Dihedral in flying wing design?

[David Hall 9]

Thinking of an own design flying wing that could be used for F5J thermal comps in the 2m class. There are one or two out there that look to be credible airframes... might be fun to try something in Depron and ply (composite?).

I wondered if some dihedral might help stability, but I don't recall having ever seen a flying wing that incorporated dihedral.. is this not feasible for a flying wing?

Dave...

 

Edited By David Hall 9 on 29/11/2017 16:34:46

[Peter Miller]

I have never designed a flying wing but a couple of thoughts cross my mind

Most flying wings have quite a lot of sweep back on the leading edge. This works in the same way as dihedral

I see no reason why you can't incorporate a little dihedral as well ifnyou want to.

There used to be a rule of thumb about how many degrees of sweep back equal how many degrees of dihedral but I can't remember what it was.

[Dave Bran]

All my own design aerobatic flying wings and deltas (and that is a lot), whether wings are swept or "plank" in plan, have the two wing halves glued together upside down, so the tapering section from root to tip creates some dihedral.

[Frank Skilbeck]

The full size Fauvel AV36 had 2.5 deg dihedral on the outer panels, https://scalesoaring.co.uk/VINTAGE/Documentation/AV36/AV36.html

Flying wings are good fun but not the most efficient, one I have (I didn't build it thought) is 2nd on this video, at around 1:38 https://www.youtube.com/watch?v=EGkoz4oHvjc

[Piers Bowlan]

Swept flying wings achieve lateral stability by virtue of their sweep. Longitudinal stability is achieved with washout. If you add dihedral in addition to sweep, dutch roll will likely become a problem due to the increased yaw/roll couple. The greater the sweep the more laterally stable they tend to become, hence some actually incorporate anhedral to counter dutch roll. These generalisations don't just apply to tailless aircraft (flying wings) but also to conventional aircraft that are highly swept. Highly swept airliners normally incorporate yaw dampers as well as other aerodynamic fixes to counter dutch roll.

The AV36 is an unswept flying wing (a plank) which has some dihedral for lateral stability instead of sweep. For longitudinal stability it uses a zero pitching moment aerofoil that is reflexed, plus a forward C of G.

I have built several swept and unswept flying wings over the years, not always with success!  

Edited By Piers Bowlan on 29/11/2017 18:27:26

[Peter Miller]

I must be suffering from old age.

I said that I had never designed a flying wing.

Not true, I did design a non swept aerobatic model based on the ideas of an American designer,

The model was Montezuma's revenge and published by Traplet

[MaxG]

This site may be of help.

http://www.b2streamlines.com/

Maxg

[Piers Bowlan]

Montezuma's Revenge.

[David Hall 9]

Thanks for your comments. I had to look up Dutch Roll and figure that i don't want any...

So, a flat wing possibly trying some small dihedral in the outer panels if I'm not happy with the lateral stability, Starting point is to add generous sweep angle as it's not intended to be a particularly manoeuvrable plane...

Thanks for your help... I have a further question about airbrakes for a wing, so will make a new thread. ..

[Denis Watkins]

Use Reflex David

For stability

Both Ailerons up 1mm

[Erfolg]

I have built a few wings.

This is a 2m model. It was intended as a preliminary design for a 100s, for the BARCS leuge, in the unorthodox section, so I could get more flights in per comp(in the days of 35 meg, ensuring I had a crystal clash).

It flies OK.

I have built a plank, which was a disaster, in that it it did not thermal. The section chosen being to thick, draggy (camber line) although looking semi symetrical.

One of the issues is line behaviour. My plank towed far better, the swept wing, in various versions, did not tow well, and all were very different with the same plan form.

My canard was the same. This was rewinged into an electric model of 2m from a 100s concept.

Off the line all were controllable.

For good glide all need thin section wings.

[Dave Bran]

Posted by David Hall 9 on 30/11/2017 17:47:06:

Thanks for your help... I have a further question about airbrakes for a wing, so will make a new thread. ..

