Reflex flaps and ailerons

[Bruce Tuncks]

Apparently you can get a few knots more airspeed at cruise if you adjust the flaps to be in reflex, that is with the flap trailing edge raised 20mm or so.I think I saw this posted from France.

 

This could easily be done, and the new positions of flaps would be :

 

zero=raised

 

+1= "no flap"

 

+2= landing flap,that is down flap as = +1 was before the change.

 

This has always seemed to me to be a bad idea because the ailerons, even though they are short, are not in agreement with the flaps, and in cruise they will be drooped , so this part of the wing will be making more lift than it should, and this is aerodynamically and structurally dreadful.

 

So what is required is a mechanism to raise the ailerons to line up the trailing edge at cruise.

 

Or I guess you could just raise them so they were always like that and live with the increased stall speed and take-off distance. You would be taking off with the flaps down and the ailerons raised in reflex. I wonder how big these effects on stall speed and take-off distance would be.

 

Yes I know that this stuff would be illegal on an LSA, but I'm still curious. My guess is that the benefit of increased cruise speed would not be worth the downside effects, or others would have done this long ago.

 

This brings us back to the mechanism idea, where the aileron trailing-edge would be raised so the whole trailing edge was in line.

 

For many years I flew a glider which had this feature, and it worked fine.

 

.... Bruce

 

 

[Geoff13]

Not unusual in some of the plastic fantastics that I have been looking into. Appear to be common in some European models.

 

Cheers Geoff13

 

 

[jetjr]

I do recall discussing this somewhere and someone tried it on a Jabiru, no effect was seen. Hazy memory though

 

 

[Guest Maj Millard]

Reflexing flap and aileron is one of the few 'free rides' in aviation. Reflex flap on a draggy old Lightwing is good for a free 5-6 Kts in cruise. You can also line- up the ailerons (reflex) for no aparent effect of the aileron effectiveness. It's a simple drag reducing tool.......with the surfaces streamlined with the rest of the wing the airflow coming off the top surface wants to go down, not straight back.......this is induced drag.

 

When you reflex trailing edge surfaces up the air coming off the wings is forced to depart straight back...therefore reducing or eliminated that drag....it's really that simple.

 

Anyone who has studied many of the very early airfoil shapes will see that they incorporate reflex into the rib shape at the trailing edge. Many of those early wing shapes came from people studying bird wings, many of which also incorporate reflex.

 

 

[AVOCET]

We tried reflex on jabs years ago .

 

What i noticed when the ailerons were reflexed was that in turbulence it would make the control colum shudder with gusts , pretty obvious what was going on . Jabs have a bit of slack in the comtrol cables ( push pull )

 

 

[Guest Maj Millard]

We tried reflex on jabs years ago .What i noticed when the ailerons were reflexed was that in turbulence it would make the control colum shudder with gusts , pretty obvious what was going on . Jabs have a bit of slack in the comtrol cables ( push pull )

That's a Jab problem not a reflexting problem.........maybe you had too much .

 

 

[Guest Maj Millard]

In this photo of my Lightwing at the bowser at Clifton you can clearly see the reflex on my ailerons relative to the flaps. ( the left aileron is up a little more than the right. - stick not centered). I had just flown direct from Gunadah at 8000' or higher with consistent 95+ groundspeeds.

 

When these flaps are put on the GR wing you cannot physically reflex the flaps any further up. On the GA wing which is where the come from, they can be reflexed up about 6 deg in cruise.

 

I reckon I increased my cruise speed overall about 4 kts on the Standard old GR large wing. It's more noticeable in smooth air at high altitude. 4 Kts may not sound like much, but on a trip Townsville -Temora and return it does make a difference.

 

The Europeans know all about reflex...all fancy sailplanes have it, and many of the new plastic fantastics out of Europe have it, some up to 11 deg capability.......Maj....

 

 

 

[bexrbetter]

Reflexing flap and aileron is one of the few 'free rides' in aviation.

