Typhoon v Rafale Question

[CoffmanStarter]

Good afternoon all …

OK … so my somewhat battered edition of Kermode (1972) isn’t too helpful on the topic of canards/foreplanes (no duck jokes please), so I thought some of you aerodynamicists might be able to help.

I came across a couple of cross sections recently of the Typhoon and Rafale which sparked my curiosity. Broadly speaking both aircraft are similar high performance, inherently unstable, designs benefiting enormously from the magic of FBW to achieve phenomenal maneuverability. In fact there is a degree of striking physical similarity in design other than the obvious use of canards/ foreplanes.

http://i1004.photobucket.com/albums/af162/CoffmanStarter/TPhoon_zpsa878bf2d.jpg

Typhoon Cross Section

http://i1004.photobucket.com/albums/af162/CoffmanStarter/Rfl_zps99800669.jpg

Rafale Cross Section

I was, however, surprised to see that the Rafale has a marked Dihedral of the canard/foreplane as opposed the Anhedral of the Tyhoon. I understand that pitch control is provided by symmetric operation of canard/foreplanes and wing flaperons, while roll control is primarily achieved through differential operation of wing flaperons.

So my question … Why the need for either Anhedral or Dihedral on the respective canards/foreplanes ? What is the aerodynamic advantage ?

As I say just curious ...

Best ...

Coff.

[Eclectic]

I am no aerodynamicists.
But a guess.
Maybe this is to do with what the canard vortices are used for.

[gr4techie]

I'm not 100% about canards but from what I know about wings.
Dihedral (upward angled) wings contribute to stability. Often found on passenger a/c.
Anhedral (downwards angled) wings have more maneuverability. Found in museums on the Harrier.

Maybe with the Rafale, they wanted the back end to be manoeuvrable and the nose to be stable?

The physics are explained here http://en.wikipedia.org/wiki/Dihedral_(aircraft)

[Trim Stab]

I don't think you will ever get an "exact" answer to your question, as aerodynamic design has so many trade-offs and compromises, and even more so in supersonic aircraft that have static instability controlled by FBW such as Rafale and Typhoon.

As you probably already know, more dihedral gives strong spiral stability which is generally desirable in transport aircraft. Anhedral gives manoeuvrability in subsonic aircraft - one of the reasons the SHAR had massive anhedral.

In the case of two supersonic fly-by-wire aircraft, I suspect you would have to go deep into the original design objectives of the aircraft, and then interview at depth the engineers who set out to meet those objectives. The fact that the Rafale and Typhoon have (apparently) similar flight characteristics shows that there are several ways of reaching the same end result, depending on which compromises the designers choose. I wouldn't believe any "simple" answer to your question!

[CoffmanStarter]

Thanks Trim :ok:

Yes ... I understand the basic physics behind conventional wing anhedral and dihedral along with the need to trade off/compromise in order to optimise design/performance goals. It's just that such a marked anhedral on the Tyhoon canard/foreplane is perhaps easier to understand given the marked anhedral on the aft pitch control surfaces of such legacy mil FJ' like the Phantom, Harrier etc. against the complete reverse on the Rafale.

It's an interesting question ... Let's see what others come up with ...

Thanks again ...

Coff.

[thing]

Maybe with the Rafale, they wanted the back end to be manoeuvrable and the nose to be stable?

That's a fairly unique and interesting concept...

[Lord Spandex Masher]

Like a bendy bus!

[Albert Driver]

the back end to be manoeuvrable and the nose to be stable

Those French...

[cornish-stormrider]

Nose stable for not spilling of wine from lunchbox
Tail unstable for rapid surrendering and running away while waving white scarf???

Bonjourrrrrrrrr yer cheese eating surrender monkeys - said groundskeeper willie.
And yes, I do acknowledge the bravery of our chums across the water.

[LateArmLive]

Foreplanes...not canards!

[RedhillPhil]

If my (failing nowadays) memory serves weren't the foreplanes on SaaB's mighty Viggen level?

[CoffmanStarter]

RedhillPhil ...

Yep ... Viggen foreplanes appear to be level

http://upload.wikimedia.org/wikipedia/commons/5/53/Saab_VIGGEN_AJ-37.png

LAL ... Stood corrected ... Foreplanes not Canards :ok:

[safetypee]

I too would suspect that the vortex pattern is significant; re differences in fuselage cross section.
Rafale has some main-plane anhedral, so perhaps there is no need for it on the fore-planes.
Is there any difference in the fundamental control laws – aircraft axis reference vs fight path vector reference. IIRC Typhoon is the latter.

