E-Mail exchange with Ken Kopp and Steve Coy
This email exchange discusses directional stability issues with the BD-4.
| From: Steve Coy To: Kenneth Kopp Sent: Monday, July 14, 2003 9:00 PM Subject: Directional instibility |
| Ken: I had a long talk with a former BD4 owner this weekend. He described the handling charistics of the 4 as lousy. He stated that directional stability was so bad the he was constantly working the rudder to maintain a steady course in anything but dead smooth air. He used termes like divergent stability to describe the issue, He also said that the the roll response lagged way behind the stick input. But, he said the it could have been just his plane that be bought used. I know that you have done some objective analysis in this area and would like to hear your opinion. This could be a decision maker for me. Thanks Steve Coy |
| From: Kenneth Kopp To: Steve Coy Sent: ?? Subject: RE: Directional instibility |
| Hi Steve, The difficult thing about making comments on handling qualities for experimental aircraft is that no two are alike and therefore handling qualities may differ significantly. Having said that however, words like "Divergent" are very strong and have very specific meanings. If this gentlemen's BD-4 truly was divergent in the lateral or directional axis I would submit something was really wrong with the construction and not the design. A lot of airplanes have dynamically divergent modes of stability; for example the P-3 Orion is longitudinally statically stable but its long period or Phugoid mode is dynamically divergent (the amplitude of pitch oscillations increases as a function of time). In this case it is not a big issue because the "time to double amplitude" is so long the pilots will be in the loop making corrections before the mode diverges excessively. My BD-4 is both statically and dynamically stable in the lateral directional axis. In the lateral axis is exhibits very strong positive dihedral effect and neutral spiral stability (wings tend to hold AOB over time). Its directional stability is a bit weaker than say a 172 as evidenced by significant adverse yaw and requirement to add significant rudder during turns. One of the biggest issues I felt was the rudder sensitivity. With relatively weak directional stability and an overly powered rudder (the degree of rudder deflection per inch of rudder travel is way too high: its like a gear ratio) make for more challenging directional control. I reduced the gear ratio to require more pedal per degree of rudder travel by almost twice and found it to be an improvement. Probably the most problematic mode in my view is the enormous dihedral effect which is roll due to sideslip. Because of the sensitive nature of the rudder pedals and the weak directional stability only slight rudder inputs create large sideslip angles which then create large roll moments. So...in gusty or crosswind conditions making rudder corrections requires constant aileron corrections as well....net effect is an increase in workload. Its fun and it is manageable if you know your limits and don't push to hard to fast. Hope this helps! Ken LCDR Kenneth G. Kopp MH-60R Strike Helo Test Pilot |
| From: Steve Coy To: Kenneth Kopp Sent: Wednesday, July 16, 2003 6:22 PM Subject: Re: Directional instibility |
| Ken: Yes, that did help very much. I have been following you comments on the newsgroup for quite some time and respect your scientific approach toward analyzing the aircraft as a whole. The man I spoke with did say that his bird had the large rudder and that he got very little feedback from the rudder pedals. After what you said, I am beginning to think that his problem was a combination large rudder, high "gear" ratio and over controlling due to the lack of feedback from the rudder pedals. I doubt that the aircraft was truly divergent as he made a number of X country trips in the six month he owned the plane. It seems that if it were truly divergent it would have gotten away from him at some point. Thanks again for you input on this issue, it sounds like the problem is not very hard to correct. Steve Coy |
| From: Kenneth Kopp To: Steve Coy Sent: ?? Subject: RE: Directional instibility |
| I agree..I doubt his bird was truly divergent in the directional axis.
With this type of aircraft configuration it would actually be very difficult
to design this type of instability into the plane even if you wanted it.
Because of the large Yaw moment created by the distance the vert stab
is from the CG and the size of the Vert Stab there would have to be an
aerodynamic structure forward of the CG to counter the restoring moments
created by the Vert stab and to a lesser extent the fuselage structure
aft of the CG. This would be a very odd looking configuration!
One point about stability.... when we talk about stability we really need to divorce ourselves from discussing the control associated with the axis in question. When characterizing and aircrafts stability it is helpful to think of the control as being fixed and analyzing the response to a gust input to the axis in question. The controls are there to create moments that OPPOSE the aircrafts natural stability so separating them in the characterization process is important. Flight test techniques are developed to initiate the moments using the controls and then freezing them to allow the airplanes response to be evaluated. If the pilot remains in the loop during the maneuver then the true stability CANNOT be evaluated. If you wanted to take the stability discussion to a much higher level you could evaluate the aircraft and pilot as one complete system rather than separately and then consider the pilot as a control function providing the closed loop feedback needed to enhance stability. This has less value than true classic handling qualities evaluations since depending on the pilots skill, even naturally unstable aircraft would APPEAR to be stable with the pilot making the required control corrections to alleviate any static and dynamic instabilities.....does this make sense? Ken |