TPI Threads

[Anonymous]

As a moderately experienced glider pilot and inspector I feel able to talk a bit about brakes on gliders. I’ll concentrate on modern glass fibre gliders rather than what went on in the days of wood and fabric. Actually glass fibre is a bit of a misnomer, as most modern gliders are built using carbon fibre and kevlar, with glass limited to low-stress coamings and covers.

 

All modern gliders have airbrakes. These are a paddle that rises vertically from the wing, usually on the top surface only. On earlier gliders there were paddles top and bottom, but this led to sealing problems when the paddles were supposed to be closed. The airbrake serves two purposes; it increases drag and severely reduces lift over part of the wing. The result of all this is a decreased glide angle, as John correctly says. Airbrakes also serve a secondary purpose of being speed limiting. On old British gliders they were speed limiting whatever you did, even in a vertical dive. Very useful if you lost control in cloud. On modern gliders the airbrakes are speed limiting up to a 45° dive, after that you are on your own. Possibly quite literally as the wings might have come off! Airbrakes vary from being over-effective to having sweet FA effect, depending on the glider type. Here are some numbers as an example. My open class two seat glider has a glide angle of about 60:1 with airbrakes closed. With airbrakes open it drops to about 25:1. Quite a difference, but still a respectable glide angle, so you definitely need to be thinking a long way ahead of the glider on approach.

 

Most modern gliders have a retractable wheel at the front and a small fixed wheel at the rear. Even once you’re on the ground the glider will continue to roll. So, all modern gliders have wheel brakes, as well as airbrakes.

 

I’ve added a picture in my odds ‘n’ sods album to illustrate the daft places we go to in gliders. The picture was taken at about 12000 feet in the French Alps.

 

Regards,

 

Andrew

[Ian S CParticipant @iansc]

Andrew I’v posted a photo of a glider , that was owned by Steve Fosset, and used in NZ for an altitude record attempt. He even borrowed a pressure suit from NASA, he did’nt quite make it, think the best he could do was some thing over 50,000ft, conciderably less than he wanted. It an Ash 25 2 seater, Owned by Terry Delore, and last year broke a world distance record of 2501km in 11hrs, at an altitude of 28,000ft and speeds up to 200kmh. Terry and Steve Fossit broke 11 world records with it. Ian S C

Edited By Ian S C on 31/10/2010 12:10:05

[WolfieParticipant @wolfie]

I see gliders quite a lot. I live not too far from a place called Sutton Bank where they fly from. If I go anywhere north or west I have to pass it.

[BogstandardParticipant @bogstandard]

Terry,

 

Many thanks, we strive to entertain.

 

Many years ago, I worked in the computer peripherals manufacturing industry and the company was American owned, and seeing as we made Maxtor hard drives (a big one then was a full height 5 1/4″ with a storage capacity of around 160MB), the company used mainly UNF and UNC fasteners and a few BA. Then things changed over to English ownership, and we then started to develop our own sytems.

 

Imagine the confusion we had over about a two year transition period where we changed over from the American system to metric, where we were building and supporting basically the same sorts of peripherals in parallel, but using different fasteners. Luckily our mainly female workforce soon picked up the differences.

 

It was the same in the aircraft hanger when we had both US and French designed helicopters. Everything had to be kept totally segregated because of the different thread and tooling problems.

The US designed, but built under licence in the UK had BSF, BSW, BA and Unified in use, and of course the French was all metric.

 

As you can maybe realise, putting the wrong bits in the wrong holes could have caused a catastrophe in both cases. The first financially, the second, potentially a lot more serious.

 

 

Bogs

[TerrydParticipant @terryd72465]

Posted by Bogstandard on 31/10/2010 16:54:26:

Terry,

 

Many thanks, we strive to entertain.

 

 Hi Bogs,

 

you don’t just strive, like a toothless budgie, you succeed!