[NutLoose]

I would also think the proximity of the intakes would also come into play, as they actually are beneath the Foreplanes, unlike the Tiffie where they are below the fuselage. The cross section view does no show that well.


Photos: Dassault Rafale M Aircraft Pictures | Airliners.net (http://www.airliners.net/photo/1373686/L/)

[Rhino power]

Canard foreplanes... :ok:

-RP

[NITRO104]

Coff,
Rafale's anhedral wing is good when you wanna play dead (or surrender :}), quickly.
Ducklings take dihedral to blow where they are supposed to, that is, whenever they don't blow under the wings.
Typhoon is a completely different design and just because they both have tails on the wrong side of the aircraft, they are nothing alike really.

[fltlt]

Maybe someone can correct me if I am wrong. Memory says the reason for the Harrier anhedral was the Pegasus required a shoulder mounted wing and the max length of the outriggers decided the final angle.

[FoxtrotAlpha18]

I suspect the Harrier's anhedral may have also assisted with that aircraft's ground-effect hovering ability. John Farley?

[The Oberon]

Somewhere in the mists of time and Merlot, I seem to remember that the Phantom's tail was originally designed flat and was ineffective, it wasn't until the anhedral was added that the tail worked properly. Maybe something along similar lines ?

[LowObservable]

First you decide where you want the canards to be, in the vertical and longitudinal axes, which is a complicated ballet of size, moment arm, area ruling and visibility, even before you get to canard/wing interactions.

Anhedral and dihedral may have something to do with the interaction bit, but more importantly, you don't want large gaps opening up at small deflection angles. I talked to one of the lead designers on Gripen last year, and he said that he caught a lot of flak for making the inlets square (versus oval on Viggen) but it was for exactly that reason.

Also, the "dihedral" on Rafale's canards is sometimes a factor of viewing angle:

http://kovy.free.fr/temp/rafale/rafale_show9.jpg

The Toom's tail anhedral IIRC was to get it out of a bad vertical location relative to the wing (pitch up issues) and I believe the Harrier's anhedral had too do with avoiding excess roll stability, although JF may correct me.

[Guzz]

Maybe the designers just thought it looked cool to put the canards on at jaunty angles?

Guzz.

[sandozer]

Rafale dihedral not so obvious especially when the foreplane is not in streamlined position.
Rafale | Flickr - Photo Sharing! (http://www.flickr.com/photos/sandozer/9720895351/)
Also when loading this picture, noticed it has radar reflecting edges built into the trailing edge of the foreplane. Not immediately obvious but looking closer on the pic will show the characteristic zig-zag.

The radar reduced reflection edges | Flickr - Photo Sharing! (http://www.flickr.com/photos/sandozer/9741418839/)

[cornish-stormrider]

I must be sad and joining the old farts club but I am actually finding this interesting - purile and childish comments (by me) aside.....

do keep it up - I are learnding
wibble

[CoffmanStarter]

Glad you like Cornish :ok:

It would be good to have a steely eyed TP chip in ... not too much heavy duty maths though :ok:

[NutLoose]

http://www.pprune.org/tech-log/65099-dihedral-anhedral.html

http://www.pprune.org/tech-log/163977-anhedral-low-wing-aircraft.html

http://www.pprune.org/tech-log/120555-question-anhedral-airflow.html

[CoffmanStarter]

Cheers Nutty :ok:

[Courtney Mil]

You might find this quite informative.

http://www.ltas-aea.ulg.ac.be/cms/uploads/FlightDynamics02.pdf

:ok:

[ORAC]

Dassault Rafale analysis (http://defenseissues.wordpress.com/2013/08/24/dassault-rafale-analysis/)

[CoffmanStarter]

Many thanks chaps ...

Armed with a couple of cups of strong tea ... I've now read the material you guys posted ... what a fascinating topic :8

A special thanks to Courtney for the Wednesday afternoon maths workout ... which was a bit painful in places. :ok:

So in short summary ...

It would seem an important aspect of close-coupled canard/foreplanes is the production of vortices which interact/excite/energise other vortices (eg wing generated) to benefit, amongst other positives at sub/supersonic speeds, the max angle of attack achievable which in turn provides the aircraft with extensive post stall maneuvering capabilities. It would seem that the use of anhedral or dihedral applied to canard/foreplanes has a lot to do with positioning/optimising these vortices.