I have only once needed to add speed brakes, this was to a pod front delta wing. I fitted a servo which projected two rods each side out of the pod fuselage and pushed tape hinged plates out either side close to the CG.

This was quite effective , but not really needed generally as most of mine were powered, and if you run a prop slowly, it causes a lot of braking drag.

[Harry Twist]

Hi Just thought I would chip in with my recent experience with a scratchbuilt flying wing. I recently built the model shown below - its a 58" wingspan PSS slope soarer (Horten 229) mainly in correx. It has now completed many flights. I did a fair bit of reading beforehand before deciding on the wing layout . Basically it has a little of everything!

Around 20mm dihedral each at each wingtip, around 4mm washout and flies with around 2mm reflex on the elevons. It occasionally demonstrates a little Dutch rolling but this is mainly induced by me when I am trying to fly in conditions that are a little on the light side for the model or when I try to fly it too slowly (hanging on the up stick too much!).

In good conditions (say 15mph plus ) no dutch rolling is ever evident.

P.S. Not saying this would work for all flying wings though and as I think about it I cant recall seeing dihedral on flying wings elsewhere......just my get out clause!

Anyway just my 2pence worth of comments.

[Erfolg]

My canard, is completely flat winged. In turns, when travelling fast it can exhibit wing rock. Not sure I would call it Dutch rolling. Although not a wing, initially directional control was appalling. Eventually controlled by very much bigger end plates.

Although, a very much reduced span than the 100s, it can thermal, in quite modest lift.

My swept wing, does thermal, an astounding ability with a rearward CG. The downside was that it would enter a flat spin, if handled carelessly or suddenly powered up. A pussy cat with the CG forward, the stall being a nodding, no stall to speak of.

[Piers Bowlan]

If you have a generous sweep angle it should be laterally stable, adding dihedral to the outer panels as well may not be beneficial. I have seen swept flying wing designs with anhedral on the outer panel. Don't forget some fins on the wingtips or midspan for directional stability. Reimer Horten always maintained that 'the wing was enough' but then his wings usually featured ailerons and spoilers (drag rudders). I have found fins to be necessary so I am very impressed by your Horten 9 Harry.

On an unswept wing (plank) reflex is essential and the C of G should be well forward, around 20%. On a swept wing reflex should not be necessary if there is sufficient washout, although I usually end up adding a small amount anyway.

I agree that thinner sections seem to work better with tailless designs. That is just a subjective observation on my part. It is fun experimenting.

 

Edited By Piers Bowlan on 30/11/2017 21:54:52

[Harry Twist]

Hi Piers, I agree with your observations. I should add that I have flown the model with and without small removeable fins just outboard of where the mock engines are.Overall I reckon they do help with stability.

[Mark Kettle 1]

 

Ludwig Prandtl 1875 -1953 did some research and recorded it in the 1920's -and in his 1933 paper some great research came out and this has something to do with current 2017 aerodynamic thinking and findings. Prandtl, he is often referred to as the father of modern aerodynamics

Hi David Hall 9, I bet you are wondering what the above has to do with your 'dihedral on a flying wing' question? Well a little dihedral can be put on your flying wing and it will cause no problems. To prove this point at the end of my thread entry I’ll put a linked video on that will show a model flying very well with dihedral plus the video may help you with your design and give some fresh ideas, especially if you want your model to find thermals and soar well.

Before I type any more I’ll call out a ‘caution’, everything you thought you knew about aerodynamics is about to be tested and you will be surprised and amazed with ‘not new’, but ‘re-found’ Prandtl fundamental principles that are currently being used on a new flying wing for aerodynamic research and at present it is all being tested on radio controlled airplanes.

When a flying wing is controlled in a turn, the roll into the turn can manifest a un-wanted yaw effect, however the design team you will see have yaw control from the tip elevons that is propulsive and reduces drag at the tips turning into more thrust, yes I said more thrust.

Design problems and ideas visited by the team from NASA lead by Albion Bowers who is the chief scientist at NASA Armstrong are listed below as -

The main key point was to lower drag, minimum induced drag for a flying wing,

• Yaw becomes a propulsive, called proverse yaw
• Birds don’t have a spin problem so design this into the flying wing.
• Structure of the wing much like birds, like the Horten brothers past design wings.
• Improvement on the Whitcomb winglets idea / design
• Control roll and yaw without any drag making device on the wing tips.
• Geometry of the plan form gives a forward C of G at the 12% point with no trim change
• Why birds don’t have vertical tails, copy this and it will reduce drag on real aircraft.

And for anyone who is interested in research of advanced aerodynamics you must watch this video, Called the Prandtl Project.

 

 

 

 

Edited By Mark Kettle 1 on 30/11/2017 23:46:2

[Mark Kettle 1]

David I'm always interested in flying wings and love getting them to fly better, I hope the video helps with your design.

Apology for the advents in the middle for propellers, flights sims, transponders and aircraft batteries.

Avro Vulcan model that flies nice in varied wing speeds.

 

Avro Vulcan model that flies nice in varied wing speeds.

 

Edited By Mark Kettle 1 on 01/12/2017 00:16:55

[Tom Thomas]

Posted by Erfolg on 30/11/2017 18:38:28:

I have built a few wings.

Erflog, I've had my eye on one these for a while now, did you leccy it up yourself?

[KiwiKid]

I have been enjoying this discussion as I am a bit of a fan of flying wings. They can be a versatile model being great fun for park and club flying, thermaling and slopin' - sometimes all four

Here's a few of my wingy things to show the variety that can be enjoyed - they all have sweepback to varying derees.

Zeta Phantom with wing extensions

3.2m "expanded" combat wing

Horten Amerika bomber

Blohm und Voss high altitude fighter

Zupair Z1 (with B2 removable motor pod)

Zupair Zulu

Own design Sloper

Own design sloper

 

Edited By KiwiKid on 01/12/2017 04:50:38

[Piers Bowlan]

If you watch the lecture by Albion H Bowers, in my next post, this is how the 'proverse yaw' is achieved by using non-linear wing twist. (extract from Bowers US patent).

'This invention meets these and other objectives related to more efficient aircraft wing design by providing an aircraft that produces adverse yaw control without a rudder. This is accomplished by a wing design having a span load that changes from downwash to upwash at a location from about 60% to about 80% of the way from the aircraft centerline to the aircraft wingtips. Such a design comprises a total wing twist percentage of about 10 to about 30 times the aircraft's design lift coefficient. This total wing twist is nonlinear such that from about 10% to about 35% of the total wing twist occurs from the aircraft centerline to about halfway to the wing tip and the remainder of the total wing twist occurs along the remainder of the wing.'

See lecture below!

Edited By Piers Bowlan on 01/12/2017 05:51:35

[Piers Bowlan]

Prof. Albion H Bowers explains 'proverse yaw'. Fascinating lecture for AMA. Here is an extract of the US patent that explains the non linear wing twist to create the bell shaped lift distribution and proverse yaw. Looks like it may be tricky to build!

[Mark Kettle 1]

Hi Piers I watched these video's some months ago and like you found them fascinating !

Early this year I already started on making a building jig to make a wing with twist in for my own use.

 

And below this model made from cardboard that flew with a included twist and it's built on jig board with the twist build in.

No dihedral mind on this one. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Edited By Mark Kettle 1 on 01/12/2017 06:41:06

[Engine Doctor]

The Zagi by Avicraft had a small amount of dihedral built in as wings were joined on  a flat surface/table with top of wing flat on table. The taper of the wing section gave it the dihedral on the bottom surface.

Edited By Engine Doctor on 01/12/2017 09:49:15

[Dave Bran]

Posted by Mark Kettle 1 on 01/12/2017 06:37:55:

No dihedral mind on this one.

No, anhedral due the wing taper with washout.