Gentlemen,

 

<8===V=^=======v=

 

Where:

< is the spinner

8 is the prop

V is the CG

^ is the wing center of lift (Cl)

v is the inverted wing (h-stab) center of lift and

= is the fuselage and, most importantly, it is flying from right to left.

 

So, in level flight at a constant speed, the “h-stab“ is exerting a negative lift (v) to balance the airplane against the snapshot CG (V) and the current wing center of lift (^). 

 

At first, let us keep the conversation strictly to the 320/360 Lancair (it’s the only one upon which I can perform experiments) with the small tail, short engine mount.

 

The horizontal stabilizer is not symmetric – it is an inverted wing.  In my airplane the angle of incidence is about  negative 1* (negative to the longeron, not the main wing angle of incidence which is a positive angle(about 2.5* in the flap area and washes out to about 1* at the aileron) to the longeron).  The Aspect Ratio of the small tail is 3.45, similar to a jet fighter wing, but not as low as a flying saucer. Low AR wings induced drag is sensitive to the lift load.  Induced drag increases with a decrease in speed.

 

The main wing NLF (Natural Laminar Flow) flapped airfoil has some interesting drag reduction, lift and pitching moment characteristics – especially when the reflexed flap is moved out of reflex. Thanks to Ian for pointing to the NASA technical paper describing the wing with tests at .1 mach (about 64 kts).

 

Suppose we have a 53# weight distributed on the seat next to me and the header is full with about 10 gallons in each wing for a gross weight of 1645 pounds.  For my plane, this would result is a CG of about 25.2 or 17% MAC.  If I moved the weight to the back of the baggage compartment at about FS 84, the CG moves back to 26.7 or 20% MAC.  According to popular thought, this would reduce the negative lift load on the tail (and, consequently, the “weight” the main wing was carrying), the induced tail drag and thus allow the plane to go faster.  Note that if the h-stab negative center of lift is located 7 feet further back, the weight movement is equivalent to about 20#  less exerted on the tail and 20# less the wing has to carry.  The questions to be answered are:

 

1. With the expected trim change, does this reduce the angle of attack of the main wing and increase the AOA of the h-stab even though both are carrying less aerodynamic weight (above example is 20#)?
2. Is this significant enough to be reflected in an increase in speed?
3. If the speed increases, is the main wing AOA further decreased?

 

Relative to query 3 above, we all note that as speed increases, more nose down trim is added.  This means less negative h-stab lift, less aerodynamic weight on all the wings still attached.  If you think upside down for a moment, the extreme nose down trim at max cruise speed is actually beginning to put the h-stab wing flap, the elevator, in reflex!  This is, of course, reducing the negative lift on the h-stab. 

 

Why does the pitch change occur?  Must it be due to the speed related pitching moments of the main wing with the flaps in reflex i.e. nose up, tail down.  Or, is it a change in the center of lift (center of pressure) closer to the CG?  If it is a change in the Cl towards the CG, that would reduce the load on the tail.  If we reduce the load on the tail enough, moving the CG rearward may have less of an effect on the tail download, or maybe more leading to potential instability?   Should we install Jim Frantz’s AOA on somebody’s tail (upside down, of course)?  Do I see any hands raised?  Volunteers to calibrate it?

 

In any event, the experiment is set up with 53 pounds of weight as above.  A digital level is affixed parallel to the longeron and my airplane is equipped with the digital AOA.

 

In level flight (under auto pilot control, smooth air) at 8500 feet (Baro=30.19, temp=2*C), WOT 23.3” MAP, 2500 RPM, 9.3 gph, 74% power

 

175 IAS, 197 TAS AOA=2.4*, Longeron (L)=0* and

 

then the weight is transferred to the FS 84 Location. The auto pilot requests trim assist and the Reichel trim wheel is rotated forward (nose down) about 1/6^th of a turn.

 

176 IAS, 197 TAS AOA= 2.2* to 2.4*, L=0* to -.2*.

 

Leaving the weight in the back, the power was reduced to 21” MAP and 2300 RPM, 7.1 gph, 62% power

 

161 IAS, 180 TAS AOA= 2.7* to 2.9*, L=.4*

 

The weight was moved back to the front and the nose was trimmed up

 

161 IAS, 181 TAS AOA=2.7* to 2.9*, L =.4* to .6*

 

Conclusion: I won’t be abnormally shifting weight to try to increase speed.  Let’s see, during the Air Venture Cup Race, Mark’s child was heavier than Larry’s child (moving CG to the rear) yet Larry was faster.  I ran about as expected even though I had all my “stuff” in the back.  Maybe I am fast because my engine thrust line was shifted upward which reduces tail down load?

 

Other interesting numbers:

WOT @ 7500 feet 2700 RPM, 11.3 gph, 83% power

 

185 IAS, 204 TAS AOA=2.1* L=-.2*

 

Climbing out at 130 IAS (about 2000 MSL) AOA=4.1* L=7.2* AI=5* and,

Still climbing at 8000 MSL, 140 IAS AOA=3.8* L=4.6* AI=2*