Originally Posted by hpergm

To try to re-adjust the climbing tendency of the plane, a first action was to give a bit more downthrust to the firwall, however, this may not be sufficient to give a good all-round solution - decalage modifications should also be combined; for sure, more downthrust will only cause a side-effect of ballooning on landing upon throttle cut - been there done that. The solution is to check what's going on with the decalage as well.


So, the plane was false-assembled for a quick measurement with the trusty Robart incidence gauge:
- Fuz was set at 0 degrees, using the top engine box plank as a datum (here, I assume this is parallel to the main "stringer" of the plane, which would be used as a datum line if this was a kit instead of an ARF - hope I'm right!).
- Engine was @ -2 degrees (as modified - see previous post).
- Tailplane seat was @ +1.5 degrees.
- Bottom wing was measured at +4.0 degrees.
- Top tank/wing was measured at + 5.5 to 6.0 degrees.


Now, this means that there is a positive decalage of 5.5/6.0-4.0 = 1.5 to 2.0 degrees.


If, at flying attitute, we assume the stab to fly at 0 degrees, then
- The fuz will fly at a -1.5degree nose down stance (which "sounds" about right for a TM)
- The effective downthrust will be -1.5 +(-2.0) = -3.5degrees (a generally accepted angle for lightly loaded scale planes)
- The bottom wing will fly at 4.0-1.5 = 2.5 degrees
and
- The top wing will fly at 6.0-1.5 = 4.5 degrees.
- Decalage is then +2.0 degrees.


(Have a look at the attached sketch).


Question now is, at what flying speed will the plane trim out with such decalage. To me, after seeing these values, the ballooning effect is a certainty - plane will climb at idle! Also, take into account that the effective AoA of an undercambered airfoil actually is positive at zero degrees. This further exacerbates the effect.


A fair bit of Googling around (i.e. literature survey) for decalage suggestions from both full-size and scale builders has indicated the following facts:
- Positive decalage is used to incur a benign stall. This has to do with the downdraft of the top wing affecting the actual AoA of the bottom wing (increases it). This pushes the bottom wing to stall first, "hanging" the bipe only from the top wing which is still flying. In a positive stagger design, this will induce a pitch-up (i.e. unstable post-stall behavior). By increasing the incidence of the top wing, the designer makes sure this stalls first (and makes for a "mushy" stall).
- Negative decalage is used to promote sharp stalling, usually used in aerobatics.
- Full size Tiger Moths are rigged in different ways for different purposes. For aerobatics displays or cross-country flights, pilots rig the plane with zero/negative decalage to increase the neutral-trim-cruising-speed and/or to allow for nice clean break upon spinning. For pleasure/training flights, the plane is rigged with positive decalage for increased safety margins in the stall.
- Many modellers have radically improved the behavior of their planes by using negative decalage.


Clearly, the H9 designer has decided to make the plane benign and compromised it by allowing a very small cruising speed (i.e. idle). Anything else, its a homesick angel.


One final snack for thought.


We have all seen the error in the H9 manual regarding the CG values - a pretty basic piece of data for the plane and one that can destroy a new plane. Pretty bummer from H9.


Now, consider the piece of info regarding the mounting of the top wing tank from the same manual (page 16, see attached pdf). If somebody made a mistake on the CG, then it is wildly possible that also this little step has been mixed-up. Just as a check - if one swaps the 235mm strut with the 230mm strut (front-to-back), the top tank sits nicely (without any stress) at +2.5 degrees, instead of +6.0. This changes the decalage from +2 to -2.5 degrees.


With this modification, at a flying attitude (i.e. stab @ zero, fuz @ -1.5), the plane will "sit" as follows:
- Total effective Downthrust -3.5 degrees
- The bottom wing +2.5 degrees
- The top wing will fly at +1 degree.
- Decalage -2.5 degrees.


I am willing to make a bet that this solves most of what we perceive as ballooning (i.e. down-trim at a slow cruising speed) while allowing for crisper aerobatics...


My plane will take months to complete - Easter maybe, so we will no know until then.


Is anybody willing to quickly try this strut swap and see what happens to the behavior of the plane? Worse case, it will need to hold a bit of up on finals and landing and possibly snap a bit easier if pushed!