Some very complicated trade offs and compromises appear to be at work :D

For those interested ORAC's reference ... see "Aerodynamics" section which covers the topic in more depth (which I suspect is a lift from other work).

Best ...

Coff.

[The Helpful Stacker]

ORAC -After having a quick scan through that blog article my spidey senses are screaming Top Trumps, a copy of Jane's 'All The World's Aircraft' and a combat flight sim game of some sort are his primary reference material.

Some selected gems include,

"Wikipedia puts flyaway cost at 90 million....."

An in-line reference to Wikipedia. Sweet Jesus, how to destroy any academic credibility in one simple swipe.

"Typhoon has larger radar, but that doesn’t matter because noone sane is going to use radar in air-to-air combat anyway...."

Yeah I'm sure he is right, they're just more convenient to install than a big lump of concrete.:ugh:

[Trim Stab]

You might find this quite informative.

http://www.ltas-aea.ulg.ac.be/cms/up...Dynamics02.pdf


Not really - that is just basic undergaduate flight mechanics - it only covers non-compressible flow and does not even touch on FBW stability augmentation.

The link provided by ORAC is more interesting, but still a bit limited - e.g.:

"Mid-wing vertical arrangement is more laterally stable than low-wing arrangement, especially when combined with Rafale’s wide body. As too much lateral stability can cause severe Dutch roll and excessive roll response to lateral gusts, Rafale’s wings are designed with anhedral to reduce stability."

Any fule no that, so I am surprised they put that in the article.

If I am to stick my neck out, I would say that the Rafale designers have prioritised low-speed handling characteristics to allow it to land on carriers with reasonable safety margins. This has meant optimising the airframe for non-compressible flow - hence fairly conventional lateral stability augmentation by increasing anhedral. They have probably done some clever stability augmentation to take care of the compromises in other parts of the flight envelope, particularly supersonic flight.

On the other hand, Typhoon designers have possibly aimed to optimise performance in compressible flow. Anhedral is not desirable in supersonic flight as it worsens shock wave separation from the extrados of the wing exacerbating mach tuck, and drag. So they have possibly gone for a flat wing to give good stability and range in supersonic flight and used stability augmentation to improve handling in subsonic flight.

Just guesswork - you would really have to interview the software engineers to get a proper answer.

[CoffmanStarter]

Thanks Trim ... looking at your particulars ... is your discipline Aero Engineering ?

Thanks again for helping out :ok:

[henra]

According to the following paper there are only two locations for the foreplane that don't cause excessive interference with the main wing and thus deterioration of lift/drag ratio in a Delta Canard configuration:

Low and far forward or high and close to the wing.
It is stated that this was found out in years of Investigation prior to finalisation of the design.

You see exactly these two layouts in the Typhoon and Rafale, respectively:
Rafale has the canard close to the main wing. Dihedral serves to move the tip of the canard high above the main plane without messing up the fuselage shape/cross section.

Typhoon has the other layout: Low canard and with anhedral they move the tip down to the level of the main plane despite the chin inlet for the engines.

Dihedral/anhedral in both designs is not related to lateral stability. It is rather used to move the foreplane higher/lower matching to the longitudinal position of the foreplane without compromising general fuselage layout.

http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADP010499

ETA: The difference between both layouts points indeed as @coffman has stated to the design priorities:
The Close Coupled Canard of the Rafale will help to control airflow over the main wing at higher AoA and thus be helpful for low speed performance/lift which is crucial for Carrier operations.
Long Coupled Canard has some advantages at supersonic speeds as it allows for a more negative longitudinal static stability margin and allows the use of a somewhat smaller foreplane size.

regards,
henra

[CoffmanStarter]

Many thanks for your contribution Henra :ok:

[Trim Stab]

Henra - very interesting paper - would like to see an equivalent essay on the Rafale!

I thought that this passage was quite interesting:

"The demand for linear aerodynamics for the flight control system is always present with every aircraft design. However, there is also a view that, because the FCS is clever, it can cope with very non-linear aerodynamics. The phrase often heard was "never mind, the FCS will take care of it!" In the case of Eurofighter, the level of instability that was required for the overall aircraft was such that linear aerodynamics became a much firmer requirement, together with good aerodynamics from the flying controls. "

I have to say that I thought that FCS would be able to take care of any anomalies in the aero-design.

