Decalage - Good or Bad?

[Guest]

Decalage: The difference in incidence between the upper and lower wings of a biplane.

Lots of designers have some decalage on their biplane designs. The reasons given are usually pretty feeble to my way of thinking, especially at model sizes.

Usually, biplanes which are only equipped with ailerons on the lower wing have the top wing set at a higher angle than the lower wing. The reason given is that the top wing will stall first (true) so leaving the lower wing unstalled and therefore maintaining aileron control, as (theoretically) the lower wing is still flying.

In reality, when the top wing stalls and stops producing lift, the entire weight of the aircraft is suddenly transfered to the lower wing, which, as you would guess, stalls instantly on being burdened with all this extra weight.

Personally, all my bipes have no decalage, and others that have come into my possesion, have flown better for having the decalage taken off. One in particular, had some three degrees plus of decalage and was a very pitch sensitive machine with unpredictable stalling habits. On removing the decalage the machine was transformed and became a sweet handling aircraft.

So why do designers persist in using it?

[Guest]

cool post wowbagger

dont s`pose you could shed dome more light on wing stall or even wing design`s loading etc , for us layman

keep up the posts m8

Adz..

[Guest]

On no you don't! Aerodynamics is far too contentious a topic to get into wrangles with other people about. Too much dogma and not enough common sense! Anyone who sets themselves up as an expert is asking to be shot down in flames! :cheesygri

I have my methods, won by hard and painful experience, and they keep me out of trouble, but I'm not going to debate them on here. Nevertheless, some things that are accepted as the right thing to do are, nonetheless, puzzling, like decalage, which is why I made the post.

[Guest]

no no no you dont get of that easy

only kiddin m8.

i was looking for more info on the difference between high wings and lo wings . especially when beginners are recomended high wing aircraft. to do with wing loading ???

and clarify what "wing stall" is

and difference in characteristics in flying between different wing designs.

but if it`s too open a minefield then no probs

[wonwinglo]

Re: Decalage good or bad ?

Like a lot of things in aerodynamics things are not quite as cut and dry as they at first appear,decalage has been used on biplanes since they were first designed however things are not quite as straightforward as they may first appear ?

For our subject we will take the humble little Tiger Moth,a machine which I am very familiar with as back in the sixties I used to rebuild them,quite a lot of knowledge was acquired about rigging these little beasties over that time,for example did you know that there are about a dozen ways to rig a Tiger ? if you used them for spraying crops which they did do by the way,then you would need to incorporate more decalage ( more about why later ) for air racing especially the Tiger club machines very little upper wing incidence was required as speed was the essence,so they sacrificed the incidence for more speed ( a bit more about this later ) if you wanted to get the best from each aeroplane then you needed to rig her then fly her,every aeroplane just like our models would fly in a very different way.

Lets take a general look at the rigging of a biplane so we can understand better why designers see fit to incorporate a few degrees more on the upper surface as opposed to the lower wing,the tail is normally set zero incidence in relation to the aircraft datum line ( bear with me a while here ) everything that determins the rigging of any aircraft revolves around that datum line,the first thing you do is to trestle up the machine lay a large straight edge across the datums and drop a plumb line,now and only now can we commence to set up the relativity of the two wings in relation to the known datum,however instread of thinking of this as two seperate wings as wowbagger has we must think in terms of collective lift of the two surfaces at their optimum flying position in relation to everything else in the complex set up,when Mr De Havilland designed the Tiger he was like all designers trying to get the very best from his design,this like all design criteria means compromises,lots of them,so we now begin to look at the aircrafts thrustline in the case of the Tiger in line with a lot of similar designs the engine thrustline is parallel to the tailplane incidence,the aircraft has a tendency to rotate around the centre section hence when the engine is opened up,the nose will raise just like any aeroplane will,but with the biplane it is a bit more complex,the designer needs to incorporate some form of fore and aft trim,in the case of the Tiger this is a simple device which places pressure onto a spring to give the respective feel to the stick with either a forward or backward force,simple but ingenious.

