Turns at low IAS

[old man emu]

4 minutes ago, RFguy said:

moderate speed steep descending < 30 deg turn

Just enlighten me here. Are you saying that the Base and Finals turns are not Rate 1 turns, but higher Rate turns?  I'm not looking for an argument. I just don't know. I do know that a Rate 1 Turn in an aircraft flying a low speed only required a bank angle of a few degrees.

[Garfly]

I don't think we have the same understanding on this, RFguy.

To me, to accept the 'parcel of air' concept is to accept that 'wind' only refers to this parcel's movement across the land below.  Therefore, with a given wind value, making a 180 degree turn within our parcel (all else being equal) has no effect on the airspeed - and thus, the approach of the stall speed - whatsoever.

Edited January 16, 2021 by Garfly

[facthunter]

It's a bit hard to "GET" this, but when you are turning in a parcel of Air it's all the plane knows, same as if you are circling in the middle of a flowing river at a constant speed through the water. From the bank you look to be slowing upstream and going faster downstream. Getting out of the bottom of the sky and back into it you get the problems or when there's windshear .Nev

Edited January 16, 2021 by facthunter

[old man emu]

43 minutes ago, Garfly said:

to accept the 'parcel of air' concept is to accept that 'wind' only refers to this parcel's movement across the land below.

This debate can only reach a sensible conclusion if we define the Frame of Reference that the air mass and the aircraft are in, since "airspeed" is the speed of an aircraft relative to the surrounding air.

 

If we take as our reference point an observer on the ground, then what is easy to observe is the aircraft's ground speed. If, however our observer is in the aircraft, then the observation is of airspeed, and that is relative to the observer in the aircraft.

 

So, in my opinion, turns from headwind, through crosswind to tailwind and their effect on airspeed have to be considered from the frame of reference of the aircraft.

 

Let's take the simplest case - nil wind and constant height.

The aircraft travels on a heading of 180 M at an airspeed above the stalling speed. If a turn is made to 090 M, the airspeed remains the same throughout the turn. This turn takes time, and the track made good is a curve. Likewise if, after flying on the 090 M heading for a few minutes, another turn is made to 000 M then the airspeed still remains the same.

 

Now, let's observe that flight from the frame of reference of an observer on the ground. Since the air mass the aircraft is in is not moving, the observer on the ground will see the aircraft fly a neat rectangular course, with curved corners.

 

Now, let's give the air mass a velocity, relative to the observer on the ground of, say 10 kts. In the frame of reference of the aircraft in the air, the airspeed remains the same as in the nil wind situation because the airspeed is a measure of the speed of the aircraft relative to the surrounding air mass. The aircraft can fly the same manoeuvres as in the still air condition and the airspeed will be constant.

 

HOWEVER, to the observer on the ground, as the aircraft flies into the wind, it will take longer than before because the aircraft is moving in the air mass in a backwards direction over the ground. It's groundspeed has been reduced. As the turn to 090 M is made, the airspeed remains the same, but the ground speed might increase as the wind is no longer coming from the side (I'm open to comment on that statement). Finally as the next turn is made and the air mass and the direction of travel align, the groundspeed will increase, but the airspeed will remain the same.

 

From the observer on the ground's frame of reference, the aircraft flies slowly into wind then makes a turn that results in a track at an angle towards the observer, and an increase in ground speed. Finally, the aircraft turns to travel with the wind, relative to the observer on the ground, and the aircraft's ground speed increases. The track made good is a little bit like a triangle.

 

At no time during either example does the airspeed change as a result of the aircraft's heading relative to the direction of movement of the air mass. Therefore, if an aircraft is flown at the recommended airspeed for its weight, angle of attack and angle of bank, it can't stall.

 

 

[RFguy]

agree 100% on tailwind landing.

Now in the case of a pure crosswind landing :  (for the example, left hand circuit, wind on landing will be on your right so you have a headwind on base) 
 

Going from base leg with headwind (say 100% headwind) to final (100% crosswind) -- that will be going from some headwind to zero headwind  and THERE you could come unstuck if you have insufficient airspeed margin and you are not watching what you are doing- 

 

The turn will take maybe 10 seconds, so unless you have completely gone to sleep and not watching the ASI (or feeling wind in your hair) , there will be plenty of time  to add power, or push nose down, or relax the turn  (or a combination thereof) .  

 

[old man emu]

3 minutes ago, RFguy said:

that will be going from some headwind to zero headwind 

I know that you were writing this as I was writing mine, but if you have read mine, can you repost yours, indicating the frame of reference, please?

[RFguy]

(my reference is the ground)
"making a 180 degree turn within our parcel (all else being equal) has no effect on the airspeed - and thus, the approach of the stall speed - whatsoever."
 

