glider side slip

[justinpalma]

Side slip or what ?
A question to the well composed instructors out there. I got kindly dragged into doing a weeks gliding. Normally feeling jet blast i thought it may be an intersting change but one aspect has failed my clear lack of areodynamic understanding.

side slipping a glider i did what is expected but then got asked to pull the stick right back and hold it. i suspected a stall was on it's way but no. So my question is even though the glider has a pitch up attitude, full back stick and unco-ordinated, why doesn't it loose airspeed /stall because it seems to keep going happy as larry. The AOA seems high. No glider instructor came close to helping me understand it....just the classic murmour of .....bloody propellor heads!

If anyone can answer our query in plain english i would be most gratefull.

[will5023]

Hi sounds like you where in a glass ship ? in which case holding aft stick will prevent it from speeding up, and hold it's speed on the side slip. Also due to the wing span and the different seating position, the angle can look completly different to what you are used to.

Will.

[homeguard]

The sideslip

The stall is a result of the angle of attack of the wing to the relative air flow (RAF). The aircraft may be pitched up in an attitude that would normally result in a stall when the RAF is along the longitudinal axis. However in the situation of the side slip the RAF is not along the longitudinal axis but from one side. In the extreme and the aircaft is slipping 90 degrees to the longitudinal axis the aircraft is actually falling but the rate of descent is resisted by drag.

[BackPacker]

What speed was the glider trimmed for, and at what speed did you do the sideslip?

[Doodlebug]

Since every approach comes without the option of a go-around you'd better not be low, however, what to do when you're high? The slip comes from the days of gliders without spoilers or funky ASW 20-style flaps.
Why the high nose? You're going sideways (ok, forward and sideways, but work with me here) Lee-side tailplane and wing are negatively influenced by the fuselage which is now in the airflow. Elevator has become less effective and therefore the need to hoik the stick back in compensation.

Justinpalma, did this gliding take place in Spain? Please check your PM, thanks.

[RatherBeFlying]

What Kind of Glider?

They're all a bit different.

Anyway the modern glass ships generally have such powerful spoilers that you don't need to bother with sideslips.

In many you can simply pull full spoilers and point it at the chosen landing spot if you're high enough. The added airspeed adds drag. In such cases, you do need to maintain sufficient airspeed and perhaps reduce spoilers to half during roundout to avoid a heavy landing -- depends on the glider. The manual will tell you.

However many of the older two-seaters such as Blaniks don't have such powerful spoilers; so, sideslips can become necessary.

[nonrad]

Same happens in powered a/c

You need back stick, and sometimes lots of it, when sideslipping powered aircraft, not just a function of a glider. Especially noticeable in Cubs, Pitts, Extra etc.

Also more back stick is needed on some a/c as the tailplane gets blanked and its effectiveness is reduced while slipping.
.

[gpn01]

To respond to a couple of the commentators:

"at what speed did you do the sideslip?" - In all probability the answer will be that the ppilot doesn't know because there is so much disturbance to the airflow that the ASI will read completely incorrectly
"What speed was the glider trimmed for" - Unlikely that trim position will have much bearing (except that if it's trimmed forward this may give you slightly more elevator authority)
Spoilers - In the UK most gliders have airbrakes (which generate drag)
"modern glass ships generally have such powerful spoilers that you don't need to bother with sideslips" - Believe me, if you're going into a small field in a slippery glider then you may need anything that helps decrease the distance to travel to lose height when trying to squeeze into a field that you're at risk of overshooting
"In many you can simply pull full spoilers and point it at the chosen landing spot" - most airbrakes are speed limiting only up to about a 40 degree downline. Beyond that the glider may continue building up speed.

Homeguard looks like a good summary of how it works.

[astir 8]

It's a very good question. Trouble is, I don't know the answer.

Having done a lot of heavily sideslipped approaches in our old T21 Sedbergh (the spoilers don't increase sink that much and don't prevent the speed increasing if you lower the nose to try to arrive at the desired spot on the ground so sideslipping is often essential) I can only certify that it works!

You usually sideslip when at normal approach speed for the day (c. 40 knots for the T21) and when you kick it straight, the speed is still about 40 knots. But during sideslipping ASI reading is of course highly inaccurate.

[jxk]

If best glide speed means that the glide range is 5 miles from 2500 ft then what is impact of increasing/decreasing this speed by say 10 knots???

[cats_five]

If the range at best L/D from 2500' is 5 miles that suggests the L/D is 12 or thereabouts. Any genuine glider with that as a best L/D probably has a polar like a camel's hump. 10 knots slower and it will stall out of the air, 10 knots faster and it will fly about half the distance.

A more modern 40:1 ship has a glide range of about 15 (nautical) miles from 2,500. Flying faster probably won't make a huge difference as the polar is probably fairly flat, flying slower will reduce the range but not by as much as for whatever the glider you were thinking of. It will probably be a longer duration though as 10 knots below best L/D speed will probably be close to min. sink.

Notice all the 'probably's in the above? Unless you state what the glider is, so the polar is known, any answer can only be a slightly educated guess.

[JBGA]

Originally Posted by jxk

If best glide speed means that the glide range is 5 miles from 2500 ft then what is impact of increasing/decreasing this speed by say 10 knots???

The answer to your question depends entirely on a) the aerodynamic properties of the glider, and; b) the speed range over which the 10kt difference is applied.

