Bilge keels are good.

[chippie]

Thankyou for a lucid explanation. Can you explain why fin keels work as wings as they are symmetrical in section whereas an aircraft wing has a rounder top surface and corresponding longer distance for the air to travel over when compared with the lower surface.
Thanks

 

[peterb]

Symmetric sections

Aircraft use asymmetric wing sections because for most (if not all) the time their lift is required to be in one direction. But some aircraft (particularly stunt machines) which may spend a significant part of their flying time upside down do use symmetric sections.

Almost any section, even a flat plate, will produce lift if it is held at an angle to the air or water flow. Symmetric sections are used for keels because we don't know in which direction they will be required to give their lift.

With splayed bilge keels on a heeled boat the leeward keel will be more vertical than the windward keel, and may be producing most of the 'lift' necessary to balance the wind forces. When the tack changes, the direction of 'lift' required also changes, so each of the keels will have a preferred direction in which to provide its 'lift'. Some designs therefore use asymmetric keel sections, with the 'upper', more rounded, surfaces inwards.

 

[chippie]

Re: Symmetric sections

Thanks for that, I didnt know that stunt machines were built that way.
Re the flat plate example is that not just a case of deflection as opposed to true lift?, or is that the same thing?

 

[peterb]

Re: Symmetric sections

I suspect that you are suggesting that the lift on a flat plate is due to the airflow being deflected by the lower surface. Strictly speaking, all lift is caused by a deflection of the airflow. A flat plate isn't very good at it, because the flow becomes turbulent on the upper surface. But even with the turbulence, the pressure on the upper surface is reduced compared with the pressure in the free stream. A proper aerofoil section helps to reduce the turbulence, thus reducing drag and increasing lift. It's even better with some camber (i.e. where the centre line of the aerofoil is curved convex upward) but that is usually difficult on a keel since it usually has to work in both directions. But I believe that some of the America's Cup boats used trim tabs or flaps at the trailing edges of their keels to give the effect of camber.

 

[gtmoore]

Re: Symmetric sections

Fascinating explanation - I had never realised any of that

Thanks!

Gavin

 

[pugwash]

Hang gliders?

Wonderful information -- thank you! I guess one piece of kit that provides aerofoil lift in different directions as required is a sail, but it has limitations aloft. Does this explain why hang gliders are so dangerous? Is it true to say that when a hang glider stalls it effectively goes about and on the other "tack" proceeds downwards faster than you've ever sailed your yacht? You'd never get me up in one of those things but I've always wondered.

 

[BrendanS]

Re: Hang gliders?

No, hang gliders are very advanced bits of kit, and the sail will not invert. The entire leading edge is built of two layers of cloth, with foam in the very leading edge, and the entire sail is shaped and held in profile by shaped aluminium or carbon fibre 'stringers' so has a profile similar to a typical aircraft wing

Modern hang gliders can do aerobatics and can fly inverted in loops and similar maneouveres

 

[pugwash]

Re: Hang gliders?

So when they stall is the effect like that of any other wing, or does something additional happen which makes them more dangerous to fly than other aircraft? Can a hang glider in a nose-dive be recovered?

 

[BrendanS]

Re: Hang gliders?

They have quirks just like any other aircraft, but yes quite safe. Stalling is in fact how you land them (push control bar right out as far as it will go, stall wing, just as feet touch ground). Nose dives are fine, in fact an accepted way of getting out of extreme lift conditions.

Had to do this in 'wave' on my third solo flight on a very early hang glider (22 odd years ago). Got into extreme lift that took me to over 10,000 feet and still rising, despite bar pulled right into my stomach. Tucked my legs up and dropped them over bar, so that I was in a near vertical dive, until I got out of lift. No problems, though a bit hairy at time as I'd only been a few hundred feet off ground during training. Got lots of stick when I landed as pilots dream of catching 'wave' like that.

 

[pugwash]

Re: Hang gliders?

Interesting stuff! One more question. If you can sail, if you have that instinctive feel for twitches of wind on the back of your neck, can you hang-glide? Or at least, do you tend to have an aptitude for it? Do the skills overlap in any way, besides a willingness, I suspect, to behave like a bloody fool and tear up money? I've always been amazed by the distance between boat sailing and board sailing; the latter seems utterly counter-intuitive, at least, it is to me.

We seem to have strayed a long way from bilge keels. Sorry!

 

[BrendanS]

Re: Hang gliders?

A good feel for wind helps, and so sailing skills won't go amiss. What will be totally different is the feel for three dimensions. Good glider and hang glider pilots have a feel for thermals and sinking air, and an ability to feel their way around them that no sailor will ever experience.

Electronic gadgets now tell you if you are rising or sinking and are very useful, but when I started it was all seat of the pants.

It is one of the most wonderful sensations you can ever hope to experience, flying almost bird like across open countryside, so if you have an inclination to try it, then I'd recommend it. Not too expensive to take up. What you need in this country is lots of time, as many weekends are spent on top of hills waiting for the right conditions to fly.