The relative stick forces on our aeroplane in normal cruising flight determine the best position for the wing incidence,no incidence will mean too much rearward trim will be required and the aeroplane uncomfortable to handle requiring brute force to fly,the pilot would quickly tire on anything but a long journey.

As the aircraft gathers forward speed the tail will lift and the machine will be said to be 'flying' as the lift overcomes the drag and the weight a wing without any incidence would effectively become inefficent and require a much longer take off run to become airborne,dont forget the aerofoil section on such machines will have a marked effect on the wing incidence or angle of attack as we call it,but always think of these angles as one and not separate units,the stall will occur at a optimum setting in relation to the decalage set by the designer.

Added to this and getting back to the aerofoil section we have the centre of pressure to contend with,once again wing incidence will affect how she flies,the race pilot we spoke about earlier will just want speed at the sacrifice of getting airborne quickly,he will put up with an discomfort in the handling,on the other hand the crop spraying pilot with his Agcat biplane who operates from tight strips will need to leap into the air to get his load airborne,whilst in model form these differences cannot possibly be detected easily they do make a difference,the problem is they are difficult to set up with any accuracy anyone who has wrestled with piano wire cabane struts will know what I mean ! you need to prepare a careful jig first,remember that important datum ? well you will need to determine that as well.

I build a lot of lighweight biplanes for either rubber or electric power and normally incorporate just a few degrees to make them fly well,with radio models it is possible to fly with or without any decalage it just depends on the design,short coupled designs always appear touchy and difficult to trim fore and aft,medium to long moment arms will certainly benefit in the pitching flight,however you dont really want a model like the Pitts Special to be too stable so less incidence or non at all will suffice,horses for courses.

But above all dont just write decalage off as not required,as the designer of a full size machine will opt for the best then we need to do the same with our models,the subject is highly complex and very little written about it,too many variables to say that it does not work,rest assured that any large model biplane will respond exactly as its full size counterpart as we build smaller those important settings get more critical the problem is implementing them !

Aerodynamics is a fascinating subject mostly learned by experimentation,the best way to check out any setting is to make the humble chuck glider,this will show up any features to the best advantage,and above all they dont cost much and great fun !

nowledge is all about experimenting,even crashes,but that knowledge gained will prove invaluable to your future projects.

Go forth and take flight.

[Guest]

Put simply, Adz':

High wing aircraft are recommended for beginners because they tend to be more stable in the roll axis due to the pendulum effect of the fuselage being beneath the wing. This is especially true if there is also any dihedral present. These things combined give an "auto-stability" effect, that is, if the aircraft is left alone in a bank it will roll back to wings level by itself (eventually!). If there is little or no dihedral present then the effect is reduced.

Nope, it's not much to do with wing loading, but a low wing loading will make life much easier. For a start the aircraft won't have to fly so fast to develope enough lift, which in turn means it's flying slower which always helps a beginner, and also if the pilot gets it into a stall the recovery will be much quicker than a heavily loaded wing.

It's debatable what type of model is best for a beginner to learn on. That argument could go on for ever! But generally speaking a lightly loaded high wing model with plenty of dihedral would be a fairly safe introduction, even if you won't learn much about flying from it!

If you want a safe route for teaching yourself, do what I did. I had a six foot span glider with a simple Clark Y airfoil and plenty of dihedral and an 049 glow motor on the nose. Later, when I became confident I fitted an .09. That first model was a Veron Big Impala (with plastic moulded wing ribs!) with just rudder and elevator control, and the motor was a DC "Quickstart", which used to take me about fiftenn minutes to start! I probably got more fun and more reward going down that route than the guys who were taught to fly.

Reminds me of this:

…..but I don’t think it bigoted to say that self-taught flying produced the better and safer pilot. Some did not survive, of course, but those who did, because everything was self-taught and learnt at first hand, never forgot.

