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.
