02 June 2008

Gliding like a Christmas turkey

May 2008 The last day of autumn, Saturday 31st May, was excellent flying weather; except when climbing up to 4500 ft to begin a workout session, I could see a layer of brownish light smog over the city and along the horizon. Nice!

On this sortie, one thing I wanted to do was use the big Russian attitude indicator to help me obtain the 45 degree inverted downline while flying a half Cuban 8. Competition aerobatic aircraft usually have angle sights mounted on each wingtip to assist pilots obtain the proper upline and downline angles.

But since no self-respecting warbird soils its wingtips with such untidy items, the non-tumbling attitude indicator comes in handy for obtaining the desired pitch angles when flying such manoeuvres. All it took was a quick glance to confirm the big bird was pointing 45 degrees downhill while inverted before stopping the pitch input and then rolling back to the upright wings level position.

I flew a number of consecutive full Cubans and reverse Cubans until I was comfortable using the instrument with its non-conventional (by western standards) reversed sky and ground display.

Next came flying a few Immelmans (roll off the top of a loop). While the Yak-50 manual nominates a 300 kph entry speed and minimum 150 kph at the top of the loop, it doesn't nominate a G loading for the initial pull up into the manoeuvre.

So I started with a 4G pull up and found that the airspeed at the top of the loop before rolling upright wings level was close to the 1G stall speed of 100 kph. The handling was mushy when rolling upright. It seemed clear the 4G pull up didn't deliver a sufficiently fast turn rate in the vertical plane before the bird lost energy at the top of the loop.

During the next attempt, I used a 4.5G pull up and it resulted in a 150 kph airspeed at the top of the loop. Handling was reasonably good when rolling upright wings level. With a 5G pull up, the airspeed at the top of the loop was about 170 kph. Satisfied with my findings, it was time for the main purpose of the sortie - determining suitable wing reference points for use during forced landings.

The previous weekend, I used a row of wing rivets (kept over the desired touchdown point when gliding to short final) between a third and a half wingspan inboard from the wingtip for my last partially successful attempt; which seemed to work until the landing gear was extended.

To compensate for the greatly increased descent rate with the gear down, I chose to start with another row of wing rivets at about half wingspan position. Conveniently, this corresponds to the inboard edge of the aileron. That's right, ailerons on the 50 cover slightly over half the length of each wing. On the Su-31, they span two-thirds of each wing!

Following Noel Kruse's advice, I also added 20 kph to the 150 kph best glide speed to provide better control and higher kinetic energy during the forced landing glide. I was pleased when the new reference row of rivets and higher glide speed resulted in a tight but successful arrival over the selected touchdown point.
Geometrically, the 50 requires a narrower angled cone (compared to other less draggy birds) for a successful forced landing. Put another way, the Lift to Drag ratio of the beast is pretty poor due to the big paddle bladed prop! So the resulting geometry requires a tight spiral glide path to be flown to reach the touchdown point. And it becomes even tighter when a strong headwind is present.

For many years now, it has been standard practice for me to execute at least one practice forced landing during each sortie. With its Christmas turkey glide performance, the Yak-50 demands maintenance of this discipline. Being prepared is definitely better than being sorry.

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