‘Sky Fever’

Sir Geoffrey deHavilland

So, yeah, teaching yourself is far more rewarding if you begin with a PG. (powered glider)

[Guest]

guys :bravo: :bravo: :no1: :goodpost:

extreemely informative, and reasonably easy understood from a laymans poont of view :woohoo:

many thanx for the time taken to do so. much appreciated

:respect:

[Guest]

At risk of "blotting my copy book", I feel I must question the use of the term "decalage".

I have been led to believe that the term relates to the difference in incidence between the wing(s) and the tail plane - more commonly termed "longitudinal dihedral".

I have a recollection that as far as models go the difference in wing incidences on biplanes isn't too critical.

There have also been arguments as to the pros and cons of having greater or lesser incidence on the top wing compared with the lower wing, and, from what I can remember it doesn't seem to make much difference to models. The article I read looked at several examples of a particular full size aircraft and found differences between the various examples so there was no absolute "standard".

Malcolm

[wonwinglo]

At risk of "blotting my copy book", I feel I must question the use of the term "decalage".I have been led to believe that the term relates to the difference in incidence between the wing(s) and the tail plane - more commonly termed "longitudinal dihedral".

I have a recollection that as far as models go the difference in wing incidences on biplanes isn't too critical.

There have also been arguments as to the pros and cons of having greater or lesser incidence on the top wing compared with the lower wing, and, from what I can remember it doesn't seem to make much difference to models. The article I read looked at several examples of a particular full size aircraft and found differences between the various examples so there was no absolute "standard".

Malcolm

***Not quite correct Malcolm,lets go through the rigging procedure for a Tiger Moth a machine very near to me as during the sixties I rebuilt several examples some of which I am pleased to say are still flying or preserved,including Dick Emerys G-AOEL which is at East Fortune,lets talk us through the procedure-

Firstly the aircraft is set up on trestles in the flying position,to do this a straightedge is placed on two pips on the side of the fuselage which correspond with the datum line which in turn lines up with the tailplane itself,everything around this point is said to be plus+ or minius- in terms of whatever we set the wing incidence(s) next all of the flying and landing wires are slackened so the aeroplane is 'limp' and a start from scratch,gradually the dihedral angle of the lower wing is wound in via the landing wires,whatever happens now the lower wing remains constant,next a clino instrument is placed onto the datum rule which fits around the top wing from leading edge to trailing edge and the two adjusting screws are moved until the top wing is set at + whatever setting is required ( Tiger Moths can be rigged many ways,ie for air racing,aerobatics,cross country etc) next step is to place the incidence board onto the lower wing and read off the lower wing setting,this setting is always less than the upper but positive to the tailplane/fuselage datum,to confirm this setting the flying wires are then tightened up to the manual setting and the upper incidence is then re-read again.

The term decalage only refers to the relationship between the two mainplanes but never related to the tailplane,in the case of the Tiger Moth an ingeniuos device trims the machine by back pressure on the elevator only,this takes the form of a powerful spring trimmable from the cockpit which in effect places a back pressure on the stick.

To add complication to this most full size machines do not incorporate any engine downthrust as we sometimes use on our models,just thoiught that I would mention this !

I agree that on our relatively small aeroplane models any decalage can seem to have very little effect,this has always been a bone of contention as some years ago a well known modeller built a biplane with moveable decalage to do some testing.

The top wing is always made to stall before the lower but the angle of attack always collectively measured via the decalage incorporated.

I have heard the term longitudinal dihedral mentioned but do not believe it is anything to do with the decalage of the wing of a biplane,and is probably more of a modellers term relating to the incidence of a wing in relationship to the fuselage on a high or low wing aeroplane.

PS-Almost forgot to mention that the interplane wires play a very important part in the rigging of the decalage,the final check is to ensure everything is 'in safety' ie all threads are visible for the streamlined wires via a tiny hole in each clevis fitting,as a young fitter one of my jobs apart from holding that datum board was to go all around the aeroplane with a piece of wire prodding the clevis fittings ! sorry for the unweildy explanation but for completeness I thought that many will find it interesting.

[Guest]

It would appear that I have been misled or misinterpreted something along the way.

I know of the variety of differences in incidences between the main planes of bipes (and tripes for that matter), but don't recall ever hearing of it termed "decalage".

