Aerobatics - why biplanes?

[Blues&twos]

So, to the aerobaticists amongst you, why are biplanes quite common for aeros?

The only time I see them flying near my house, they're all over the place. And very entertaining too.

[eharding]

Why biplanes?

Fundamentally, because prior to the advent of carbon spars, stubby wing biplanes with wooden spars...where all of the strength came from the flying wires...was seen as the way to go.

Wooden spar techniques did move along - the Cap 10, Cap 21 and 231 are examples, but at the end of the day, longevity is still an issue. The One-Design, essentially a monoplane Pitts with a wooden spar, is a different and robust development - frankly, if the worst happens and the North Koreans start dropping instant-sunshine on our heads, then I'm going to hide under a One-Design - the spar can survive pretty much anything.

But if you've got the funds, and the time, you do your time in a Pitts, and then buy an Extra.

Ultimately, the Pitts is a 200 BHP Caterham, and the Extra is a 300 BHP+ Porsche. Take your pick, but set your budget accordingly.

[mad_jock]

Its to give it more wing area for lugging the beer bellys in the air.

By having a bi plane it moves the centre of pressure up nearer the C of G (the belly) thus decreasing the stability and improving handling.

Well thats my theory

[BackPacker]

Another factor might be that the wings don't have to span that far outwards for the same chord, wing loading and surface area. This means better roll rates.

But aerobatics is fun and there's a definite "wow" factor involved as well.

[Zulu Alpha]

Biplanes provided a very strong wing structure by using flying wires bracing a very light box structure. As Ed says, until the 70's the monoplane spars were not strong enough for high G loads without being very heavy. Nowdays the use of carbon biber has changed this and the monoplane wings win out because of reducd drag.

So your title could have been " why are old aerobatic aircraft Biplanes".

There is also an aesthetic argument. The Biplane boys and girls love them. The Monoplane men and women think that there are just too many wings!!

ZA

[stiknruda]

Ah, ZA but more wings means more chance of being seen by others in the open FIR!!:uhoh::uhoh::uhoh:

[Blues&twos]

Thanks for the replies! Nice and technical, just how I like them.

[Mike Cross]

They are also relatively slow and have high drag, which means they perform compact manoeuvres and don't pick up speed quite so quickly when pointed downwards.

Compare the size of a loop flown by the Reds with one flown by a Tiger Moth and you get the picture. OTOH unless they have a big engine like a Pitts they run out of energy fairly quickly.

[aditya104]

Another factor might be that the wings don't have to span that far outwards for the same chord, wing loading and surface area. This means better roll rates.

But aerobatics is fun and there's a definite "wow" factor involved as well.

I know biplanes have better roll rates. But how does small wing span provide better roll rates?

As I understand, a biplane with struts and braces weighs a lot less than a monoplane of the same wing area. Thus allowing a heavier engine to be installed which provides better performance as compared to a monoplane with similar weight.

http://i.imgur.com/6uiLy.jpg

Can someone give a clarification? :ok:

[Jan Olieslagers]

One more reason might be that, in aerobatics, a basic disadvantage of a biplane is less relevant. A biplane has poorer aerodynamics than a monoplane, especially a cantilever monoplane. However for aerobatics neither cruise speed nor fuel efficiency are prime factors.

But how does small wing span provide better roll rates?By having the wing's centre of gravity closer to the craft's axis, reducing the wing's momentum
(hope I worded and spelt that right).

[Zulu Alpha]

Comparing a Pitts with a C152 is misleading.

Comparing a Pitts S1T with 200 HP against an Laser 200 would be a better comparison. They have similar roll rates, the Pitts is slightly lighter but has more drag. If you compare the Pitts to an Edge 360 then the Monoplane has better performance all round

As was said earlier the newer wooden or composite monoplanes are very light for their strength.

Roll rate is a consequence of the size of the aileron not the short or long wingspan. Having aileron further away from the fuselage gives more "leverage" but aileron near the fuselage gives faster roll at low speed when its in the prop slipstream. There is also a limit due to the strength of the wing that the aileron attaches to. Again, modern composites are stronger which has allowed bigger ailerons and therefore faster roll rates.

Nowadays people have moved on to 300+ HP and all the new competition aircraft are composite. There are no biplanes competing in internationals at Advanced or Unlimited...or Red Bull for that matter.

So, Curtis Pitts aircraft were brilliant and allowed a very light structure to be very strong. However nowadays they have been overtaken by modern composites.

[The500man]

Aside from the performance comparisons I think the Pitts has a certain something that is immediately evident as soon as you leave the ground; it's definitely a rewarding aircraft to fly. It's also reasonably affordable, particularly if it's kept on a permit, and you can homebuild some models too. The design is quite old now but they do seem to last once their built with a reasonable amount of care. I think that is why there are still so many kicking about today and hence their popularity as a cost effective competition ship.

[stiknruda]

I flew the first part of a two sector delivery for a new lady Pitts owner yesterday. 170mph at 2700rpm in an 180hp biplane!

Stik

[aditya104]

Thank You all for the succinct insight. :D

[Guzzler]

Why?

I think the Caterham analogy is spot on.