Is it not because you are pitching the wing back therefore reducing some of the energy required for the HS to hold the tail down?

 

 

[peter]

Reflex flaps with normal ailerons work well on my Cobra, subtle variations also achieve trim control,

 

Peter

 

 

[Camel]

That's a Jab problem not a reflexting problem.........maybe you had too much .

Maj, Yes a Jab problem, if the ailerons are reflexes there is more movement in one than the other, if you place aileron in neutral level position or down there is little play in cable, put the aileron in up position and there is play in the cable due to angle, so if one is level and other up the up one will vibrate due to play in cable.

 

Avocet, how far did you adjust them up and find vibration ?

 

 

[Guest Maj Millard]

Also noticable in this photo.......

 

Is it not because you are pitching the wing back therefore reducing some of the energy required for the HS to hold the tail down?

Don't think so BB..the wing is the same until the airflow departs the trailing edge............

 

 

[Head in the clouds]

Is it not because you are pitching the wing back therefore reducing some of the energy required for the HS to hold the tail down?

Yes, you are right, the change in the pitching moment caused by the reflexed airfoil shape results in less downforce required from the HS. Consequently the reduction in total lift requirement is what reduces the induced drag, not the effects of a very localised change in the downwash angle.

 

The total downwash volume is reduced by an amount commensurate with the lift reduction and the downwash angle is also reduced by that very small amount - not by an angle anything like the amount that the flaps or ailerons are reflexed. The control surface effect on the flow angles is very localised and does not extend to the 'larger picture' i.e. 300mm, 500mm, 1m or 2m above/below the wing. The overall downwash angle is virtually unaffected by the reflexed surfaces, except to the extent caused by the pitching moment/lift requirement change.

 

 

[Dafydd Llewellyn]

For what it's worth - aircraft that need to be able to fly slowly - which includes all ultralights that have a restricted stall speed, and gliders, which have to be able to turn with a small radius of turn, to stay in a thermal - have highly cambered wings. A highly cambered wing wants to turn nose-down - in technical terms, it has a negative pitching moment. That pitching moment increases in proportion to the square of the speed; so the download on the tail to balance it also increases in proportion to the square of the speed, which generates considerable structural loads. The tail load also increases the tail induced drag, and since the wing has to carry the tail load as well as the weight, it increases the wing induced drag a bit, too.

 

If the wing uses a laminar-flow airfoil (as gliders do), there is a second effect; laminar airfoils have a reduced drag over a limited range of angle of attack; this is sometimes referred to as the low-drag "bucket" because of the shape of the lift/drag curve. Outside that "bucket", laminar airfoils usually have a higher drag than the conventional airfoil on the jabiru etc. Reflexing the trailing-edge surfaces moves the low-drag bucket, and can therefore increase the usable speed range in which low drag occurs.

 

Both these things can be a reason for the designer to use "variable camber" via symmetrical movement of the flaps and ailerons. Doing so helps keep the fuselage close to its minimum-drag angle. However, the total performance gain is relatively small - and it greatly complicates the task of certification, because all possible geometries have to be investigated.

 

An example of this was the Pik-20B glider, which had a crank-handle on the left of the cockpit to change the wing camber. The sensation that gave the pilot was spectacular - on leaving a thermal, one did not shove the stick forward to gain speed, one wound the crank, and the airspeed shot up with no perceptible change in attitude - it was almost as though you were cranking the ASI needle around the dial. However, the actual performance was really much the same as its contemporaries without this feature; it just felt faster. Most current gliders use it, to squeeze the last bit of performance - tho I wonder whether it's more a case of squeezing the last dollar from the consumers.

 

 

[AVOCET]

It

 

Maj, Yes a Jab problem, if the ailerons are reflexes there is more movement in one than the other, if you place aileron in neutral level position or down there is little play in cable, put the aileron in up position and there is play in the cable due to angle, so if one is level and other up the up one will vibrate due to play in cable.Avocet, how far did you adjust them up and find vibration ?

was a while ago now , from memory want much , theres not that much adjustment without having to move the outer cable clamp position .

