting a landing, must be coordinated with back pressure on the stick.

Drastic reductions in throttle setting during landing can also be problematic. During the flare sequence upon landing, pulling back on the stick to flare and reducing the throttle setting at the same time, will often cause the plane to rise and slow down simultaneously.

The power reduction at the high thrust line causes the nose to pitch up increasing the angle of attack. This also increases drag and, momentarily, increases  lift. But then the aircraft slows down approaching stall but at a height to high for a smooth landing. This is a pilot induced oscillation and not the fault of the aircraft. This often results in a hard landing.

This is not to say that the Challenger is a difficult plane to fly. It is not. It is simply different from a Cessna or a Piper or a Mooney or an RV. Any pilot new to Challengers is wise to take conversion training with an ultralight instructor until proficiency is attained.

Lots of Rudder Input Required

I first learned to fly in gliders. Those big long slender wings needed lots of rudder input to bring them around in coordinated turns. It was good training. I learned early to fly with my feet.

Often I hear or read of pilots criticizing Challengers and other ultralights for the amount of rudder input needed to control the aircraft. They likely learned to fly on GA aircraft that require little or no rudder input except during taxiing. To convert to Challengers they need to learn to use their feet more. (Note: The new Challenger LSS, XS-50 and XL-65 models have reduced the amount of rudder input required by incorporating a differential aileron modification to the control system)

Hopping into a Challenger without proper cross training may prove embarrassing, if not fatal. Unfortunately, pilots foolish enough to attempt such flights will usually blame the plane and not themselves for the subsequent accident. A few of the incidents that I have read have involved pilots who took absolutely no type training before climbing aboard their new Challenger and promptly crashing it.

The need for lots of rudder input is not because the Challenger or its other ultralight cousins are bad designs. The gliders I flew were not bad designs. It is simply that these designs require rudder input coordinated with control stick input to overcome the adverse yaw of their simple wing design and the drag and inertia inherent to those big long wings. This is a skill that should be learned under the supervision of an instructor experienced in the aircraft type. Take ultralight conversion training.

Oddly enough, I never really noticed that the Challenger required rudder input because I was so used to using my feet when I flew gliders. The look out front seat of the Challenger reminds me of the same unobstructed view I enjoyed while flying Swietzers and Blaniks. When I first strapped on the Challenger, I naturally used my feet. If I had been solely trained on Cessna 152’s, the scenario would no doubt have been quite different. I would have needed to learn a new skill set for my size 10’s.

Some changes to the new light sport model Challengers have reduced the need for as much rudder input as required on the older “Classic” model Challengers. The new LSS, XS-50, XS-65, and XL-65 model Challengers have larger taller tail fins along with a fin extension over the rudder. This larger tail area improves the tracking ability of the aircraft, that is, its ability to maintain a steady course without requiring corrective rudder inputs.

Secondly, the new light sport model Challengers are equipped with differential ailerons. Differential ailerons reduce the need for rudder input during turns by reducing the effect of adverse yaw.

Adverse yaw occurs when an aircraft turns and the down going aileron produces lift to raise the wing on the outside of the turn. Generating lift causes drag. The upward travelling aileron on the inside of the turn does not create as much drag. This unbalanced combination of drag from the opposing ailerons makes the aircraft want to turn left when it banks right, and turn right when it banks left. This tendency, called adverse yaw, is overcome by using the aircraft rudder.

There are a number of ways that various aircraft models use to reduce adverse yaw. Differential ailerons are one such method and is the method used on the new Challengers. Differential ailerons are designed so that the aileron travelling upwards moves much further in proportion to the aileron that is travelling downwards. This tends to balance the drag forces on the opposing ailerons when the aircraft banks for a turn. This results in much less required rudder input in order to accomplish a coordinated turn. Yet it may still be more rudder input than many GA pilots are used to.

Gust Response

The lift produced by a wing is directly proportional to the square of the air speed. In other words, if you double the airspeed, you quadruple the lift generated by the wing.

The wing used on the Challenger is a high lift wing that results in an aircraft with a very low stall speed. This means that small increases in airspeed result in a larger relative increase in lift compared to aircraft with higher stall speeds.

This can prove problematic in gusty wind conditions.The situation is a matter of physics. Note how the mathematical equations below illustrate the large changes in lift produced by the small changes in airspeed on a typical ultralight with a stall speed of 30 mph.

