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Design => Stunt design => Topic started by: Wolfgang Nieuwkamp on June 06, 2025, 08:56:15 AM
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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:
(https://thumbs-eu-west-1.myalbum.io/photo/1k0/27557d9a-5ceb-4f90-97a5-88c215465518.jpg)
To test this, this winter I built a twinboom model with only one flap. 58 years ago I already did:
(https://thumbs-eu-west-1.myalbum.io/photo/1k0/53ea2b30-0d1a-4f31-9c86-0623c81d7a9b.jpg)
The new one is called Gemini:
(https://thumbs-eu-west-1.myalbum.io/photo/1k0/8d673c59-3c9b-4ccc-8945-5c854d4c94a6.jpg)
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.
(https://thumbs-eu-west-1.myalbum.io/photo/1k0/1cc38bf1-3703-4cfa-9620-9b2e7b4a0695.jpg)
If interested, I could post a video of the last flight.
Comments welcome.
Regards,
Wolfgang
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Thank you very much, Wolfgang.
Your constantly demonstrated search for knowledge and innovation for a seemingly lost matter is a valuable contribution to our common cause.
Keep it up, dear friend
Peter
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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
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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?
Regards,
Wolfgang
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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
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That flow direction change is called downwash. I worried about it as a youth, along with other things I needn’t have worried about. Now as an old man I worry about flap hinge moment, which is roughly proportional to the square of flap chord, and which leads to long, narrow flaps.
I think the main advantage of stunt plane flaps is direct lift control: the whole airplane doesn’t have to rotate to change wing lift. That enables one to fly a path more accurately. A secondary advantage is less turbulence response for a given amount of lift. Flaps enable a higher wing loading for a given cornering capability.
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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) :- )))
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flap hinge moment, which is roughly proportional to the square of flap chord, and which leads to long, narrow flaps.
That is right, it is square of flap chord ... but only if you extend overall flap area. If you keep the same area and you extend chord at root and limit span to keep that aera constant, it is linear I think. But still something we do not want.
IMHO, there is necessary liitle concentration of flap area in the middle of wing at fuselage, but only because of limiting tip stall effects (rolling in extremely tight corners). That means wing tip to wing root ratio should be larger than flap tip to flap root ratio (including optional flat tap on outer flap).
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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).
(https://thumbs-eu-west-1.myalbum.io/video/480_h264/8161c4c8-7ff8-4b78-947b-57378b0ada5d.mp4)
Regards,
Wolfgang
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" sometimes it works like that, and other times, it doesn't "
When you exit the wingover inverted , and the pushrod breaks aft , the first time you apply control it lifts the nose . The second time the flaps work as elevators , so you create a trench where it slides in ,. S?P H^^
Ahem . Snapped when ( the plane ) was level . AT the exit , level . nose started dropping , mild lift to level . nose started dropping , BOTH Flap & Elevator observed way down . Aircraft did imitation of plough furrow .
Unfortunately I didnt observe as acutely the first control input , but I belive it was flap down & elevator trailing even , then dropped a few degrees - which is when the nose drooped again .
Probly the same day Urtnowski got hold of the throttle on Mr Zambilli's plane . The funny graunchy noise from the engine , was the split at the rear bend in 3/32 wire Elev. Horn . Irvine 40 was fair tramping .