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 and messed around enough with the "eccentric cams" on the "Strutted Model Aircraft" to compile any "real data" that would suggest this is a true or accurate statement. Regarding speed gains by drooping or otherwise altering eccentrics in any way outside of producing equal Wing Lift as close to a neutral state as possible does not seem to make any significant gain. I have done a bit of this "lift reduction" during standard climb characteristics with the "neutral Trim" on the 182P However, getting an Aircraft to fly straight is always the priority. After rigging properly it nearly always picks up performance so after that there is not much further change that needs made in my opinion to give any significant speed gains above the book performances that can be obtained with the eccentrics 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” Trim Tab at the Elevator/Stabilizer with all streamlined.

I have 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. Unfortunately I never did get any good feedback as to actual numbers 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 and picked up a tremendous speed gain just by alignment of controls and eccentrics (about 10-11KTS).  

I don't know that between the initial rigging and the latter adjustments 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 these 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 speed" again. I usually see some very mis-rigged Airplanes so the focus is to make them right.....

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 as 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 a Cessna Flap System and secondly if you did this on the Aileron System there are several other complications as a resuly including non-conforming control movement travels however it is possible mechanically accomplish this to some extent.