Ive found that the ailerons feel firmer if there set slightly down , a mm or 2 .

 

Not reflexed .

 

What i have found is that winglets on the j250 wing has given me 4/5 knts increase .

 

 

 

[bexrbetter]

You do some lovely work Mike.

 

Thanks HITC.

 

 

[AVOCET]

The left and right hand winglets arnt exactly mirror image , sculpting plugs by eye and a blunt sharpie pen.

 

When the 3 d printers up and running i plan to experiment with a few winglet shapes.

 

notice ive made these winglets removable .

 

Its amazing the difference in slow speed aileron control , which if you remember was a contributing factor in my prang .

 

Mike

 

 

[Bruce Tuncks]

I like the look of those winglets too. And I reckon its a good idea to make them easy to adjust. On a glider I know ( an ASW27) the winglets plug-in vertically to the wingtip, and you can see a joggle where they adjusted the winglet toe-out by a few degrees.

 

Now you guys have got me wondering about winglets on my SK. 4 knots would sure be a worthwhile increase, Maj is right about that.

 

But my understanding was that winglets improved the low-speed handling at the expense of high-speed performance. This is true I think with gliders, where the term"cross-over speed" has been used to describe the speed at which winglets begin to detract from the performance.

 

Now a Jabiru wing sure differs from a glider wing, and maybe the cross-over speed is above the cruise speed... How would an ordinary person determine this? I guess you can only do your best with the design, especially that toe-out angle, then put them on and see if you like them.

 

... Bruce

 

 

[AVOCET]

I like the look of those winglets too. And I reckon its a good idea to make them easy to adjust. On a glider I know ( an ASW27) the winglets plug-in vertically to the wingtip, and you can see a joggle where they adjusted the winglet toe-out by a few degrees.Now you guys have got me wondering about winglets on my SK. 4 knots would sure be a worthwhile increase, Maj is right about that.

But my understanding was that winglets improved the low-speed handling at the expense of high-speed performance. This is true I think with gliders, where the term"cross-over speed" has been used to describe the speed at which winglets begin to detract from the performance.

 

Now a Jabiru wing sure differs from a glider wing, and maybe the cross-over speed is above the cruise speed... How would an ordinary person determine this? I guess you can only do your best with the design, especially that toe-out angle, then put them on and see if you like them.

 

... Bruce

Hi bruce , hope your over your wog ,

Joelle lost her vioce on friday ,

 

I mentioned it was unusually quiet this weekend , then she pointed out the tv wasnt on .

 

Any way , when the 3d printers up and running , i plan to experiment , as one does , with wing shapes and different tip sections ,

 

I havnt noticed any decrease in cruise , if anything its seems to get to 95 knt at lower revs , still early days yet and i still haven't wound the Avocet out ,

 

I went to go for a fly yesterday , after replacing a couple of coolant lines ,

 

Taxied out and green coolant ran off the turret and through the door-seal on to my shoulder .

 

I figured that it was some residue spillidge , which is was ,( after taxiing back to the hanger )

 

To windy again to day ,

 

Cheers Mike .

 

 

[albert997]

Pipistrel Uses 5% negative flaps in cruise on their Virus model for additional speed....

[skippydiesel]

9 hours ago, albert997 said:

Pipistrel Uses 5% negative flaps in cruise on their Virus model for additional speed....

As does all ATEC aircraft. Unfortunately I have no data on with/without the slight negative set of the flaps - I just set them to factory specifications, which results in a negative/reflexed flap on 0/nil flap setting.

 

Both Pipistrel Virus SW & ATEC range have class leading high cruise and very low stall (an unusual combination). The low stall of these aircraft "puts to bed" the idea that reflexing flaps causes a deterioration in stall speed.