Biplanes

[Mounty]

Setting the incidence on Biplanes[:b]
my name is joe and new to this type of discussion but here goes, I like to experiment and to this end I would like to know how to set both wings at the correct angle when building or adding a second wing to a model thanks in anticipation. Joe

[Terry Rigden]

A lot depends on what sort of flying you are doing and the wing sections used.

One shcool of thought is to set the lower wing at a higher incidence so that it stalls first. The drag from the stalled wing gives a nose down moment and helps recovery. Typicly used on a trainer with flat bottomed wing section like the Tiger moth

Other people set the top wing at a higher incidence and alledge that it helps with snap manuevers. Often used for aerobatics with a fully symetrical section say on a pitts

I set my aerobatic bipe up with the two wings at the same incidence. So its stall charecteristics are the same upright and inverted.

I dont think a dgree or so either way makes a huge difference.

Regards

Terry

[Tony Jones]

Joe
Do you mean 'what is correct in terms of incidence' or 'how do I physically set it up once I've decided on the incidence'?
Assuming the second, an incidence meter is the best way without doubt, but some people just measure from the leading and trailing edges of the top and bottom wings. A long strip attached to the bottom surfaces by laccy bands and eyeballed a foot or so away from the tip to ensure they are parallel is also surprisingly accurate.
Hope this helps.
Cheers
Tony.

[Mounty]

thanks to those who took the time to help, what i am looking for is how to physically set the incidence but the comments thus far are great. I have never used an incidence meter but if thats what i need then great. Cheers, Joe

[Tony Jones]

Joe

You'll also find it useful for checking/setting decalage: thats the difference in angle of incidence of the main wing(s) and the tailplane.

Best wishes
Tony

[Terry Rigden]

NO declage is the difference between the two wings on a biplane. the difference between the wing and tail is the wing incidence.

Terry

[Tony Jones]

Terry,
Sorry, but you are wrong on this. Go here: http://www.fatlion.com/sailplanes/decalage.html
for confirmation. Googling 'decalage' will provide several other sources of information confirming the same thing.

Wing incidence is the angle between the zero lift line of the main wing and an arbitray 'horizontal' line down the fuselage. Its the datum from which mainplane and tailplane incidence is measured.

Best wishes
Tony

[Terry Rigden]

Tony thaks for the links and yes they do support your definition of the term. However it seems that with the demise of the biplane that the term had changed its meaning - I still hold the view that the original use of the term is to describe the difference in the angle of incidence of the two wings of a biplane. But modern (mis)use of the term has changed it meaning much like "wicked" now means something is good.

Regards

Terry Rigden

[Mike Rolls]

Terry is right. 'Decalage' comes from WW1 time and IS the difference in rigging angle between the upper and lower wings of a biplane. Over the years it has been misused to describe the difference in rigging angle between the tail and wing of a monoplane. You will also see this referred to as 'longitudinal dihedral'. Perhaps the most logical term is Tail Setting Angle (TSA) whihc you will also see used and which at keast has an obvious link to what is under discussion.
Mike

[Tony Jones]

I accept your statement gentlemen, but I have to say that I have only ever seen the term used for the relationship between the mainplane and tailplane incidences. And I have some quite old reference books! For sure, it has been used in the later manner for so long that the former meaning must surely now be regarded as antique.

If you could quote a official example I'd be most interested. Incidentally (sorry) as far as I know most biplanes have their lower and upper mainplanes set to the same angle of incidence.
Cheers
Tony.

[Mike Rolls]

Tony
If you want a recent comment, there is this in Ailsdair Sutherland's 'Model Aircraft Aerodynamics'. page 140:
"The legitimacy of the term 'longitudinal dihedral' has been a matter of some dispute. It was apparently introduced by Professor N.A.V. Piercy in lectures and advanced texts during the 1920s and 1930s. It was used and given wider circualtion by A.C Kermode in his well known book 'Mecahincs of Flight' late editions of whihc continue in use. The term 'decalage' strictly refers to the difference in rigging angles of the upper and lower wings of biplanes and should probably not be used in other contexts'.

Couldn't have put it better myself.

Incidentally a lot of full size biplanes do not have the wings rigged at the same geometric inicdence.
I used to have a publication (lost yonks ago) which set out for RFC mechanincs the making and use of an incidence board for checking the angles of the wings. It was a War Department publication - wish I still had it.

