Some time ago Peter Germann told me that there is a difference between theory and practice: Large flap deviation down  in theory would cause a downward pitch, but in practice it helps to  give upward pitch.
My explanation woud be that the main effect of a flap is not downward pitch, not increased lift and drag, but it increases the angle of attac of the elevator. Drawing:


To test this, this winter I built a twinboom model with only one flap. 58 years ago I already did:


The new one is called Gemini:


Only three flights until today, but I like the flying, especially the sailing!

The airplane can be started with my iPhone, the microcontroller has acceleration sensors, rotation sensors (not used), and a bluetooth antenna.


If interested, I could post a video of the last flight.

Comments welcome.

Regards,

Wolfgang

Some time ago Peter Germann told me that there is a difference between theory and practice: Large flap deviation down  in theory would cause a downward pitch, but in practice it helps to  give upward pitch.
My explanation woud be that the main effect of a flap is not downward pitch, not increased lift and drag, but it increases the angle of attac of the elevator.

    Just from observation, sometimes it works like that, and other times, it doesn't. What it does suggest is that the damping is reduced around zero attitude and rate.

      Brett

What it does suggest, is that full span flaps do not make sense. The span should be somewhat larger than the stabilizer.
Do you agree?

   Not at all, and I am not sure how you come to that conclusion, particularly from this observation - it's interesting, but not the most significant issue.
 
     If you imagine that the flaps are there to add to the Cl, while not causing you to have inordinate hinge moments, you want the flaps to add Cl while contributing the least to the hinge moment. That seems to indicate you want the entire wing to have a flap, so you don't have a large Cl ad the root and low or nothing (or negative) at the tips. Short flaps also give you an AoA that varies wildly from root to tip, like having an inch of wash-out.

   With respect to your observation, I don't see a tremendous value in creating a stab AoA from downwash, when I could get the same thing without it. If you re correct, it has the effect of reducing the pitch damping around zero, which may be beneficial from an initial acceleration standpoint.

     Brett



   

there is a difference between theory and practice

Right, and practice IMHO says that the smaller the flaps, the higher angular rate in corners. That is why we need CG moved far forward on flapeless models,  why I limited flap movement and flap size in my MAX serie to get tighter corners models and why we do not use flaps on indoors. And that is probaly also why I crashed when my elevator pushrod disconnected on my small .15 model when I was kid (I just remember i gave up handle and model turned hardly down).

So what is says about theory? (and which theory) :- )))

The Gemini has a center section chord of 250 mm (10 inches)- The flap chord is 80 mm ( pi inches ), which is 32 %.

But the cornering is normal, as can be seen at the attached video of the 3rd flight (taken by a bystander).


Regards,

Wolfgang