[jhb] Re: Pireps
- From: "Paul Reynolds" <paul@xxxxxxxxxxxxxxxxxxx>
- To: <jhb@xxxxxxxxxxxxx>
- Date: Mon, 10 Sep 2007 21:23:12 +0100
Ah but then there's Ground effect. If my understanding is correct, this is
the air displaced downward hitting the ground and coming back up underneath
the aircraft thereby increasing the lift. This is more apparent in low wing
aircraft for obvious reasons and will impact on both landing and take-off.
Ou would need to be aware of it's impact because as you climb it disappears
extremely quickly or conversely, as you try to land it will be resisting.
Should you be doing a flaps/slow speed/ high AOA take-off, the need to gain
speed could be critical to compensate for the loss of ground effect.
Paul
-----Original Message-----
From: jhb-bounce@xxxxxxxxxxxxx [mailto:jhb-bounce@xxxxxxxxxxxxx] On Behalf
Of bones
Sent: 10 September 2007 16:51
To: jhb@xxxxxxxxxxxxx
Subject: [jhb] Re: Pireps
Lowering flaps effectively changes the camber of a wing and therefore its
lift properties. As you continue to add flap it creates more lift but from
about 15 degrees the drag it produces is more obvious. Don't be fooled by
this - if more flap didn't increase lift then flaps could not lower the
stalling speed. Drag alone doesn't lower the stall - which is why flaps are
better in this respect than air brakes.
Let's look at this more closely.
You are flying along at 70kts trimmed level flight. Lower flaps to the first
notch. This produces more lift and you start to climb - but as you want to
stay in level flight you therefore have to lower the nose. This lowers the
AoA so less lift is created - it cancels out the lift created by the flaps.
So rule one is that adding flaps results in a lower Angle of Attack for a
given speed. This is good as it gives you a better forward view during
approach and landing.
Rule two is that flaps also add drag so to maintain the same airspeed you
require a bit more power.
Adding further flap requires more power to maintain a specific speed but
also results in further nose down attitude as you trim out the lift. In
other words your AoA continues to reduce as flap is increased.
Aircraft don't stall at a given airspeed. They stall because the wing's
Angle of Attack has exceeded the CLMax. As speed is reduced lift is lost and
can only be replaced by increasing AoA - but only up to CLMax and then the
wing gives up and stalls. So, in level flight, for a given speed there is a
relative Angle of Attack. More speed, less AoA and less speed means more
AoA. But this is not a SPECIFIC speed because it varies with the weight of
an aircraft. If you are heavier you need more lift to take off so you either
need more speed or more AoA. So, although AoA and speed as directly relative
this relationship is altered by the weight of the aircraft.
As flaps reduce the AoA it therefore means that in slowing down it takes
longer to reach the critical angle of attack to stall the aircraft - which
means a lower stalling speed.
As flaps produce lift as well as drag it is good in that stall speed is
reduced but there is a penalty. Once flaps are lowered on the approach it is
EXTREMELY inadvisable to raise them again - the loss of lift is significant.
That is why power becomes the force needed to overcome drag. In this respect
speed brakes are better because they only create drag and you can increase
or decrease these at will to control speed. Speed brakes don't create lift
though so are rarely used on aircraft - flaps are better for giving aircraft
sensible approach speeds.
bones
-----Original Message-----
From: jhb-bounce@xxxxxxxxxxxxx [mailto:jhb-bounce@xxxxxxxxxxxxx] On Behalf
Of Gerry Winskill
Sent: 10 September 2007 08:47
To: jhb@xxxxxxxxxxxxx
Subject: [jhb] Re: Pireps
I eventually got around to examining the effect of increasing the landing
flap angle, on the TB21GT. It worked!
Before trying the mod I did a series of tests to check the actual stall
speeds, at 30 deg and 45 deg, with the gear down. Under the same temperature
conditions repeats show that it stalls at 56 kias and 52 kias, for the two
settings.
