HP-18 Center Stick Project

I test flew my HP-18 with the center stick installed on Saturday, 7 July 2001 at Air Sailing gliderport. The flight lasted about an hour and twenty minutes, and demonstrated that the new control system worked as expected and did not introduce any bad habits into the aircraft behavior. The flight was conducted at a calculated CG position of 32.7% MAC at a total calculated weight of 697 lbs.

The aircraft was inspected by both an A&P mechanic and by an FAA official from the Reno FSDO the day before the first flight. The A&P looked at it for its annual condition inspection, and the FAA guy looked at it so he could reissue its airworthiness certificate with a new set of operating limitations and a 5-hour test period. No discrepancies were noted in either inspection.

The flight started with a normal takeoff and a 4600' agl (8900' msl) tow. I used -4 degrees of flap on the initial roll, and cranked over to +10 degrees after the tail came up. An initial wing drop from the towplane wake was easily countered with aileron.

Off tow, I first performed incipient and full stalls, straight and turning, at three flap settings, and also explored general aircraft behavior at flap settings from 0 to 90 degrees.

Satisfied that the aircraft behaved well enough to allow a safe landing, I retracted the landing gear and indulged in some general soaring flight to get a better feel for the aircraft. Problems with the vario display and with the TE compensation precluded any serious thermalling, and problems with the radio hampered communication with the ground. However, I did get up to about 13000 feet, and managed to perform several inter-thermal runs to explore the upper-middle part of the speed range.

At the end of the flight, I circled down over the airport with about 45 degrees of flap, and then performed a normal left-hand pattern and landing to runway 3.

Some impressions about the flight:

• In general, the new control system worked just fine. All the controls were crisp and positive. I was impressed how much more control response there was than I am used to with the HP-11. I could make thermal corrections on a dime. In general, I found the experience much, much more comfortable than with the original side-stick.

• The stick travel in the pitch and roll axes work well with the stock rudder travel. In general, I think that the control harmony is quite nice.

• With the stick limits set to achieve the control surface deflections specified in the plans, there was plenty of stick travel in all directions for all of the maneuvers I performed. I did not ever touch either the forward or aft stick limits in flight. I touched the roll limits only while maneuvering in turbulence on tow, and later while timing the roll rate from 45-to-45 (about 3 seconds at 60 knots).

• The aircraft seemed to thermal very well. With the flaps at around +10, it felt very comfortable at 45 knots in a 45-degree bank. It seemed to take a lot less work to thermal than the HP-11, which complains somewhat about thermalling at any less than about 52 knots. In general, it felt like a much lighter glider than the HP-11, which is funny because my HP-11 generally runs about 120 lbs lighter than my HP-18.

• I think that the pitch forces are perhaps a bit too light; with the very neutral handling of the HP-18, it is easy to overcontrol in pitch, especially with neutral or negative flap. In the next few flights, I'll probably try a more authoritative pitch trim system to introduce more 'feel.'

• I did perform a few pull-ups and pushovers to get a feel for the potential for PIO feedback from the stick handle geometry. It did not feel like there was any problem there; the stick load per G seemed very linear throughout the part of the range I tested.

• The control stick did feel like it might be a bit too tall, and perhaps a bit too far forward. I can easily shorten the stick; but moving the handle aft would be problematical, since that would increase the potential for PIO feedback. The next few flights should let me get a better feel for this issue.