Q&A: How do wing eccentric cam adjustments affect A/C performance?
"Is it true that having the wings adjusted up on a Cessna 185 will give best speed and having the eccentrics adjusted down will give better low speed performance?"
I don't have the 180-185 experience as some although I rigged quite a few Cessna 180-185 Aircraft. I've accomplished enough "eccentric cam adjustments" on the Cessna "Strutted Model Aircraft" in the past however have not attempted to compile any "real data" that would suggest this is a true or accurate statement. Regarding speed gains by decreasing lift or drooping to increase lift it would be questionable just how much actual impact this might have as an end result? Altering eccentrics in any way outside of producing Equal Wing Lift as close to a neutral state on both L & R.H Wings does not seem to make any significant gain. The highest speed gains will be obtained through a streamlined Airframe.
I have done a bit of this "lift reduction" during standard climb characteristics with the "neutral Trim" on the 182P in an attempt to maximize cruise configuration speed gains. However, getting an Aircraft to fly straight is always the priority. After rigging properly it nearly always picks up performance speed gains so after that there is not much further change that needs made in my opinion. Significant speed gains above the book performance would be doubtful with altering the eccentric movements alone.
I have found on nearly all the “Legacy” Cessna single line that with the factory eccentric cam setting in "neutral" the acft has a tendency to climb at 200-500FPM with the elevator Trim tab in a mechanical "neutral" state. Aircraft that have the 3-500FPM+ range climb are nearly always the ones with a leading edge cuff, Horton STOL, Engine HP increase W/ 3-Blade Prop or modified in some way. Utilizing a standard Wing, I believe Cessna incorporated in that design a standard 200-300 FPM Climb rate when trimmed for “neutral”. That is, Elevator/Stabilizer Trim Tab all streamlined.
The "Restart" series changed up the Airframe regarding Stabilizer/Elevator therefore it takes more Trim Tab deflection in slipstream for a coordinated climb.
I had set up a 182 once after rigging to have a Level Flight Attitude with the Trim Tab Streamlined in cruise configuration by decreasing Wing lift & bringing the trailing edge "up" to maximum eccentric position while keeping L & R Wing with equal Lift. Unfortunately I never did get any good feedback as to actual speed calculations afterwards only "I think I gained a little more cruise speed". The Plane was from East Coast and when it 1st came in it was very out of rig. That particular Aircraft had picked up a tremendous speed gain just by alignment of controls and eccentrics (about 10-11KTS).
Between the initial rigging and the latter adjustments I don't know that they had enough time to really evaluate those changes well enough?
I have a 172H & a 182Q that I hope to play around with one day and get some actual numbers on the subtle changes. I am going to also evaluate some other speed mods and get some real data if possible since I get asked these type of questions quite regularly. I just haven't the time to take a well rigged, good flying acft and change it up additionally due to my normal schedule focusing on getting "book speeds" again. I usually see some very mis-rigged Airplanes so the focus is to make them conforming not necessarily chasing additional performance.....
I spoke with one of the leading experts that does the 180/185 mods several years ago. I believe he mentioned 1/2" of L. Aileron below wing tip and 5/8 on the Right or something similar. I never followed through with the idea as I base my adjustments on actual mechanical degrees in relation to the various Flight Control Surfaces. I then evaluate items such as the entire wing symmetry and bring things as close to neutral alignment as possible within a "group". I have had very good results with this procedure and factor into this the Up/Down Control travels. You can’t do this with an arbitrary measurement to one specific control setting.
Another OWT I hear is rigging the Ailerons & Flaps "Up" will increase cruise speeds. For one, there is no way to accomplish this on any Cessna Flap System. Secondly, if you did this alignment alteration to the Aileron System there are several other complications as a result including non-conforming control movement travels. It is possible mechanically to accomplish this in some extent however you will have non-conforming deflection travels concerning the TCDS and Aircraft certification standards.