Post by Allen Peterson on Aug 16, 2010 17:09:33 GMT -5
Another of my DUH-uh events.
A while back I set up a Harmon Rocket in FS9. During the set up I downloaded a Van's RV-4, which the Rocket was developed from, and a RV-8. Then I found and downloaded a Harmon Rocket and - best of all - a pilot's manual on the Rocket. The manual had all of the dimensions for crew, etc. including the empty weight CG position. After plugging in the dimensions in the .cfg file and making a few tweaks in AirWrench it was flying pretty well. I sent it down to the son of a friend of mine to check out. He flies the real plane. He said that my sim flies very well, he was impressed. He said that the real plane slows down a lot faster when the throttle is closed than the sim, and that the sim should be a little more sensitive in pitch. I changed the prop from 2 blades to 4 blades and it didn't make much difference in the slow down rate so it looks like the sim doesn't model prop drag.
Anyway, I then set up the RV-4 and RV-8 and got them flying OK.
I set the weight of the pilot to 200 lbs., and the passenger to 180 lbs.
Recently I purchased the book "Simplified Aircraft Design for Homebuilders" by Daniel P. Raymer. The book explains and applies a lot of the stuff that Tom has been teaching, so it was really neat going through the equations and methods - simplified, of course - to set up a real plane. Since the book had some info on the RV-6, which I had downloaded some time ago, I decided to set it up again (I made a copy of the .cfg and .air files first).
That took off and on about a week and I learned a lot in the process. One of the things was that the term "aerodynamic center" in the book and the term "Center of Lift" in AirWrench refer to the same thing. Also the center of gravity should be forward of the neutral point for stability.
AirWrench has a Direct Edit window on the Balance page where you can set the CoG and the Center of Lift. Long story short, after making sure all of the dimensions and weights were set up correctly, I set the Center of Lift to the 1/4 chord ref. datum (0.0) and the CG to about 0.5 and that also set the empty weight CG about right.
After flight testing with the AP on to check the HP and airspeeds, I turned off the AP altitude hold and the plane stayed in level flight. But when I turned off the HDG hold the plane wanted to roll to the left. I could correct it with some aileron trim or by turning the HDG back on, but if left alone the plane would keep rolling to the left and "depart controlled flight".
I added a Aileron deflection gauge and an Aileron trim gauge and confirmed that with the HDG on the AP required about 3 degrees of aileron to maintain the heading, and by adding aileron trim I could get the aileron deflection to 0.
Neither the RV-4 or the RV-8 had the problem. So what was wrong? I calibrated the yoke and rudder pedals, reduced the prop torque, increased the roll stability, nothing helped.
I compared line by line the RV-4 .air file to the RV-4 and RV-8 .air files, nothing of any significant difference.
I was setting there, trying to figure out what to do next when it dawned on me that in the RV-4 and RV-8 the pilot and passenger are seated in tandem while in the RV-6 they are seated side by side. So in the RV-6 I had 200 lbs. in the left seat and 180 lbs. in the right seat. DUH-uh!
After the fix the plane was flying level and on course without the AP. I was feeling pretty pleased with myself so I explained in detail my accomplishment to my wife. She just looked at me, and, in all of her wisdom, said "Well, you should have known that!" She was right, as usual.
A while back I set up a Harmon Rocket in FS9. During the set up I downloaded a Van's RV-4, which the Rocket was developed from, and a RV-8. Then I found and downloaded a Harmon Rocket and - best of all - a pilot's manual on the Rocket. The manual had all of the dimensions for crew, etc. including the empty weight CG position. After plugging in the dimensions in the .cfg file and making a few tweaks in AirWrench it was flying pretty well. I sent it down to the son of a friend of mine to check out. He flies the real plane. He said that my sim flies very well, he was impressed. He said that the real plane slows down a lot faster when the throttle is closed than the sim, and that the sim should be a little more sensitive in pitch. I changed the prop from 2 blades to 4 blades and it didn't make much difference in the slow down rate so it looks like the sim doesn't model prop drag.
Anyway, I then set up the RV-4 and RV-8 and got them flying OK.
I set the weight of the pilot to 200 lbs., and the passenger to 180 lbs.
Recently I purchased the book "Simplified Aircraft Design for Homebuilders" by Daniel P. Raymer. The book explains and applies a lot of the stuff that Tom has been teaching, so it was really neat going through the equations and methods - simplified, of course - to set up a real plane. Since the book had some info on the RV-6, which I had downloaded some time ago, I decided to set it up again (I made a copy of the .cfg and .air files first).
That took off and on about a week and I learned a lot in the process. One of the things was that the term "aerodynamic center" in the book and the term "Center of Lift" in AirWrench refer to the same thing. Also the center of gravity should be forward of the neutral point for stability.
AirWrench has a Direct Edit window on the Balance page where you can set the CoG and the Center of Lift. Long story short, after making sure all of the dimensions and weights were set up correctly, I set the Center of Lift to the 1/4 chord ref. datum (0.0) and the CG to about 0.5 and that also set the empty weight CG about right.
After flight testing with the AP on to check the HP and airspeeds, I turned off the AP altitude hold and the plane stayed in level flight. But when I turned off the HDG hold the plane wanted to roll to the left. I could correct it with some aileron trim or by turning the HDG back on, but if left alone the plane would keep rolling to the left and "depart controlled flight".
I added a Aileron deflection gauge and an Aileron trim gauge and confirmed that with the HDG on the AP required about 3 degrees of aileron to maintain the heading, and by adding aileron trim I could get the aileron deflection to 0.
Neither the RV-4 or the RV-8 had the problem. So what was wrong? I calibrated the yoke and rudder pedals, reduced the prop torque, increased the roll stability, nothing helped.
I compared line by line the RV-4 .air file to the RV-4 and RV-8 .air files, nothing of any significant difference.
I was setting there, trying to figure out what to do next when it dawned on me that in the RV-4 and RV-8 the pilot and passenger are seated in tandem while in the RV-6 they are seated side by side. So in the RV-6 I had 200 lbs. in the left seat and 180 lbs. in the right seat. DUH-uh!
After the fix the plane was flying level and on course without the AP. I was feeling pretty pleased with myself so I explained in detail my accomplishment to my wife. She just looked at me, and, in all of her wisdom, said "Well, you should have known that!" She was right, as usual.