I then increased the Landing Flap angle to 60 deg. The first pleasant
surprise was, seen in spot view, that the angle did indeed increase. Equally
surprising was that the increased angle was supported by the deployment
brackets etc.
A repeat of the Stall checks showed the 30 deg figure to be unchanged but
the 60 deg flaps reduced the stall speed by 3 kts, to 49 kias. I'd thought
it might increase drag, without increasing lift.
Next to see what difference it made to landing attitude. With a landing
completed, with power slowly pulled back during the hold off, a landing was
completed, then played back. On Short Finals the attitude was slightly
flatter. In the hold off the AOA increased realistically, before it greased
itself on with no tendency to get close to a tail strike, as it had before.
A far better result than I'd expected. Although there was no crosswind to
complicate matters, I had it set with no headwind either.
It's going to be worth trying on the other couple of the of GA aircraft
that I use and it might confirm my impression that the Lionheart TB21GT is
above average.
Gerry Winskill
bones wrote:
>Slow speed handling in FS is especially rubbish if you don't fiddle
>with the values properly.
>
>The average aircraft wing stalls at 15 degrees AoA (forgetting
>supercritical sections and flaps). As you rightly observe some FS
>aircraft adopt much higher nose up attitudes before the stall occurs
>which means the wing is still flying at crazy AoA's. There's not a lot
>you can do about this unless you want to rewrite the Section 404 data.
>
>Your idea of adding more flap drag is a work around but you have to be
>careful as both lift and drag values create pitch moments. I'm
>surprised that drag coefficient is a fixed figure across all flap
>settings though - where are you finding this value and what are you
>editing it with?
>
>Don't forget that you can cheat by altering the flap selection settings
>in the cfg file by stating higher deployment angles than in real life.
>If the default values are 0, 10, 20, 30 you could change this to 0, 15,
>25, 40 or even 0, 15, 25, 60 (or more).
>
>Note that in the .air file in Section 320 there is a Max Flap Angle
>value in radians. I've no idea what this is for or whether it is now
>redundant.
>
>bones
>
>-----Original Message-----
>From: jhb-bounce@xxxxxxxxxxxxx [mailto:jhb-bounce@xxxxxxxxxxxxx] On
>Behalf Of Gerry Winskill
>Sent: 08 September 2007 07:39
>To: jhb@xxxxxxxxxxxxx
>Subject: [jhb] Re: Pireps
>
>
>Because I include the Vspeed gauge in my commercial aircraft, together
>with a Plaque giving Vref and throttle settings for various weights,
>I've had to carry out the exercise you describe, for about half a dozen
>aircraft. I soon discovered that the stall warning is a dodgy FS
>parameter and adopt your other suggestion, of using altitude drop. My
>multipage check list gauge allows me to watch for the point at which
>rate of descent starts to increase. It becomes an enjoyable exercise in
>its own right, even though quite time consuming.
>
>The overall time required is greatly reduced if, in each case, a
>situation is saved, so that you can recover to that situation, rather
>than starting from scratch. For instance I might save a situation with
>max landing wt, gear and flaps down and already within about 20 knots
>of expected stall speeds, at my test altitude. With one data set
>aquired I recover to the saved situation and start again.
>
>I do the same sort of exercise with the takeoff regime, to get some
>idea of required thrust v TOW. It's easy to just apply max thrust but
>with low TOW for a short sector, a decent bizjet model would have you
>off the ground in an unrealistically short time and extremely pushed to
>get it down to 250 kias without climbing vertically. Your raising the
>matter is going to force me to stop overlooking the effect of
>temperature. I do hope my SWMBO doesn't bear you too much animosity!
>
>Once the approach is adopted then I admit the workload just escalates.
>Because of the variation in thrust settings, I was forced to make a
>percent throttle setting gauge, etc, etc.
>
>Another area where I've started to do a bit of fiddling is GA aircraft
>landing speeds. In some, as delivered, it's quite difficult to land by
>the numbers. Sometimes the stall speed proves to be significantly lower
>than stated. Even worse, if you look from the outside, you find that
>holding off to the stall always results in a tail strike because the
>change of attitude doesn't produce an acceptable speed reduction..
>Sometimes this can be improved by increasing the flap drag. Which is
>where another limitation of FS surfaces. You can't increase the drag of
>one flap setting, in isolation. So it's possible to get the landing
>speed down to a realistic level, overcoming the equally unrealistic
>slipperiness of something in the Spam Can bracket, only to find that
>the first stage of Take Off flap increases the take off run to
>unacceptable levels. I tried increasing the aircraft drag instead, but
>this requires the available engine power to be taken beyond realistic
>levels, to maintain correct cruise performance.
>
>If you chuck this on top of the other bits in which we get involved,
>then getting up at 0600 becomes essential. For no pay, you ask? The
>blokes a nutter! I suppose we also forget how little we pay and what
>flight dynamics accuracy we can expect to recieve, for the money.
>
>Gerry Winskill
>
>
>
>bones wrote:
>
>
>
>>Wind and speed - two of the most important factors on the approach and
>>landing.
>>
>>A 10kt headwind can reduce landing run by 20% or more and because this
>>sort of wind is about normal for the UK most pilots get used to the
>>short landing it produces. A zero wind landing can still come as a
>>shock for the length of runway it uses up, the apparent lack of
>>deceleration after touchdown and the sloppy control reactions.
>>
>>Speed is more critical. I've already mentioned this in respect to
>>Gerry's farm strips but it applies to all aircraft regardless of size.
>>Excess speed on the approach is going to result in excessive landing
>>distance. Speed is mostly relative to weight, which is why airline
>>pilots calculate the approach speed for EVERY approach. It is also
>>relative to other factors like flaps, temperature and pressure so a
>>complex graph is used to determine the right speed (or the FMS on
>>posher aircraft).
>>
>>For light aircraft the published approach speed is usually a single
>>value and based on the maximum weight the aircraft is authorised for.
>>Technically this means you are safe at any weight (you won't stall if
>>heavy) but the lighter the aircraft is from MAUW the lower the
>>stalling speed and the more you will float on flaring the aircraft.
>>
>>A PA28 181 has a flaps down stall of 49kts. Approach speed is 1.3 x
>>Stall so this is 63.7kts (usually rounded up to 65kts). This book
>>figure is based on the MAUW of 2550lb. With just one person on the
>>aircraft and 10 gal fuel the weight is down to 1800lb - a good 750lb
>>lighter - and this drops the stall speed by 7.8kts to 41.2kts. At this
>>weight an approach could be flown at 55kts safely. The important point
>>is that if the approach is flown at 55kts at 1800lb it results in the
>>same landing run as flying at 65kts at 2550lb - because the lift
>>generated on the approach is the same in both cases.
>>
>>If you don't adjust for weight and fly at 65kts on the approach at
>>1800lbs then you are too fast. In flaring the aircraft it has to
>>decelerate from 65kts down to 41kts before the wing no longer
>>generates lift to fly and that eats up a huge chunk of runway.
>>
>>It may be hard to apply this learning in FS because you need to know
>>the MAUW and the flap down stall speed of the aircraft you are flying
>>and this isn't always available. If it is then, as a rough guide, for
>>every 100lbs you are below MAUW subtract 1kt from the stall speed.
>>Your approach should be made at 1.3 x Stall.
>>
>>Airliners are far more complex and the approach speed can vary hugely
>>with weight. I doubt many FS pilots use anything other than a nominal
>>book figure for approach speed but it would never work in real life.
>>Excess speed in an airliner on landing has far more serious
>>implications.
>>
>>Even on big jets the same basic rule applies - approach speed is
>>determined as 1.25 x stall. If you are used to banging in the same
>>fuel amount on a jet each time you fly you can see from the above that
>>a short flight of 30min is going to need a faster approach speed
>>(little fuel used so heavier aircraft) than taking the aircraft across
>>the pond on a 7 hour trip. Which leads to a nice little exercise in FS
>>for you..
>>
>>Take your favourite jet and prepare it for flight as you might
>>normally do. If you never fiddle with fuel amounts so much the better.
>>Before you start this trip write down the normal speed you use for the
>>approach. Now get airborne, fly a circuit round to a 10nm final on the
>>ILS and settle on the approach in normal landing configuration at your
>>normal approach speed.
>>
>>Set the Alt Hold to 500ft so the aircraft levels off at this height.
>>DON'T touch anything - keep the gear and flaps in approach settings.
>>Now, slowly start reducing the speed on AT so that the aircraft slowly
>>slows down - it has to be slow for a good reading. Forget that you
>>have overflown the airport as that was only to get you settled into
>>correct landing configuration with flaps and gear down. As you bring
>>the speed back watch for one of two things - either the stall warning
>>or the aircraft starting to lose height. One or the other will happen
>>first - it's a mark of how well the perf file has been written. Make a
>>note of the speed at which this occurs.
>>
>>This exercise should show (roughly) the stall speed of the aircraft at
>>your normal loaded fuel weight. OK you may have spend 20 min doing
>>this so you could say it would represent a short flight. If you
>>multiply the observed speed by 1.25 this should be your approach speed
>>AT THAT WEIGHT - and it would be interesting to see if it is anywhere
>>near the speed you normally use.
>>
>>I could ask you to repeat the exercise except this time you take the
>>aircraft for a pretty long run so a lot of fuel is burnt off. It may
>>be easier just to dump fuel using the menu. Regardless, if you
>>repeated the exercise but with a great deal less fuel on board (and
>>maybe an emptier
>>cabin) then when you come to reduce throttle to get the aircraft to
>>stall you may be very surprised to find that the figure is
>>substantially less
>>
>>
>than
>
>
>>when you flew with a lot more fuel on board.
>>
>>For 90% of our flying we don't do any of this in FS and that is where
>>the division starts to break down between FS and real world flying. FS
>>is good for showing us system management of aircraft but it's pretty
>>much goldfish when it comes to teaching airmanship. Very few FS pilots
>>learn to appreciate the relationship between weight, speed, angle of
>>attack and approach handling.
>>
>>One thing I may have misread from Mike's post but you should never
>>vary approach speed with wind strength. The only time you EVER do this
>>is in gusty conditions when you should add 50% of the difference
>>between Max and Min wind speeds as given by ATC.
>>
>>bones
>>
>>-----Original Message-----
>>From: jhb-bounce@xxxxxxxxxxxxx [mailto:jhb-bounce@xxxxxxxxxxxxx] On
>>Behalf Of Mike Lucas
>>Sent: 07 September 2007 12:31
>>To: jhb@xxxxxxxxxxxxx
>>Subject: [jhb] Re: Pireps
>>
>>
>>Something I discovered landing a F100 at EGNS last night. Bones had
>>given me "Winds calm"; I had the approach set up perfectly, usual
>>speed, full flaps ... but it just felt wrong coming over the
>>threshold. I have never experienced a medium jet going "floaty" on me,
>>but that's the only way I can describe it. Fortunately I had enough
>>margin to plonk it down after using rather more of the runway than
>>usual, and I stopped before the end - but only just; I had to
>>backtrack for Taxiway B. I could/should have knocked 10 kts off my
>>final approach speed.
>>
>>Mike L
>>
>>bones wrote:
>>
>>
>>
>>
>>
>>>Wind is your friend - with a 10kt headwind your
>>>landing run is greatly reduced but pilots get used to this. Landing
>>>with no wind eats up more runway but this is exceptional in the UK
>>>and so pilots don't realise what the normal landing performance is -
>>>they become accustomed to windy conditions and it affects their
>>>perception of landing distances.
>>>
>>>
>>>
>>>
>>
>>
>>
>>
>>
>>
>
>
>
>
>
>
>
Other related posts:
