HP-18 Aileron Roll Control
Problem Is Solved
By David Colling
(Article provided by Albert "Buster" Stone.)
A new set of ailerons has increased the roll rate on my HP-18 by 300 percent. It is a well-known fact that the HP18 has poor roll control characteristics. Being the proud fifth owner of HP-18 #35, 1 soon realized the reason for the artificially low cost and why there were so many previous owners of this otherwise beautiful sailplane. My particular HP-18 had suffered major damage in the past by one of its previous owners on their first flight due to poor roll response. After numerous ground loops Between my sailplane partner and I, both on takeoff and landing, and very poor cross wind capability, we finally came to the conclusion that something had to be done.
Being an Aerospace Engineer, I began the task of figuring out the problem and designing a fix. I received lots of advice from other glider pilots. Some claimed it could not be fixed because it was due to the V-tail, others claimed it was the side stick. There was a modification in the past which added a section of the flaps to the aileron thus increasing its area. This however does not work since the added section not only interferes with the aileron push rod which binds up the controls, it also increases stick forces because of the increased area.
The reason for the poor roll response was due to flow separation on the top surface of the aileron. When comparing the dimensions of the actual blue prints, the problem is obvious. The wing trailing edge spar is about 3/16 inch taller than it is suppose to be while the aileron is exactly, the right size, according to the plans. If a straight edge is placed on the top wing skin, the straight edge is suppose to also touch the top skin of the aileron as shown in Figure 1. On my wings, there was almost a 1/4 inch gap between the straight edge and the aileron top surface at the wing tip. Since the trailing edge spar was pre-drilled at the factory, the flaw was therefore with the HP-18 kits in general. Even though the problem lies with the wings, a new set of ailerons had to be guilt since it is easier and more economical to replace the ailerons than it is to replace the wings.
The new set of ailerons have an increased thickness to match the thickness of the wing. The beryllium/copper seal was also trimmed back to the trailing edge spar. The seal not only covered about 1/3 of the aileron top surface area, a big gap would also open between the sea] and the aileron causing flow separation in all down deflections as shown in Figure 2. The beryllium/copper also stiffens the aileron controls which prevents the proper amount of up deflection. By removing the beryllium/copper from the ailerons, the controls are smoother and more precise, it also recovers about a square foot of aileron area from each wing. The new thicker ailerons have been fitted with a leading edge radius. This allows a smoother path for the airflow at all deflection angles as shown in Figures 3 and 4.
This leading edge radius was bonded to the top skin of the aileron with 3/4 inch overlap. Care was taken to ensure that the leading edge radius can in no way jam or rub against the wing. I used the exact same construction for the new, ailerons as specified in the plans. The work was also supervised by an A&P mechanic and the structural epoxy and rivets were purchased from Airmate Co. (Bryan Aircraft) in Bryan, Ohio.
With the new ailerons, roll control is achievable at speeds as low as 5 mph which alleviates the need for fast wing runners. The roll rate has creased from 30 degrees in six seconds to 30 degrees in less than two seconds. The aileron control is smoother and more precise which makes for more enjoyable thermaling and a feeling of control not present before. The new ailerons are only slightly heavier than the old. So far I have had my HP-18 up to 120 mph with no signs of flutter.
In summary I believe the roll control problem with the HP-18 has finally been solved. With the new ailerons, the HP-18 is more enjoyable and less of a challenge to fly. Even though ten weekends of soaring were lost while designing and building ailerons, I feel it was worth the time and effort.