The rate of descent of a glider does not increase uniformly with speed (Because drag increases exponentially). At very slow airspeeds (Just above the stall) the rate of descent is quite high. As airspeed increases, the rate of descent reduces to a minimum point known as the 'minimum sink speed'. After that, as airspeed increases further, the rate of descent increases (almost)exponentially.

So, your rate of descent at 5kts below the minimum sink speed is probably about the same as it is at 5kts above the minimum sink speed. In other words, increasing speed by 10kts has no effect whatsoever on your range. At higher speeds, the 10kt difference will be much greater, especially in older gliders whose performance at extremities is poor. You might expect your rate of descent to increase by 1m/s when going between 80 and 90kts for example.

The easy answer to your question is that in a typical glider at a typical cruising speed, an increase of 10 kts will result in a small reduction in range - perhaps between 5 to 10%.

Incidentally, 5 miles from 2500ft is pretty poor. The average glider will do 40:1 so that's about 20 miles from 2500ft. Unless you factor in circuit and landing (Assuming you won't glide straight in to your intended landing site) in which case 10 miles is probably reasonable. In still air of course.

[BackPacker]

From what little I picked up in a weeks gliding course....

Much more important than the actual minimum sink or best glide speed of the aircraft is the vertical movement of the air you're flying in. Because of this, gliders are fitted with a MacCready ring around the VSI. This tells you the speed to fly given the sink speed of the air and is based on the polar diagram for the aircraft. In other words: the ring is tailored for each aircraft type.

The idea behind it is that you want to leave an area of sinking air as soon as possible, but the faster you fly the more inefficient your flight becomes. So it's a tradeoff between the amount of time you fly in the sinking air vs. the sink you incur with the aircraft itself by flying fast.

Wikipedia has the full explanation: Speed to fly - Wikipedia, the free encyclopedia

So...

If best glide speed means that the glide range is 5 miles from 2500 ft then what is impact of increasing/decreasing this speed by say 10 knots???

Depending on the vertical speed of the air over that five mile range and how the pilot handles it, the glider might be on the ground well before that five miles, or be at 3500ft or even higher. And his cruise speed might have been varied between 80 and 160 kph or even more.

Oh, and gliders don't fly in non-thermal conditions. Not for long anyway. (Ridge lift and such excepted.)

[jxk]

Hypothetical ?

My question was rather hypothetical. Given that you've entered a patch of still air and a landing is inevitable what impact does increasing/decreasing the speed have on the gliding distance? If you push the nose down and increase the speed to say 20 knots above best glide speed and then reduce speed for landing, have you shortened the range or is it the same as flying at best LD ratio?

[cats_five]

A landing isn't inevitable because a glider has entered still air. That's the time to fly at best L/D speed (unless one is intending landing) to where the next bit of lift might be. If one flies into still air (and there's very little of that - it's usually going up or down, and there is often wind) at 4,500' a 40:1 glider can go a long way before the pilot has to even think about picking a field, let alone planning a circuit. And not only does the sink or lift in non-still air alter the best speed to fly, so does the headwind/tailwind component.

But the impact of changes in speed on distance flown can only be accurately answered once the glider type is known, as then it's polar can be used to answer the question. The McCreedy ring mentioned by 'backpacker' has to be calibrated to the glider type - it's no use putting one from a instructional 2-seater into a 40:1 single-seater.

[jxk]

Skytype Mk1

OK the glider is a Skytype Mk1, there are no thermals etc., the best glide speed is quoted as 60 knots, you fly either 5 knots faster or slower than this.
Does the 5 knots either way increase or decrease your ability to get back to base for a landing assuming that the distance is at the optimum?

[ProfChrisReed]

OK the glider is a Skytype Mk1, there are no thermals etc., the best glide speed is quoted as 60 knots, you fly either 5 knots faster or slower than this.
Does the 5 knots either way increase or decrease your ability to get back to base for a landing assuming that the distance is at the optimum?

Assuming still air and nil wind, yes. Both decreas the distance you will fly through the air before the ground rises up to meet you, and as it's nil wind it will have the same effect on your distance over the ground.

5kt either way will not make much difference in a modern glider - perhaps only a field or six short of your destination, if that bothers you.

However, head and tail wind do make a difference. Into a head wind you want to fly somewhat faster than best L/D (the easiest way to understand this in your example is to imagine a 60kt headwind - then best L/D achieves no distance over the ground whilst flying faster makes at least some progress), into a tail wind somewhat slower but never slower than minimum sink (for your example glider, probably 45-50 kt).

Calculating these speeds precisely requires some form of glide computer. As a rule of thumb, adding 1/2 headwind to best L/D gets you fairly close. Sod's law means you will never have a tailwind on a marginal final glide.

As you approach the airfield, whether it is rising up the canopy or sinking is the best guide - but if it's rising you're already so low that you'd better have picked your field already.

[shortstripper]

Looking at the times, I would guess was in the process of writing that when you posted. Therefore, he hadn't read your answer before he answered in the same way .... happens to me all the time!

SS

[jxk]

Another scenario

OK here's another scenario: I'm on finals to land and think I'm going to overshoot - I stick the nose down let the speed build up to above normal approach speed, I then pull up to regain normal landing speed. Would I still overshoot or because I've effectively gone outside of the best glide speed reduce my range to touchdown?

[Crash one]

I would think you would most certainly continue to overshoot, at a lower level maybe but faster.