[jhb] Re: Pireps
- From: "bones" <bones@xxxxxxx>
- To: <jhb@xxxxxxxxxxxxx>
- Date: Sat, 8 Sep 2007 09:36:17 +0100
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.
>>
>>
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