[NITRO104]

...would like to see an equivalent essay on the Rafale!
Perhaps not an equivalent, but does shed some light on the Dassault's pursuit for lift.
Download Rafale_demonstrator.pdf from FileFactory.com (http://www.filefactory.com/file/5wh2w5wmqah1/Rafale_demonstrator_pdf)

[Machdiamond]

I do not believe the Eurofighter foreplane anhedral is motivated by any particular stability criteria, in this case, but simply to stay away from the pilot field of view. The paper referenced by Henra alludes to it.

[Bevo]

I have to say that I thought that FCS would be able to take care of any anomalies in the aero-design.

Based on my involvement (management not engineering) with some unstable designs I believe a flight control “system” is limited by the slowest element in that system. Looking at the inputs to the FBW system you need to be able to see the change first in the sensors. Then the system needs to react to the sensor inputs. The computers need speed, but the limiting factor in many cases is the flight control actuators. Hydraulic actuators are not capable of instantaneous movement and in the pitch axis that tends to be the limiting factor.

Another limiting factor is the airframe structural rigidity. The inertial sensors providing inputs to the FBW computers are ideally located at the aircraft CG, however, there will still be some input from the structure bending under load. If the FBW system has very high gains to keep up with large amounts of aerodynamic instabilities the structural response can be large enough to affect the sensor inputs. In the extreme this “feedback” can cause the system to diverge (not good). It is also very important that the actuator mounting and connection to the flight control surface be as rigid as practical as it can have a similar effect.

(Based on the subject of spelling and grammar in some threads on this forum I apologize in advance for the “American” spelling and any other grammatical errors others may find offensive.)

[John Farley]

Why are aircraft the shapes they are?

Somebody mentioned the Harrier.

Senior design chappie to a Hawker’s ideas man- late 50s: “You want to put an engine that is how big on the CG? Well it will need a very big hole to get it in and out. Said hole will have to be on top as the fuse will need strengthening round the hole and you might as well use the wing as a nice strong cover plate.

The wing will have to have sweep for the speed you want, so the rolling moment due to sideslip will be too big ‘cos of the sum of the sweep effects and the high wing low CG setup. So standby for some anhedral to keep the L&D gust response sensible (you did say you want to aim this thing at targets didn’t you?)

With the wing so far off the ground a pair of tricycle main legs will be stupidly long and heavy and will have to retract into pods as the wing is the only place for fuel thanks to this bloody engine you started with. As for those hot rear exhausts what do you want to use for tyres? Use a bicycle gear and it might work. Tough tit about the reduced braking effect - I thought you said it was a VTOL so who needs brakes?

Don’t forget to keep the outriggers short enough to not hit the ground until the main leg has taken the landing loads. If you don’t want it to lean after the main leg expands following touchdown well fit the main leg with an oleo that has no rebound (I’m sure you can stuff the oil somewhere until it is needed for the next landing). Go away I’ve got work to do.

Foreplanes

As to why the foreplanes on the OPs aircraft are like they are I have no insider knowledge but I would be pretty sure that the designers had two main aims:

1 Improve the wing aerodynamics - especially at high AoA by feeding it helpful vortices

2 Help longitudinal control in general and especially in unstable parts of the envelope

As to whether they have anhedral or dihedral remember they have to be positioned pretty carefully with respect to the wing because of point 1 above. If at that particular height on the side of the nose it is not easy to provide pivots and actuators then stand by to move the attachment point up and down a bit thus needing anhedral or dihedral to achieve a compromise relationship with the wing.

Aircraft design is full of such compromises and as with sex one thing always leads to another.

[CoffmanStarter]

Many thanks John for taking the trouble to help :ok:

Now wouldn't it be interesting to do a "fly off" between both aircraft ... but I doubt that will ever happen.

Best ...

Coff.

[safetypee]

Coff, the problem with ‘fly-offs’ is to agree what might be compared and in what circumstances.
The aerodynamic comparison has been very informative, but so might an operational requirements comparison, adaptations during development, or just the compromises made – ‘one thing always leads to another’.
Or would you like a (realistic) speculative comparison on future developments and operational roles.
Realistic; who would have imagined a Lightning flying 30mm guns ground attack.