I bow to your greater experience - as an airframe rigger you must have far more knowledge than I.

As for the Tiger Moth, as previously mentioned, I have one under construction from Phil Smith's plan.

This came about following a "trial flight" in a Tiger Moth courtesy of Classic Wings based at Duxford. This was arranged as a surprise birthday present by my spouse, an experience which I treasure and think over many times.

It was a surprise to me that the plane was flown almost exactly as I would fly a model - turns were accomplished on aileron and elevator and the rudder was only used during taxiing and on the final landing approach.

For much of the flight I was in control and, once airborne and at altitude had hands and feet on the controls umtil touch down.

I recommend this to anyone who has any interest in flying for a totally unforgettable experience.

It beats flying in a Jumbo any day.

It re-awakened my desire to build a "Tiggie" and I want to finish my model in the colour scheme and using the markings of the one in which I flew.

Malcolm

[wonwinglo]

It would appear that I have been misled or misinterpreted something along the way.I know of the variety of differences in incidences between the main planes of bipes (and tripes for that matter), but don't recall ever hearing of it termed "decalage".

I bow to your greater experience - as an airframe rigger you must have far more knowledge than I.

As for the Tiger Moth, as previously mentioned, I have one under construction from Phil Smith's plan.

This came about following a "trial flight" in a Tiger Moth courtesy of Classic Wings based at Duxford. This was arranged as a surprise birthday present by my spouse, an experience which I treasure and think over many times.

It was a surprise to me that the plane was flown almost exactly as I would fly a model - turns were accomplished on aileron and elevator and the rudder was only used during taxiing and on the final landing approach.

For much of the flight I was in control and, once airborne and at altitude had hands and feet on the controls umtil touch down.

I recommend this to anyone who has any interest in flying for a totally unforgettable experience.

It beats flying in a Jumbo any day.

It re-awakened my desire to build a "Tiggie" and I want to finish my model in the colour scheme and using the markings of the one in which I flew.

Malcolm

*** The problem with biplanes Malcolm is a lot of these riggers tricks of the trade are now a thing of the past,would you believe that we were using tips gleaned from World war 1 mechanics who knew just how to tweak a few wires,I recently had the opportunity to renew my aquitance with the good old Tiger Moth when a friend who owns one of the two last flying Thruxton Jackaroo aircraft ( Tiger with a new enclosed cabin and three seats) said that his aircraft was flying left wing low ( you can see how I derived my handle now everyone !) when I looked at the pushrod exit to the aileron which is a unique blend of cam via sprocket it was offset,this led me to check the starboard wing which as suspected had a few degrees of warp from someone over doping the fabric,the old riggers trick here was to apply a bias by doping on a strip of conduit onto the trailing edge of the offending wing,although crude the low speed of these aircraft lend themselves to tricks like this and above all it works !

Here is another interesting story relating to early aeroplanes,Old Warden have the replica Bristol Boxkite in the collection which was built by Miles Marine down at Shoreham,if you look at the machine you will see that the entire aircraft is held together with piano wire rigging which is twisted around metal fittings,when they built her a member of the Miles family ( Jeremy Miles) whom I had the pleasure of meeting last year told me that they were struggling to accomplish the twists in the wire satisfactorily,one day an ex WW.1 rigger rolled up on his bicycle with a beautifully polished wooden box strapped to the crossbar ! here is what you want he said a war department 112/2 Mk.4 wire twister,he had kept it under his bed all of these years as a souvenir ! needless to say it was quickly pressed into action and the end result can be seen on the airframe,what a lovely story,next ime you see the triplane take a look,it is a work of art.

Unfortunately the word decalage has long since disappeared from the aviation bible 'British civil airworthiness requirements',the few skilled engineeres left will take their knowledge to the grave with them,such a pity.

You are quite right,you have not lived until you have been in a Tiger and experienced real flight,something that you never forget,it is real seat of the pants flying ! thanks for giving me the chance to discuss the rigging bits,Barry. :terrific: