About a year ago, I acquired a partially-built DE project and am working to complete it. In the process, I've developed some questions, each of which I will post separately. Here's the first one: what DE horizontal stabilizer incidence angle relative to the fuselage has been found to work well? The plans don’t say, so I’m looking for input from those who have built and flown DE’s.
First, a little of my background. I'm an engineer and since the late 1960s, I have built, designed & built, or restored more than 20 different airplanes and have had some experience with formal flight testing. Additionally, for the last 18 years I’ve been an FAA DAR certifying EABs, SLSAs, and ELSAs and conducting FAA-accepted ELSA Repairman courses. In short, I don’t want to present myself as an expert, but I do have a fairly deep knowledge of the subject.
Discussions of stabilizer incidence angle are intimately tied up with weight & balance considerations. I’ve read all the weight and balance information on this site and found some of it to be very good and very useful, but some to be misguided. So, I want to present a discussion of how stabilizer incidence angle, weight & balance, aircraft control, and aircraft stability are intertwined. So, here goes my long-winded post:
AIRCRAFT STABILITY
To have good longitudinal stability, the aircraft’s loaded center of gravity (CG) location must be forward of the wing center of pressure (CP) location at all angles of attack up to stall. That produces a nose-down effect that must be balanced by a downward force on the tail. (It is not intuitively obvious why that should be the case, but it is true).
The downward force on the tail is produced by the stabilizer/elevator combination. That means the stabilizer/elevator must be operating at a negative angle of attack. Oftentimes the stabilizer is mounted on the fuselage at a negative angle of incidence, but that is not universally true because with some designs, the airflow off the wings is defected downward enough to create a negative angle of ATTACK even though the stabilizer may be mounted at a zero angle of INCIDENCE relative to, say, the top fuselage longeron.
The most efficient (i.e., least drag) stabilizer angle of ATTACK is produced when, in a cruise configuration, the downward force on the tail exactly balances the nose-down CG/CP (i.e., the aircraft is in “trim”) with the elevator in trail with the stabilizer.
WEIGHT & BALANCE
As we all know, the loaded CG location is very important for good flight characteristics. Over the 100+ years of airplane design, a general rule of thumb is that:
1) If the loaded CG is aft of about 20% of the wing chord, there will likely be enough elevator power to rotate on takeoff and flare for landing (even with the engine/propeller at idle).
2) If the loaded CG is forward of about 30% chord, there likely won't be stall recovery or flat spin problems.
That means a good assumed CG range for an airplane that has not been extensively flight tested would be 20-30 percent of the wing chord. I've built and flown several homebuilt airplanes and I've always tried to make sure the loaded CG is right about the 25% chord position for my first few flights.
For a Double Eagle with a wing chord of 55 inches, the resulting CG range would be 10.8 to 16.2 inches aft of the leading edge. A couple of references on this forum have quoted 14" aft of the leading edge (which calculates to be 25.5% chord) to be the most aft limit, with anything forward of that being okay. However, other references point to the fact that because of heavy engine installations, builders often experience problems having adequate elevator authority for flare for landings. One builder reported that in a dead-stick emergency, he did not have adequate flare authority even at 50+ mph airspeed; that would be expected if the loaded CG were anywhere closer to the leading edge than about 10.8 inches.
I also read about builders incorporating more negative stabilizer incidence to try and correct a nose-heavy condition--in my opinion, the wrong approach, because it introduces extra drag and may result in decreased elevator authority at low airspeeds during the landing flare—some builders reported having to produce a propeller blast over the tail to successfully flare for landing.
So, with that admittedly longwinded explanation, does anyone have any response at to what stabilizer angle of incidence has worked well?
Thanks!
Mike