Lift Generated at a Stall Speed of 30 mph is 100% of aircraft weight
(Va)² = L = 100%
(Vs)² W
where:
Va = Your Actual Approach Airspeed
Vs = Aircraft Stall Speed
L = Lift Generated expressed as a ratio
W = Aircraft Weight

Lift Generated at 35 mph is 140% of aircraft weight
(35)² = 140%
(30)²

Lift Generated at 40 mph is 170% of aircraft weight
(40)² = 170%
(30)²

This shows that a wind gust of 10 mph will produce a very big change of lift on a typical ultralight. This can make for a very bumpy ride on final approach on a gusty day. Throw in a cross wind and you are going to need some skill and finesse!

The solution is to increase approach speed on gusty days. A higher approach speed will reduce the effect wind gusts.

A 10 mph gust is 33% of the 30 mph stall speed. But it is only 17% of a 60 mph approach speed. And the change in lift produced by the gust at the higher approach speed is proportionately smaller according to the square rule. In other words, it will not affect the aircraft as much if the approach speed is higher.

So the solution is to fly a higher approach speed in gusty conditions and be very careful in controlling your airspeed. Of course, on really gusty days, smart Ultralight pilots stay on the ground and tell outlandish stories of their past aviation exploits.

The Potential of Misuse of Flaperons

The Challenger uses flaperons for two purposes. The first is as full span flaps which are especially useful when flying on floats or getting into small short airfields. The second purpose is for trimming the plane in the pitch axis. For both of these purposes the flaperons must be used carefully.

Flaperons and Airspeed

Even without flaperons the Challenger is a STOL aircraft. Using the flaperons improves the STOL performance even more. However, like many other high wing aircraft with flaps, the use of flaps (or flaperons in our case) must be kept below a specific airspeed. This is not because the flaperons will blow off at high speeds, but rather that the nose down force produced by the flaps at high speeds will overcome the ability of the elevator to keep the nose up. At high speeds the with the flaperons down, the control stick forces are also increased. This may take an unwary pilot by surprise or even overcome a pilots ability to pull the stick back t raise the nose.

Therefore, when using flaperons in a Challenger, the pilot must watch and control the airspeed to keep it below the maximum full flap speed of 65 mph. Above this speed, the aircraft may pitch down despite maximum back force on the control stick and a maximum upward elevator position. This is particularly important on take off and landing because the aircraft is so close to the ground and there will not be enough altitude to recover safely.

On take off, with the throttle at full power, the Challenger may quickly accelerate through the max flaps extended airspeed. Watch the airspeed. Keep the nose up on take off with full flaperon extension.

On the landing approach with full flaperon extension and the nose down, the airspeed may creep up, especially if full throttle is applied to execute a go around. Watch the airspeed. Keep it below 65mph with full flaperons. In fact, keep it below 60mph, for a good margin of safety. Approach and climb speed in a Challenger is usually 55 mph.

Safely Using the Flaperons for Pitch Trim

Flaperons accomplish the trim task by changing the shape of the wing. This in turn changes the center of lift on the wing. As a result the nose will tend to pitch up or down. With the flaperons down, the nose will pitch down. With the flaperons up – actually extending above the trailing edge of the wing like a negative flap – the nose will pitch up.

A problem may arise with this latter trim situation. This is often used when the aircraft is nose heavy such as with two heavy passengers. Although the negative flap position causes the nose to pitch up, it also kills lift. This affects climb performance and increases the stall speed. The danger is not necessarily on the take-off as usually the trim is selected to neutral for take-off. The danger occurs during the landing phase of the flight.

So picture this situation that illustrates the danger when a number of factors combine:

1. two heavy passengers
2. full fuel tanks
3. a hot summer  day
4. high humidity
5. trim has been selected to full nose up after a one hour flying tour (maximum upward deflection of the flaperons)
6. the aircraft and its aviators return home to land

Now as the aircraft comes in to land, the stall speed will be higher than normal due to the high gross weight and the negative flaperon position. The hot humid weather also affects the aircraft performance. As the aircraft flares on final approach the higher stall speed may be reached while the aircraft is still well above the ground. The result is a hard landing or worse. The aircraft may suffer damage to the landing gear.

The solution is to set the trim to neutral for both take-off and landing. Also to be aware that gross weight and the temperature and humidity affect the stall speed. Increase your approach speed when its hot and humid and the aircraft is heavy.

Always re-trim your plane when entering the landing pattern.  Set the flaperons to the neutral position if you have been flying with them set in the negative (trailing edge up) position because of being nose heavy.

Of course, flights at gross weight should be avoided in hot humid weather. Take your XL sized friends flying when its cooler and when your fuel tanks are 1/2 full.

Conclusion

Ultralights, like the Challenger, are obviously quite different birds from their GA cousins. They have different characteristics that demand different procedures. This requires training from an experienced ultralight instructor. GA pilots converting to ultralights must take conversion training no matter how experienced they think they are.