Hi Guys,
I think I'm adequately "on record" in the several threads we've had touch on this subject as having concerns similar to those Lynn and Gregg expressed. That said, common sense tells me to go over the issue in the very finest of details when my "belief" is the opposite of Ed's!
In an earlier round, I suggested maybe Ed's plane was not truly rigged at 9º up limit; a suggestion Ed positively denied as being possible.
There are some other possibilities that came to mind reading these latest exchanges, primarily because I BELIEVE Ed observed and documented to his satisfaction that which he says. So maybe it's possible for everyone to be "right"?
Let's examine some "proper" glide speeds.
ESM 19 discussed the trim system effective serial numbers 213 and up (actually only to 1622, all Model Cs ). It explains that the system used on earlier Ercoupes had "...no reserve method of longitudinal control in case part of the elevator system becomes disconnected (and) the aprplane trims at a speed of approximately 130 MPH, which is obviously too high for a safe landing." "With the new trim...the natural trim speed in a glide with the controls and trim device disconnected is now approximately 70 MPH..." "The present method (through serial number 212) provides that the glide speed be established by using the trailing edge of the elevator as a fixed trim tab. It states that the "full up" elevator position is plus or minus one degree in describing "...thirteen degrees (Twelve to fourteen degrees allowable range). A rigging procedure is described that should yield "...a glide speed of 68-70MPH..." with "nose-up" trim. Note: These Ercoupes did NOT have movable trim tabs...the elevator was trimmed
ESM 25 discussed the (new) movable trim tab installed effective serial numbers 1623 and up (actually only to 2622, all Model Cs ). With the trim control full forward (and, presumably full power), "...the ship is trimmed for...130 mph" (sorry, Ed, the factory did get that big fat wing going that fast!) With the trim control full back, "...the airplane is trimmed for a power-off glide at 58 mph and a full power climb of 55 mph.
ESM 38 discussed the trim system fitted to Ercoupes 2623 and up (all Model CD's). With the trim control full forward (and, presumably full power), "...the ship should be trimmed for...115-125 MPH" (sorry, Ed, the factory did get that big fat wing going that fast!) With the trim control full back, "...the airplane is trimmed for a power-off glide at 65-75 MPH and a full power climb of 60-70 MPH."
ESM 35 specifies the "...allowable tolerance for elevator up-travel is +1/8" (which would presumably extend to later models in the absence of factory data or recommendations to the contrary). Of great interest is the revelation that "Effective about Ercoupe No. 3882, the elevator trailing edge has been bent downward 3º on production aircraft. It was found that this modification makes it easier to bring the airplane within (to?) the desired glide characteristics. (See Service Memorandum No. 19.)"
ESM 35A stated that the power off MINIMUM airspeed is 60 MPH with the "E" Model "split elevator".
ESM 55 explained the necessity of being able to retard airspeed to 60 MPH in flight. Failure of the trim tab wire on a 415-C or CD (and, presumably a D model) could result in elevator flutter and stopping the flutter required slowing to approximately 60 MPH.
Now, what variables can we throw in here? Well, most everyone has added 12 pounds right on the nose with a metal prop. If you look at the coupe as a big "free-flight" model (as the factory certainly did by incorporating downthrust and sidethrust to achieve desired power-on characteristics and by bending the elevator trailing edges to achieve desired power-on climb and power-off glide characteristics), extra nose weight increases glide speed (as the c.g. goes forward). While our planes left the factory weighing just over 800 lbs., most weigh 850 lbs. and up today. Increased weight also increases glide speed (from that possible at a lighter weight) for increased lift.
So what, you say? Well, if our mechanics were "on top" of the situation, when a metal prop went on, the trailing edge of the elevator might have required tweaking to maintain desired flight characteristics. The same would be true as weight goes up. Trim wires break (or seize) and are replaced...how many mechanics are even aware of these parameters to which they should test (annually!)? I'd like to hear from as many of you as possible about stabilized glide speeds (at idle, which can and should be tested at 4000' or so) and your model, c.g. and weight.
I know that I preferred to land with "cruise" trim retained, personally "flying" the full flair to scrub off excess landing speed just clear of the ground, and now recall that there just wasn't much in a "D" to flare with. Truth be known, flying the VASI in a coupe (which I did more than I like to admit) is almost certain to keep you from making the runway if the fire in the engine suddenly goes out.
On the other hand, I could have dialed in full nose up trim and pushed the nose down to maintain my speed until ready to "scrub it off" in the flair. This would have made limited "elevator" movement more effective in the flare. Alternately, I have "flared" to a slower touchdown by moving the trim tab to the rear once I ran out of elevator. Unwilling to risk settling in sewage settling ponds or an involuntary trip through the chain link fence separating them from the runway, I steadfastly avoided the nose-up "low and slow" descent maintained with power!
There remains the considerable difference between the speed a ship will glide, power off with full nose-up trim at a steady rate of sink, and the speed it lands at once that sink is arrested, and all elevator and trim movement available have been used to slow down prior to touchdown. I don't have any way at present to test the "D" flare-touchdown speeds at the two trim positions described, and would prefer no one risking bent metal in experimenting with lowest landing speeds!
Regards,
William R. Bayne
<____|(o)|____>
(copyright 2002)
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On Jul 8, 2004, at 12:12 PM, Ed Burkhead wrote:
Jeeze, Lynn, there’s no way the 13 – 9 degree change can make a 20 mph difference in touchdown speed. With my D, the touchdown speed was around 53-55 mph compared to a C model’s 47-52 mph touchdown with 13 degree up travel.
Anyone who can’t get their D model down well below 60 had better do some airspeed indicator repairs.
-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]
Sent: Thursday, July 08, 2004 12:04 PM
To: [EMAIL PROTECTED]; [email protected]
Subject: Re: [COUPERS-TECH] Model C to D conversion paperwork
If it was me, I would go ahead and modify the elevator to the later version. I think the Coupe handles so much better when landing at 50 mph rather than 70.
Lynn
