



Discussion
Tail Volume and Biplanes
I was just wondering how to calculate the tail volume for a Biplane.
I'm aware of the rough rule of thumb of using 85% of wing area in general calculations to account for reduced efficiency. But is there a correcting factor for the tail volume coefficient? Failing that can anyone give me a heads up on the differences (or normal parameters) in practice? 





A work around that has come up a couple of times to allow using the online CG calculators was to use the forward most leading edge and rearmost trailing edge as the limits of an "equivalent single wing chord" and then to adjust the single wing equivalent span used for the calculations to produce about 80 to 85% of the total surface area.
Needless to say there's precious little proper theory in this method. But folks have used the CG location given by the calculators using this work around and the models have flown successfully. 





The biplane efficiency rule is really a lousy rule because it misleads .
The "rule " makes it sound like there is something wrong with biplane wings.and causes them to be less efficient. What is true is that where sum of the areas is the same as a single wing, You are looking at the RN difference OF EACH of the bipe wings. vs the monoplane You end up with same area but worse RN and increased drag in struts intersections etc So ,sizing the tail volume and the horizontal tailplanes should take into consideration the total area  just like a monoplane. On the Bucker Jungmann the designer made the moment arm from CG to the tailgroup quite long  and got in my book  the best of the bipes for his efforts. The longer arm requires less tailgroup area to do the job better low speed authority and drag at speed is not too high.. I have no textbook stuf to back this up  don't ask  





Dick, the whole thing shows up in the stall and landing speeds. The closer the gap in chord distance between the wings the less effective the wings are and the higher the stall speed compared to two wings with a TALL interplane spacing or an equivalent area single wing.
From the reading I found many years back in a source I simply can't remember the amount of interference was minimal with an interplane spacing of 1.5 wing chords. This reduced to something miserable like the wings acting like a single wing of 60% of the total area by the time we get down to a 1C spacing such as found on the Curtis racers. Add on the drag of the wires and struts on that Curtis biplane racer and it's no wonder that the racers went over to monoplanes as soon as they figured out how to build the wings strongly enough 





I thot the question was about tailplane size of mono vs biplane
No? The gap issue  gap is a mixed bag  a wide gap setup can have some really screwy change in vertical drag depending on thrust line everything gets pitch sensitive I thot the setup on the Bucker was " about right "  actually better than any others I flew 





www.rcgroups.com/forums/showthread.php?t=67873
Prandtl's biplane theory from 1911 nicely calculates the wing efficiency of a biplane with any separation and stagger. The advantage of a monowing over a biplane isn't that straightforward. If you are span limited, a biplane can get two wings of twice the aspect ratio so they start with twice the span efficiency in a given span length. For slow flight, where induced drag is high, the higher aspect ratio wings could be the more efficient option despite the interference effect. When you also look at the huge structural advantage, a biplane might be the best option in some circumstances. And they look cool! Of course for model sizes, the Re of two smaller wings is a problem. Kevin Edit: "The induced drag of a multiplane is lower than that of a monoplane of equal span and total lift because the nonplanar system can influence a larger mass of air, imparting to this air mass a lower average velocity change, and therefore less energy and drag. For a biplane, if the two wings are separated vertically by a very large distance, each wing carries half of the total lift, so the induced drag of each wing is 1/4 that of the single wing. The inviscid drag of the system is then half that of the monoplane." http://aero.stanford.edu/Reports/VKI_nonplanar_Kroo.pdf 

Last edited by kcaldwel; Nov 01, 2012 at 03:29 PM.




Quote:
Thanks Kev, Bruce, Richard. I do have limited span (it's a 16" max span dime scale model) The gap and stagger is pretty standard: gap is 1.1 x the chord and the stagger is 0.4 x chord. I'm not specifically trying to calculate the efficiency, although the effective wing loading is of interest. (It is good for a dimer, even with the 85% fudge) My specific task is to correctly size the tailplane for this model. I use the tail volume coeffoicient widely in monoplane free flight designs but this my first bipe. I normally go for a Vht of around 0.6 which suits most scale models. I was just wondering whether using Vht = (Tail Area * Tail Arm)/ (Wing Area* Wing Average Cord) should I use the actual chord or the span/area? Come to that how do you describe the aspect ratio of a bipe? How do the pitching moments compare between a monoplane and biplane of the same span and area? 






Use the standard formula and tail volumes. The area is about the same for a bipe and an equivalent monoplane, the average chord = 1/2, but there are two wings, so it all comes out in the wash.
The aspect ratio is the span/ mean chord, for each wing. A bipe typically has near twice the aspect ratio of a similar span and area mono wing, but loses out from the wing interference, and all the struts and wires on old designs. A model has even bigger issues with the Re of the wings is basically half a monoplane. The pitching moment is based on the mean chord of the wing, so a bipe will have about 1/2 the moment with half the chord, but with two wings, it comes out about the same as a monoplane with the same area. That is basically why the tail ends up being the same volume too. If it wasn't for the Re and the parasitic interference drag penalties, a bipe would be a marvellous thing. Kevin 





that sounds familiar 






Quote:
For FF scale I'm just looking for sufficient tail volume for good stability and to avoid trimming issues but without too much of a tail enlargement for scale reasons. I usually go for an zero loaded or slightly lifting tail at min sink. This seems to work out with around a Vht of 0.6 and the CG around 3540% and 2 or three degrees of decalage, on normal configurations. Looking at your interference chart: Is the span ratio the ratio of upper to lower wing spans? ie the amount of 'sesquiplaneage' If so my model comes out with a span ratio of 0.875 and a gap/span of 0.166, putting the interference factor at 0.55ish. How does one incorporate the interference factor into the equations for lift and drag then? And in the case of comparing wing loadings with a monoplane (and in the speed equation) is the 85% area fudge (don't like the word rule ) reasonable in practice? 






For biplanes with stagger, u is the ratio of the aft wing span/ front wing span. There is another chart for when the aft wing has more span than the front. And those charts are for elliptical span loadings. Things can be made better using uniform loadings, or even constant chord wings  looks familiar...
Munk's original paper on biplane stagger theory is online, and will explain it all better than me. Biplane is also covered in good detail in von Mises "Theory of Flight", which is a book worth having. http://naca.central.cranfield.ac.uk/...report151.pdf The tandem wing airplanes like the Q2 and Dragonfly are covered by Munk's stagger theory, which is pretty cool! Kevin 





You can go as low as 15% of wing area and still have a decent setup
BUT 20% is a better number Not from tunnel results  just a lot of cut n try . review specs on Jungmann  these have very good ratios and moment arms even for models  





Quote:
Here's the little chap: the Avro Baby. It has quite a good tail moment so the dynamic stability should be ok. The little vertical stab might prove interesting though... Jon 



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