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Bruce Tuncks

Reflexing flaps and ailerons

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8 minutes ago, M61A1 said:

I'm pretty sure that the man made equivalent of feathers is a set of Fowler Flaps.

Ahh, but I want heaps better low speed control than Fowlers can deliver!

8 minutes ago, M61A1 said:

 

 

Edited by Old Koreelah

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1 hour ago, Old Koreelah said:

Ahh, but I want heaps better low speed control than Fowlers can deliver!

Sounds like you need a helicopter.

Do you have VGs on your Jodel? I found they made a significant improvement in the low speed handling of the 601.

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 Yep, sure do. They help, but I doubt they reduce stall any more than three Kts. They only delay the inevitable.

At stall, wool tufting shows air flowing forward over the inner half of the wing!

Edited by Old Koreelah

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35 minutes ago, Old Koreelah said:

 Yep, sure do. They help, but I doubt they reduce stall any more than three Kts. They only delay the inevitable.

At stall, wool tufting shows air flowing forward over the inner half of the wing!

What type? Where are they positioned?

I went (using JG's Stolspeed VGs at 7% of chord from LE) from 38kt indicated power off stall down to around 30 indicated. Hard to tell exactly what it is now as is doesn't really break before I run out of elevator authority. But the what I thought was the best improvement was the increase in aileron effectiveness, particularly at lower speeds.

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Wow, M61, that's a big improvement! I guess it's largely because of the 601's fat wing, which seems to be designed for slow speed. I've seen some Zenith's with an upside-down aerofoil for a tailplane; is yours like that? Couldn't work out why it's like that.

 

I have the same sort of VGs, fitted at about the same location, after a couple of redesigns.

My wing is much thinner, so I guess the VGs have less effect. I did add 10mm to the chord, but I can't say it made much difference.

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Tailplanes lift downwards, old K. Years ago, on a record attempt, this glider was oversped at the start-run and ...Bang, the tailplane broke with the tips going down. Being made out of fibreglass, the thing held together and was landed safely.

The reason tailplanes lift down is because the main wing has a downward pitching moment. And yes this creates drag, which is smaller for a longer fuse than a shorter one.

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Yes Bruce, I've had that explained to me, but most tail feathers seem to be symmetrical aerofoils, relying on slight AoA to generate whatever lift is required.

 

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Going back to the original "reflexing flap/aileron" question - my Zephyr has both flap & aileron (not flaperons) reflexed. In the Zephyrs case it's only about 4 degrees for the aileron (can't remember the flap but similar). I understand this is quite a common arrangement, supposed to give slightly better cruise performance. I have no intention of experimenting with the factory rigging specifications so have no observations of performance comparison with/without reflex.

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Hi All,

One reason the gliders have negative flaps is so that they can take away some pitching moment from the wing and unload the tailplane ( which because of it's low aspect ratio is the

highest drag generator on the glider)! With powered aircraft this is overtaken by the engine cooling drag and the undercarriage drag so as a percentage of the total drag  so it is less effective.

A more effective way is to use an all flying tailplane ( about 10% more efficient than a traditional tailplane).

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 Down load generated by the tailplane at low speeds has the same effect as putting the same extra weight on the wings PLUS any extra drag generated by the tailplane, producing that down force.. Nev

Edited by facthunter

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14 hours ago, Raytol said:

Hi All,

........................................................................................................................................................................................................................

A more effective way is to use an all flying tailplane ( about 10% more efficient than a traditional tailplane).

Interesting comment - I dont have the knowledge to discuss the 10% but I understand that although "all flying tail planes" have been around for just about ever, in small aircraft they have had operational problems. That's why we still mainly use elevators despite the drag penalty.

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Put simply, they can be too elastic as a structure and too touchy in feel. A moving horizontal stab is a very satisfactory answer. Some planes have minimum  trim change with speed change others have quite a lot. Trimming out of stick force is needed for comfort. Having authority over pitch is more a necessity. Nev

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All flying tailplanes have been used in gliders, the Cirrus, the Janus and the Schneider Boomerang are just 3 examples my club has owned.

In all cases, the tailplane was replaced with a conventional elevator setup on the next model. Like the Boomerang was superseded by the Super Arrow, the main difference being that the Super Arrow had a conventional elevator. Some people even did a mod themselves. Balaklava club modified their Mini-Nimbus to a conventional elevator.

 

On the Janus, you were supposed to notice some characteristics of the all-flying tailplane, and I pretended to in order to get converted. But most of the time, there seemed no difference to me. Occasionally, the glider would do something strange like need a bigger stick movement than you would expect.

Maybe I'm just not good at noticing nuances.

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One of the major (external) changes between the Foxbat and Vixen is the flying tail in the Vixen.

Combined with an overall more aerodynamic fuse, 10% improvement for the tail seems about right.

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On 04/10/2019 at 11:02 PM, M61A1 said:

What type? Where are they positioned?

I went (using JG's Stolspeed VGs at 7% of chord from LE) from 38kt indicated power off stall down to around 30 indicated. Hard to tell exactly what it is now as is doesn't really break before I run out of elevator authority. But the what I thought was the best improvement was the increase in aileron effectiveness, particularly at lower speeds.

The ACTUAL stall speed is different to what the airspeed says, the VG's help control with low speeds and typically increase AOA near stall. There is vitually no difference in stall speed with VG's, only slightly increased control feel at slow speed.

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Glenn, VG's affect boundary layer control in a good way and would have to lower stall speed if they are working and increasing stall AoA.  Nev

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7 hours ago, Glenn1 said:

The ACTUAL stall speed is different to what the airspeed says, the VG's help control with low speeds and typically increase AOA near stall. There is vitually no difference in stall speed with VG's, only slightly increased control feel at slow speed.

I can tell you for a fact that there is a significant difference on my aircraft, and yes I'm aware that indicated airspeed at that AoA is not accurate.

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On 04/10/2019 at 10:07 AM, Old Koreelah said:

That low-speed control issue is why I'm designing an outer wing based on feather-like extensions that allow massively changes in surface area as speed changes. Birds have it, so why shouldn't we?

You mean something like on this Stemme motor glider?

 

D9C2C085-4D5E-4EFF-8C1F-61F966E6EDCF.png
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I don't think it was a big an advantage as what they hoped otherwise it would be on every glider by now! The shape of this experiment is not correct.

The tip feathers on birds are deflected so as to use the tip vortices or reduce them. They use a cascading effect to add stability especially at high Coefficients

of Lift for landing. Each feather has it's own sensing system and can swing and rotate at the same time even when they are highly loaded ( deflected).

A possible engineering solution would be to have say 5 tip winglets that can swing and rotate to the same effects.

Maybe we need to strap a Wedge Tailed Eagle to the front of a glider to see what happens!

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On 21/10/2019 at 5:51 AM, Bruce Tuncks said:

What an interesting experiment! That Stemme is now quite a low aspect ratio plane. Did those tips work as hoped?

From my readings, yes, they worked very well.

 

Acceptance would be the issue: "they look funny   " etc.

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