Newbie Flying wing aerofoil question.

[mightypeesh]

Hi ladies and gents. Finally popped my sloping cherry yesterday, when a club mate took me up to the slope at Folkstone in Kent yesterday after a bit of flying at the patch. All I can say is WOW!!! I would not have believed that anything could fly in the howling wind up there, let alone be controlled. He lent me his Wild Thing, and after a couple of false starts I was up and away and loved every second....... Ok I admit it I am hooked, and the hanger is due to expand a bit more!

My question is this. How important is the type of aerofoil for slope soaring wings?

There were a number of wings up there, most were Zagi type ones from various makers, and obviously the wildthing that I was flying. Now the Zagi's have a very slim and elegant profile with some reflex built in (from looking at google) and the Wild Thing's aerofoil is pretty much a standard, but lardy looking affair in comparison. both seemed to fly well, though I am guessing that the Zagi's would be faster. They both have fins, be them at the tips or in the middle. In theory could you take pretty much any aerofoil as a wing, and as long as the CofG was in the right place and it had a fin added? I know they would perform differently, but would they fly?

I hope this is not too confusing or daft a question,

Mant thanks, Simon

Ps. Did I say how much I enjoyed slope soaring??!!!

Edited By mightypeesh on 29/08/2014 10:37:24

[J D 8]

A flying wing to be stable needs reflex or sweep back,most have a bit of both.I fly my Zaggi without fins [ just like it that way] and it soar's just fine although it can get a wobble on some times.A straight wing with no reflex would be unflyable,a full delta does not need any reflex. For more info look up Northrop flying wing's

[Delta Foxtrot]

Try this for a simple explanation of the role of reflex and sweep on longitudinal stability:

**LINK**

If you use a section without reflex or a swept wing without twist you will need to feed in some up elevon trim to make the wing stable.

Edited By david fillingham 1 on 29/08/2014 12:33:30

[Andy Meade]

Ah! We have converted another! Welcome, Brother Simon, to the Church of Slope.

..and John pretty much nailed it there I think.

[mightypeesh]

Thanks John, David and Andy. So if I understand correctly after my fast track tutorials the Wildthing must have reflex built into the profile as it is a 'plank' type thing with slightly tapered wings, and the Zagi type wings fly because of the combination of reflex and sweep. and those foam board type designs fly because of the swept wing as they have a flat underside, and a plank made in the same way would fail because of lack of reflex in the aerofoil? (bit of a long sentence there!)

Also in the link and in your reply David 'twist' is referred to- can you explain it in this context - is it like adding washout in a conventional aircraft? Also you refer to feeding in up elevon on a unswept and unreflexed profile. Does this mean that if you had say full length elevons on a traditional wing you could in theory use the control surface as the reflex section of the wing by dialling in some up elevator?

Thanks for your answers gents, it was my first flying wing as well as first slope experience yesterday, and I like to have an understanding of what is going on aerodynamically as well as doing the flying stuff, so all of you input is great. I also love to build my own stuff, so at least I will have half a chance if equipped with a bit of knowledge!

Cheers, Simon, the newly converted sloper.

[Andy Meade]

You would indeed use the complete full length elevon to dial in reflex, as needed, on the theortical unswept, unreflexed profile. There are some out there like this, but the tend to not be the fastest aircraft in the world, and can be very pitch sensitive. In fact, pretty much every flying wing out there is quite pitch sensitive - having a tail dampens this in a conventional layout.

[Delta Foxtrot]

Simon,

Some thoughts that might be helpful:

If you think of a straight wing in isolation, with a cambered aerofoil section, at the aerodynamic centre there will be a lift force and a nose down pitching moment. To achieve trim in pitch where lift = weight the CG has to be aft of the aerodynamic centre so that the L/W couple balances the pitching moment. This arrangement however is unstable as the lift force is ahead of the CG therefore any small pertubation in lift will result in divergent behaviour in pitch. You can see this by trying to fly a rectangular piece of paper where the CG is at 50% chord, it spins.

Adding reflex to the straight wing will in effect change the direction of the pitching moment so that to achieve trim the CG will have to be forward of the aerodymanic centre so that the Lift and weight couple is equal and opposite to the aerofoil generated pitching moment. This configuration can be trimmed and statically stable as increases in lift will result in a restorative pitch down force etc.

Adding a tail to the first example allows us to trim the wing at various CG positions forward and aft of the aerodynamic centre (providing the CG does not get too close to the neutral point). This is done by varyng the down force provided by the tail lift to balance out the residual couple from lift/weight and pitching moment. The also contibutes to the stability of this configuration, if pitch is varied it provides a corrective force proportional to the tails moment arm and area.

