It is sometimes necessary to dig further into apparently "simple truths" to reach maximum enlightenment ;<)
When additional horsepower increases elevator effectiveness, precisely how could trim authority not concurrently and similarly increase? After all, what we are talking about is an increase in airflow over the "...angle the elevator makes with the elevator chord line (see Ercoupe Service Memorandum No. 25)...". If the efficiency of either type trim system is increased, it would logically follow that less "trim input" under power is necessary for a given result. There is NO data of which I am aware that establishes any power-on difference in actual trim effectiveness that even suggests the separate tab design is sufficiently more effective than the earlier "trimmed elevator" that a change from one system to the other is logical or desirable. We also need to consider the actual difference in horsepower available in flight between the 75 hp and the 85 hp configurations. Ercoupe Service Memorandum No. 37 and 37A present propeller performance data between the standard wood Sensenich 74FC48T and the McCauley 1-A-90 that is most common today. "A typical production Ercoupe was used loaded to a gross weight of 1260 lb. for all of the tests. The instruments were calibrated, and the air conditions were very close to standard for the duration of the tests." The "typical production Ercoupe had 75 hp. The"74" Sensenich was turning at 2600 R.P.M. at a maximum speed of 121 MPH. The 73" McCauley was turning at 2500 R.P.M. at a maximum speed of 121 MPH. If one goes to the Continental Owners Manual and the Altitude Performance Curve for (both) the C75 & C85, it is quite clear that at 2600 R.P.M. more than 85 hp is being produced by the 75 hp engine. At 2500 R.P.M. approximately 83 hp is being produced. There would likely be a 2 hp variation in the maximum power output between production engines! This is the first indication that the 85 hp "improvement" in performance is a myth. Fred Weick wrote the book on propeller design in the thirties, and I happen to own the copy that was once in the Technical Information Center of the Lockheed-California Company in Burbank, California. It explains in detail why the longer 73" and 74" fixed pitch props are MORE EFFICIENT than the shorter ones specified in the Type Certificate for the C85. Ercoupers today mostly have the shorter props and 85 hp modifications. It is understandable that they are reluctant to believe original ERCO performance figures, because to do so they would have to accept that the reduced performance of their present configuration is the result of following Continental's requirements in order to modify the 75 hp Continental to an "official" 85 hp. They have been persuaded give up the steak for the "sizzle"! The ERCO discussions of the trim systems largely concern themselves with glide performance and not with full power performance. Don's trim system is discussed at length in Ercoupe Service Memorandum No. 19 of 4/26/46 (date from ESM No. 38). It says: "...the natural trim speed in a glide with the controls and trim device disconnected is now approximately 70 mph, a speed at which the airplane can be maneuvered to a satisfactory landing by throttle alone". "The use of a movable trim tab will not affect the feel of the controls". "Flight check should show a glide speed of 68-70 mph with trim unit set at "nose up". Ercoupe Service Memorandum No. 25 pertains to the "new" trim system with the separate trim tab installed on Ercoupes Serial Nos. 1623 and up with the 75 hp engine. It states: "...the angle the elevator makes with the elevator chord line trims the ship for a glide, power off, at 58 mph and a 55 mph climb with power on." Accordingly, it would appear that any difference in "trim authority" at full power would be of little, if any, significance in normal use. At full power, it is "nose down" trim that is needed. Every trim system ever fitted had more "nose down" trim available than could be effectively used. Regards, William R. Bayne . |-(o)-| . (Copyright 2010) -- On Aug 29, 2010, at 20:11, John Cooper wrote: > On 8/29/2010 5:19 PM, Donald wrote: >> I suppose someone smarter than I decided upon it, but I fail to see >> why the additional 10 hp requires a trim tab. I sure have not missed >> having one so far, and am reluctant to install one.. > Additional Horsepower increases elevator effectiveness and thus > requires > more trim authority to accomplish the same task. > > -- > John > Skyport East > www.skyportservices.net