And they look great!

http://gallery.me.com/benlovering/100250/7B5J3244_2/web.jpg?ver=12985395970001

[Guzzler]

What's not to like!!!

http://gallery.me.com/benlovering/100250/7B5J3087/web.jpg?ver=12985396940001

[Mark1234]

Isn't that more like a westfield, and a wide bodied one at that :p

(not actually being a pitts...)

[Human Factor]

The monoplane Yak 18 placed 1st, 2nd, 3rd, 4th and 7th at the 1966 World Aerobatic Championship, and was followed by the Yak 50 and 55, none of which used CF in their construction.

The Russians took a slightly different approach. Use as much pig-iron as possible and bolt a massive engine on the front. ;)

Nice photos by the way. Where's the aeroplane?

[Guzzler]

Isn't that more like a westfield, and a wide bodied one at that

(not actually being a pitts...)

Uh, yes. I guess you are right.

[aditya104]

Comparing a Pitts with a C152 is misleading.

Comparing a Pitts S1T with 200 HP against an l@ser 200 would be a better comparison. They have similar roll rates, the Pitts is slightly lighter but has more drag. If you compare the Pitts to an Edge 360 then the Monoplane has better performance all round

As was said earlier the newer wooden or composite monoplanes are very light for their strength.

How does Pitts S1T compare with Laser200 and Edge 360 in terms of weight, roll rate and wing span?

Roll rate is a consequence of the size of the aileron not the short or long wingspan. Having aileron further away from the fuselage gives more "leverage" but aileron near the fuselage gives faster roll at low speed when its in the prop slipstream. There is also a limit due to the strength of the wing that the aileron attaches to. Again, modern composites are stronger which has allowed bigger ailerons and therefore faster roll rates.

By having ailerons farther away from fuselage, what do you mean by more leverage?

Flightlab Ground School 9. Rolling Dynamics (http://flightlab.net/Flightlab.net/Download_Course_Notes_files/9_RollingDynamics.pdf)

[Justiciar]

having ailerons farther away from fuselage, what do you mean by more leverage

I would imagine this is a reference to the moment of inertia of the aircraft, i.e the rotational inertia and the torque applied to the aircraft, which is a product of the force exerted x the distance from the axis of rotation. So, the same force applied at a greater distance from the axis of rotation produces a greater torque and therefore a faster role rate. Looked at another way, the same force applied close in will produce less torque than the same force applied further out, meaning that the rate of change of angular momentum (role rate) will be less the closer in the ailerons are located. This ignores the effect of prop slipstream, which will of course be greater one side of the aircraft than the other.

The combination of wider wings and longer ailerons will generally produce a faster role rate than the same aileron on a shorter wing, all other factors being the same.

[mmgreve]

So, the same force applied at a greater distance from the axis of rotation produces a greater torque and therefore a faster role rate

Why would more torque in your mind make the roll faster? It just makes it stronger in my book. The closer you move to the fuselage, the same amount of aileron deflection would amount in a quicker roll (though less torgue, so might not happen due to inertia)

Since most aerobatic machines are pretty light, I would assume inertia not to be an issue.

[Zulu Alpha]

inertia is an issue. Getting a 1000lb aircraft to rotate and then stop quickly requires quite a high torque.

If you have ailerons near the fuselage they will create drag but lower rotation force (torque) on the fuselage. If you have them further away they generate the same drag but higher rotational force.

There is also the strength issue. the forces on the aileron can be quite large so you need a strong wing to mount them on. Pitts acheive this with short wire braced wings . Carbon monoplanes achieve it with carbon fibre spars.

So you would need bigger ailerons on a Pitts which would require a stronger wing and would create more drag.

How does Pitts S1T compare with Laser200 and Edge 360 in terms of weight, roll rate and wing span

Pitts is about 850lbs and Laser/Edges are about 1025lbs

Pitts S1T and Lasers are about 300-360 degrees per second. Edge is about 420 degrees per second. These are all speed dependent so these are for about 130/140kts

Not sure about a Pitts wingspan but the Laser and Edge are about 25 ft.

[stiknruda]

S1T wingspan is 18' (+/- a couple of inches!!)

S2 wingspan is 20'


Stik

[Happyloop]

Biplanes are draggy, rather slow, light and strong.
Biplane are quite good at knife edge flight and rotations on the 3 axes.
Even a vintage biplane with rather slow roll rate and low power is a great aerobatics performer due to his natural grace... :)
Enjoy the Bucker Jungmeister (50% scale)
https://www.youtube.com/watch?v=tnB7L-O2VJo

The Buckerholic ;)

[photofly]

Roll rate is a consequence of the size of the aileron not the short or long wingspan. Having aileron further away from the fuselage gives more "leverage" but aileron near the fuselage gives faster roll at low speed when its in the prop slipstream.
Moment of inertia scales with distance squared; rolling torque scales as the distance. That suggests that with equal aileron area an aircraft with two short wings will roll faster than an aircraft with one long wing, by a factor of 2.

Also you'd have to take into account how the mass to maintain sufficient strength goes up with the wingspan. Long wings will have higher bending moments so need to be stiffer, and therefore heavier, with higher rolling moment. Again that mitigates against one long wing and in favour of two short wings, for good roll rate. That's before you take into account the extra stiffness of a braced truss arrangement available to a biplace and the reduction on weight that allows.

The ailerons on a Pitts are still a long way outboard of the prop.