[Tony Jones]

Mike
Many thanks for that. One lives and learns. Strange that it should be so commonly misappropriated when the terms Tail Setting Angle or even Longitudinal Dihedral are so much clearer.
Best wishes
Tony

[Mike Rolls]

Thanks, Tony - you are very welcome. Out of idle curiosity I picked up a random pile of Flying Scale Model mags (everything in my workshop is in random piles!) and the first 10 biplanes (well 9 bipes, 1 tripe) showed:

Upper wing rigged positive to lower
Bristol Bulldog
Grumman F3F
Airco DH2

Upper wing rigged negative to lower
Albatros DV
Bristol F2B
Sopwith Tabloid
Siemens-Schuckert
Albatros DII

No difference (or too small to be seen on a 1:40 drawing)
Tiger Moth
Sopwtih Triplane

Regards
Mike

[Tony Jones]

Mike
Brilliant! I was about to ask why and how decalage was incorporated in some biplane designs and you have answered the second already. Now we come to the WHY. I'm 'designing' a scale biplane at the moment and am tempted to rig the lower mainplane positive to the upper so that it will stall slightly earlier. My thinking is that this should lead to a more progressive loss of lift just prior to a 3-point landing. What say you?
And finally, I'd like to apologise to Joe for comprehensively hijacking his thread for a semantic debate. At least he will now know the correct definition of decalage.
Best wishes
Tony.

[Mike Rolls]

Tony
I can't find anything definitive on this question (I know it exists - there was a long discussion on this very subject on a Compuserve forum years ago. Daryl Stinton's book "The Design of the Aeroplane" was quoted by a couple of people as covering th esubject, but I've never managed to find a copy (in this E-bay age I might have another go).

As I remember there were several aspects of biplane design being thrown into the pot:
Minimising the effect of biplane interference
Producing the most benign stall characteristcs possible
Maximising aerobatic capability.

On the question of benign stall - there two diamtrically opposed viewpoints (remember that this was a debate, not a series of fondings!). One held that the upper wing should be rigged at a greater angle, thus stalling first; the other felt that the reverse should apply. Neither convinced the other and I was confused by both!

I've only designed a couple of bipes. In both I put the upper wing at one degree greater than the lower and both had safe stall charactereists - but both were low powered stoogers. Don't know if the same would hold good for the likes of a Pitts or Ultimate. Both had flat bottomed, 'Clark Y type' sections with an average amount of stagger and a seperation between wings of slightly over a chord.

Sorry this is rambling and not terribly helpful
Regards
Mike

[Mike Rolls]

Tony
Did a Google search and Stinton's book is available from the Popular Flying Association - but it's £55 (!) so I'll keep browsing for a S/H copy some time.

Mike

[Tony Jones]

Mike
Many thanks for your input. I'd be pleased to know if you come across a more reasonably priced copy of the book.
The Biplane I'm building is a big, 'Clark Y'
STOL thingie. At the moment I'm planning to rig the bottom wing with 1 deg more incidence than the upper purely on guess work. I doubt it will make much difference actually.
Regards
Tony

[Mike Rolls]

Tony
Beat of luck with the bipe - let us know how it goes
Mike

[Mounty]

Thanks to every one who commented here the response has been great and sparked of a great discussion, thanks again Joe

[Cpt Biggles]

Hi Joe

I see your post is quite old BUT 

Did you manage to work out how to set the incidence for your bip.

i am building a Bullfrog from a RCM&E plan and need to check that the wing incidence is correct when it's finnished.

 some years ago i built a Pirrouette bip and somhow get the incidence wrong it took two very experienced flyers to work out what was wrong with the handiling it was flying like a 3 leged cammel with artheritus!

the upper wing was 0 deg and the lower was somthing like 5 deg neg so a big wedge under the trailing edge put it right.

so if you do not have an incidence meter how is it set up?????

kindest regards

Biggles 

[Mounty]

Yes I did manage to get the incidence right as follows, I bought a World Moel Ultimate Bipe which had a wing incidence jig in the kit, this worked fine so I made one and used it on the model I am experimenting with without a problem, Joe

[Cpt Biggles]

Hi Joe

You made an incidence jig. that sounds like wot i aut to do.

do you have a drawing or instructions on how? 

I am into building a biplane and will need to check the wings I was going to level the fuz then using a tape try to eyeball the incidence

Biggles 

[Mounty]

Sorry for the delay answering Cpt Biggles, I will attempt to get it down on paper and email it to you can you let me have your emal address. Joe

[Cpt Biggles]

Hi Joe, have just returned from our holidays (16th Feb) and whilst on holiday i have given a lot of consideration I think that I have a good idea how to constuct the jig but I am still very intrested in your jig as it will work my email address is [email protected] kind regards Cpt Biggles