 

Terry

[John OlsenParticipant @johnolsen79199]

There are air brakes, or spoilers if you prefer, on modern airliners that are tripped as the  wheels touch down…the intent there is quite evidently to reduce the lift to avoid bouncing off again, as well as helpingthe aircraft to stop. However plenty of aircraft have in the past had air brakes where the main intent was to enable a reduction of speed, even while in flight well away from the ground.  This has included designs where a ring of flaps could open up like a collar around the fuselage, with no direct effect on the lift of the wings. Of course, when the aircraft slows down, it will either need to increase the angle of attack to stay up, with a further increase in drag, or it will descend.

 

But even ordinary flaps have the intention of increasing the drag, not necessarily for use as brakes in the usual sense, especially not from high speed, although they will certainly slow the aircraft down. You can often feel the deceleration as they wind down more flap. (The modern type of flap will increase lift, drag, and total effective wing area.)  For a jet this can mean actually increasing the turbine speed to maintain the desired glidepath…the advantage of that is that to go around, you only have to reduce the amount of flap, which can be done faster than you can spool the turbine up from idle. You will usually hear the turbines slow down just after the wheels touch down. I used to live at a point where I could hear them wind them up as they began their approach. Also note that when the aircraft is in a significantly nose high attitude the thrust from the engines is contributing a vector component to the total lift that is not insignificant.

 

Getting back to the gliders…so if Andrew passses over the end fence at 10 metres with no air brakes on, it will be 600 metres along the runway before he touches the ground, without air brakes, and not allowing for ground effect, which would make it longer, or wind sheer, which might make it shorter and less comfortable. If he puts the nose down to land sooner, he will arrive faster…..

 

regards

John

 

[Dinosaur EngineerParticipant @dinosaurengineer]

If a design needs a reasonable strength thread in a fairly weak/soft material it’s often necessary to use the old imperial thread systems rather than the standard metric threads. The standard metric coarse threads are fine for steel but are not sometimes strong enough in weaker materials . Many aircraft part threads are “Helicoiled” for this reason but this of course is more expensive.The coarser BSW & UNC are still often used commercially in metric designs . The hard disc example quoted in a previous quote is an example of this . The hard disc mounting tapped holes are still UNC threads in some discs to give adequate strength ( 8-32 if my memory is correct).

The mixup over similar size screws is not exclusively due to metric/ imperial mix . My English 40 year old lathe has 3/16″ BSF and 2BA screws and it’s quite easy to get them mixed up !

[Chris TriceParticipant @christrice43267]

The Space Shuttle has a split rudder that acts as an airbrake in addition to the chute that gets deployed. Generally, airbrakes cause drag thus acting as a brake and can be fitted anywhere on the airframe. Spoilers are named because they spoil the airflow over the wing reducing the lift the wing generates and are thus bult into the wing structure. There’s a subtle difference although definitions can become blurred. Many WWII dive bombers had dive brakes designed to stop the aircraft diving too fast..

Edited By Chris Trice on 31/10/2010 23:21:25

[Anonymous]

Ian: Thanks for posting the picture. I have flown an ASH25, once. The glider I’ve just bought into is a Nimbus 3DT, two seats, turbo and similar span to the ASH at 25.6 metres. I was vaguely aware of Steve Fosset’s height attempts and  I have heard of Terry Delore in connection with the distance records in wave. My only, tenuous, connection with height records is being at Aboyne on the day the UK height record was set. It’s not likely to broken, now that the EU has decided to interfere in gliding in a big way.

 

Chris pretty much has the description of spoilers on gliders nailed. However, the spoilers on commercial aircraft are subtly different. They are actually lift dumpers. They do add some drag, but that is incidental. What they do is open up a hole through the wing, reducing the wing area and dumping the lift from that area. This puts the aircraft firmly on the runway so that the wheel brakes, and to a lesser extent reverse thrust, can slow the aircraft without skidding. If you have a window seat just behind the wing, when the spoilers operate you can see the runway through the wing. Or at least you hope it’s the runway!

 

I can also confirm that you go a loooooooooong way trying to land a glider without airbrakes. I have had my airbrakes freeze shut while wave soaring. Fortunately they thawed out before I had to land on a short strip. Once back at home base I tried landing without airbrakes, as we had a 10500 foot runway to play with. I used a fair amount of it.

 

Regards,

 

Andrew

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