I have modelled aircraft on and off for many years, and on retirement I now have the time to try to catch up on all the new ideas, techniques and materials that have evolved since my 'Keilkraft Days'. I am new to this forum; but from what I have seen so far, there will be those who can answer any question I can come up with; so here goes!
I have just read a lot of useful posts on the subject of biplane incidences, but an attempt to produce a design for a 30" electric R/C model of a Hannover CLIIIa, [WW1 German 2-seater with biplane tail.] has given rise to issues which I don't think have been covered.
On this aircraft both tailplanes were set at +2 degrees incidence. The Upper wing was at +4.8, and the Lower at +4.6. These values relate to a datum line based on the engine crankshaft line.
The lower Tailplane had a fairly thick symmetrical section, and the 2 degree incidence makes it's axis coincide with the propellor shaft centre; creating in effect a new datum line from prop to tail with 2 degrees engine downthrust and a tail at neutral.
Is it a reasonable conclusion to regard this line as an effective 'aerodynamic datum', relating to a level flight 'tail-up' attitude; thus producing wing incidences of 2.8 and 2.6degrees?
When related to this second datum, the above scale values are not far removed from many published plans for flying models of similar subjects; in the interests of scale outline, would it be reasonable to use them on the proposed model?
I don't know that particular aircraft, Steve, but from your description, it sounds about right.
The datum line is normally taken through the centreline of the fuz, often parallel to the top deck of the fuz behind the wing, and is considered to be zero, so the mainplanes should be positive to that, (leading edge up), the tailplane zero, and the engine thrustline down, ie front of the crankshaft pointing down. There will also be some side-thrust, again only a couple of degrees.
If the mainplane is set to zero, with positive on the tailplane (leading edge down) the aircraft tends to fly with the tail hanging down, which looks most ungainly, and is more draggy, but at the speeds these aircraft flew at, the numbers aren't terribly large!
Biplanes usually had a slightly higher incidence top to bottom, because of the interaction of the wings and the airflow over them, so they appear, to the air, to be parallel, and therefore generate equal lift.
a. On tailplane incidence... I may have got the convention wrong in describing the setting as positive. The Hannover 'Type Diagram' of 1918 shows the tailplane set with the leading edge up, and noted as +2 degrees. The lower tailplane is well below the fuselage/crankshaft datum line. Am I right to assume that in flight the incidence would produce a lifting of the tail until it was at neutral to the airflow, thus reducing the effective wing angle of attack by the same 2 degrees?
b. Several other German aircraft, notably the later Fokkers, had tailplanes on the upper longeron, ie above the fuselage datum line, set with the leading edge up. I suspect that something else is going on besides simple flight attitude, but I don't know enough to figure it out. Is it some sort of balancing act with the high lift characteristics of the thicker Fokker wing or the triplane configuration? Does this set-up work in model form?
c. I am also unsure about the reason for inverted airfoil section tailplanes, eg. the Pfalz DIII or the Rumpler C Types. I assume the effect would be a downwards force at the tail, creating a nose -up attitude, but why do some have it when other apparently similar aircraft don't? What about reproducing this on a model?
d. Hannover wing section. This is a fairly typical German undercambered section, and at 1:16 scale it has a chord of 112mm. It has a very small radius leading edge, so is it ok to use the line from leading to trailing edge to set the incidence? A number of similar model designs use a flat bottomed section. Is this for simplicity of construction, or is there any aerodynamic advantage over a scale undercambered section?... [I'd prefer to use the latter.]
I suppose one way forward would be to build a simple generic model with the facility to adjust and substitute flying surfaces to experiment with; but that rather seem like re-inventing the wheel, so I thought I'd ask first!
b), this could also be something to do with placing the tailplane out of the turbulent air leaving the te of the mainplane, with it being a short couple.
c). The real name for the tailplane is horizontal stabiliser! A plank-type flying wing will try to tumble forwards if not stabilised by reflex, which can be achieved by a weird section with concave surfaces on both sides, or by the elevons being permanently deflected upwards. The tailplane does the same thing, it does not significantly contribute to the overall lift of the aircraft, if anything the opposite, by holding the tail down! (Hence the canards!) Because we are working with comparatively low speeds, a flat plate isn't terribly efficient, so a downward lifting section is used. (If that makes sense!) Also the steeply cambered, thin wings of early a/c had a problem with a roving centre of lift - as the angle of attack was increased, the c/l moved as well, and could reach a point where the c/l was so far from the c/g that control was lost (ie Mignet's pou de ceil). Having the inverted tail partly countered this, as the c/l of the tail also moved at roughly the same rate. (Or so I understand!)
d). Simple mechanical strength! The thin wings needed a birdcage of wires to hold them together. The a/c had flying wires, which transferred the a/c weight to the wings when the wings were providing lift, and also landing wires, which directed the weight of the wings to the undercart via the fuz when the wings were NOT lifting. Also, as is now known, a small diameter/ sharp leading edge stalls much sooner than a fat blunt one, but a fat blunt wing is very draggy, so a thin wing is faster for the same power. As power was almost non-existant in early a/c, a thin wing caused less drag, and they were all based on bird wing-shapes! (Which are thin, undercambered, sharp leading edged devices). But what they couldn't copy was the infinate variable camber section, and the fantastically fast reactions to every vagary of airflow over the surface. Watch a soaring bird flutter ONE feather to alter its flight path!
The generic model - we all do that with our little scrap-wood chuck-gliders, when we are fooling around with a concept design! Even 12" to the foot designers build little models to see if it will work, before they go to the expense of building a full one. (Yes, they use computers now!)
Another thought re the downward 'lifting' tail - as the airspeed increased (in a dive) the tail lifted down harder, and pulled the nose up again! (That is terrible grammar, but how else can it be written!?)
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