The arguments in the previous paragraphs can be applied to the swept wing, think of the aft sections of the wing towards the tip as behaving like the tail on a conventional layout. With low levels of sweep reflex or tip twist will be needed and stability in pitch will be reduced.

And yes the tip twist will have the benefit of suppressing tip stalls which will induce an unwelcome pitch up to make a bad situation worse.

hope this helps and if I have any of this wrong someone wil be along to correct me

Dave

 

Edited By david fillingham 1 on 29/08/2014 14:58:23

Edited By david fillingham 1 on 29/08/2014 14:59:12

[mightypeesh]

Thanks Andy. Thanks David that clarified and embellished what I gleaned from your link earlier - must admit I got a bit lost in the maths bit on it though, but I think I am all clear on the basic theory of what is going on. Going back to the zagi type designs I have seen I guess that that is why the elevons taper out (get wider) towards the tips, creating an artificially greater reflex and amplifying the moment of the design. (Is it called 'moment' if you do not have a tail? surely it has a theoretical one even if you cannot measure it with a ruler)

On a slight diversion from flying wings and planks would adding reflex to a conventional airframe therefor i.e. straight wing with vertical and horizontal tail behind mean that you could use a smaller tailplane in a design with a shorter boom and still have good stability, or have I now lost the plot completely with the theory?

This really is interesting stuff, thanks for your time and considered answers, Simon

[Andy Meade]

Simon, yes it's still a moment - it's a force acting at a distance from the C of G. I've flown similar wings that have parallel elevons on that sort of planform, so no idea about the zagi's swollen tips - it seems to work though! There will be a point on the elevons that the force can be assumed to act as a total (MAC of the elevon I guess) and this will have a fixed distance back from the CG.

Tail volume is typically a function of projected wing area, so I'm not sure if the theoretical case you describe with reflexed ailerons would help to use a smaller tail area. I know from using spoilerons that reflexing the ailerons does usually result in quite a pitch up, with the added benefit of less likelihood of a tip stall but more drag for slower landings.

[Simon Chaddock]

Simon

Although some form of aerodynamic reflex is required for stability in a 'wing' it is working at a relatively short distance from the centre of lift. It therefore has to have a larger effect (and thus creates more drag) than it is was a long way back.

Look at modern high performance glider. A relatively small tail plane (compared to the wing!) a long way back on a long thin fuselage.

The designers do know what they are doing. It is the most efficient way of creating the required stability forces.

Having said that slope soaring can require a good degree of manoeuvrability to keep in the areas of lift so a compact less efficient airframe can actually do better.

[Mark Kettle 1]

Hello mightypeesh - you will find out more if you go on the web under 'John Roncz' either web or you tube

one of his designs with short tail boom - **LINK**

[Delta Foxtrot]

Posted by mightypeesh on 29/08/2014 15:42:26:

Thanks Andy. Thanks David that clarified and embellished what I gleaned from your link earlier - must admit I got a bit lost in the maths bit on it though, but I think I am all clear on the basic theory of what is going on. Going back to the zagi type designs I have seen I guess that that is why the elevons taper out (get wider) towards the tips, creating an artificially greater reflex and amplifying the moment of the design. (Is it called 'moment' if you do not have a tail? surely it has a theoretical one even if you cannot measure it with a ruler)

On a slight diversion from flying wings and planks would adding reflex to a conventional airframe therefor i.e. straight wing with vertical and horizontal tail behind mean that you could use a smaller tailplane in a design with a shorter boom and still have good stability, or have I now lost the plot completely with the theory?

This really is interesting stuff, thanks for your time and considered answers, Simon

Don't worry too much about the maths so long as you get the general idea and can empathise with the forces and moments involved.

On the slight diversion the answer is basically no! Adding wing reflex will change the tail moment requred to balance the aircraft at a given CG position, but you will still need the same tail area to get the same level of stability (assuming the moment arm and wing area and chord are kept the same). You can trade off tail moment arm and area, wing area and chord so long as the tail volume coefficient is maintained = Tail Area x moment arm / (wing area x chord)

As you are interested in this stuff and perhaps not too mathematically minded I can recommend a good book which you should find very helpful

**LINK**

regards

dave

[mightypeesh]

Thanks for the all info and links gents, the book is ordered - thanks for the heads up on that too Dave. I have a much greater understanding of aerodynamics than I did yesterday morning thanks to all of you on here that took the time to reply to my questions, and I look forwards to reading more when my book arrives.

Cheers again, Simon

Now, onto 'Simon's flying wing mk1' ...... pass the scapel please...........

[Delta Foxtrot]

No problem Simon!

it is great that you are taking the trouble to understand the principles of flight and stability. It is another facet of the hobby that some people are not interested in, but if you are it enriches the experience when you work this stuff out and it will pay you back when you come to design and test your own creations.

Good luck

dave