Wednesday, July 11, 2007

Liberty XL2 test flight

After a long period of inactivity, I revive this blog today as I was given the opportunity to test-flight a Liberty XL2. This plane falls in the plastic category by its engine, an AVGAS injected engine controlled by a FADEC. Nothing else is plastic in the model I tested.

There is much to say about the liberty, but all can be summarized in one point: this plane seems to be desinged to be cheap. This plane is clearly done to fly. This sounds good, but the problem is that it is NOT made to roll. Taxi, take-off and landing rolls are hard times.

A lot has already been said about the finger braking system, so I will be short on that. The nosewheel is free of rotation, so guidance during taxi and at low speed is done using brakes, which are controlled by two levers beside the power lever (don't say throttle, it's a FADEC engine).

The free nosewheel system also exists on the DA40, but it is somehow restricted, whereas the Liberty nosewheel is totally free. Even moving it arround with the towbar is not obvious.

The engine operation is as simple as a FADEC makes it, single lever, and a fuel pump that has an automatic mode. The suprising thing is that the pump is then controlled by the fuel pressure, so when throttle goes back to IDLE, typically on final, the pump starts, and it's quite noisy. When idling, the engine sounds really unhappy, so it's very tempting to taxi with more than IDLE, but this results in excessive speed.

Engine check is quick and easy thanks to FADEC, but a remark here is that the breaks locking system does now allow for holding the plane immobile during power check, and I had to manually operate the breaks at this time.

Two other strange features appear at before take-off checks time. The flaps are electrically operated with three LED to indicate their position. These LEDs are really not bright enough, so it's hard to see them from left seat.

The second oddity is about the elevator trim. It is electrically operated as well, and I must say electric only. There is no trim wheel in the cockpit. I personally dislike it, because in case of failure of the servo-trim, you are left with nothing but a plane potentially out of trim and no way to correct it.

The side visibility when at holding point is not exactly great, and its hard to check that no one comes on final before you line-up. And as you might guess, the line-up is not easy because of the brakes.

I tested it on a grass runway, and I must say that the take-off roll was just horrible. Directional control is not as problematic as during taxi as the rudder becomes operative quickly. Nevertheless I would not have to handle a rejected take-off with the finger brakes.

Anyway, what made that take-off roll unpleasant is the absence of dampers. We felt each and every bump in the grass, and I never bounced that much before take-off. I'm even wondering if this is not kind of business development for chiropracticians. Add on top of that a relatively small cockpit, and it was quite hard to fit my 1m96 in.

Things got better once in flight. The forward visibility is quite good, and as the wings are short, side visibility is good as well. The central stick is quite comfortable in hand, and the plane is responsive. Climb performance is not exceptional, but acceptable.

Flaps retraction is quite long, and as there is no position preselection, it is necessary to press the button for long seconds. Once in cruise, the indicated airspeed went up to 120kts.

We flew some steep turns, at 45° and 60° bank with no noticeable things to report. Climbs and descent are ok. Using Vy, with half fuel, we obtained sustainable climb rates of about 800 - 900fpm in clean configuration.

Approach is easy, flaps make speed control easy, and the plane behaves very well on final. The touchdown was really firm and bouncy. The directional control of ground roll was also really problematic. The goal clearly is to land and control it with rudder, and touch down slow enough to not need any braking.

As a general conclusion, this plane is made to fly, but not to taxi or go through ground rolls. The toe-brakes option is definetily a must, and a more controlled nose-wheel would be good. Better dampers would make it much better, but this is not an option.