I disagree. Let me provide an example : 

 

1) The aircraft is travelling at 100 K over the ground headed south 

2) and there is a 50 K headwind from the south   

3) therefore IAS is 150 K. yes, agree ?

now, the aircraft 'instantly' turns left 90 degrees (and we assume the turn does not cost any energy) 

4) the ground speed is still 100 K (momentum was conserved ) 

5) the headwind is now ZERO because the wind is now off the wing .

6) Now ground speed = airspeed = 100K. We lost 50 K of airspeed by turning.

 

 

 

Edited January 16, 2021 by RFguy

[Kyle Communications]

OME you wanted some figures.

Savannaj VG XL  what I do

 

Downwind usually around 65-70 kts at the end of the runway I move the trim to bring the nose up to wash off speed then once dow to 52kts I lower the flaps then start my rounded turn to final. I keep the aircraft at 50 to 52kts all the way through the turn until lined up then pull the nose slightly to wash off the speed to around 45 kts for decent then start a slow roundout to wash off the excess speed and touchdown about 38 kts or a little less depending on the headwind. 

Weight of the aircraft can vary but empty mine is 325kg..usuall no more than 70 litres fuel onboard as that is both inner tanks full and me at 107kg so flying weight at around with both tanks full is around 482kg

Flying at 600kg with a pax pretty much all the same maybe a couple of kts faster on final but 52kts is fine for the turn..sink is a little faster but not much but that has more to do with aircraft type

 

[Garfly]

4 minutes ago, RFguy said:

(my reference is the ground)

 

1) The aircraft is travelling at 100 K over the ground headed south 

2) and there is a 50 K headwind from the south   

3) therefore IAS is 150 K. yes, agree ?

 

 

 

 

No. Can't agree, because you say your ref is the ground but the IAS cares not a jot about the ground.

 

[spacesailor]

If using GPS, it will show a differance against your pito air speed.

NO IT,S NOT GROUND RELATED AS IT ORBITS EARTH.

spacesailor

[RFguy]

Yes I agree  with that .  I have sought to explain something that isnt relevant. (ground observer reference) 
 

YES I agree  if flying in a circle in the box of air, it doesn't matter ,  even if the air is at 10000 knots,  if we are flying in a circle at 100 KIAS, the airspeed does not change, the box of air  is moving . 

 

In the case of on the GROUND we are in that case fixed to the ground observer, that is different 

 

 

 

[RFguy]

I apologize for confusing myself with a irrelevant argument. We're not talking about kites  I must need more sleep.

Edited January 16, 2021 by RFguy

[aro]

44 minutes ago, RFguy said:

1) The aircraft is travelling at 100 K over the ground headed south 

2) and there is a 50 K headwind from the south   

3) therefore IAS is 150 K. yes, agree ?

now, the aircraft 'instantly' turns left 90 degrees (and we assume the turn does not cost any energy) 

4) the ground speed is still 100 K (momentum was conserved ) 

5) the headwind is now ZERO because the wind is now off the wing .

6) Now ground speed = airspeed = 100K. We lost 50 K of airspeed by turning.

 

#4 is wrong so any conclusions are invalid.

 

Momentum has a direction. Momentum is not conserved, it is changed from one direction to another. That requires a force - the lift produced by acting on the air. Forces are acting on the air, not the ground so everything needs to be worked out relative to the air mass.

 

The ground is irrelevant, apart from visual illusions.

 

 

[Garfly]

2 minutes ago, aro said:

 

#4 is wrong so any conclusions are invalid.

 

 

But get RFg on anything to do with Comms and the boot will be on the other foot!  😉

[RFguy]

Momentum is mass x velocity. Velocity is a vector .  But

the net force acting is gravity  so for a left or right turn at same height has no net change of force acting on it,

so the momentum (kgms)  would not change .

But I agree, that in the real world you are not going to execute a turn without momentum or energy cost.
 

[RFguy]

I have embarrassed myself greatly by misreading the argument.

 

and then responding and being completely  topsy turvy wrong because for a while I though I was a kite with the attached  ground observer moving 

...funny. down to earth now. 

[aro]

12 minutes ago, RFguy said:

Momentum is mass x velocity. Velocity is a vector .  But

the net force acting is gravity  so for a left or right turn at same height has no net change of force acting on it,

so the momentum (kgms)  would not change .

 

Velocity is a vector which means if you change direction, velocity and momentum have by definition changed.

 

The force to change comes from the lift of the wings acting towards the centre of the turn.

[RFguy]

No, the quantity of momentum does not change. 

the forces acting on the plane are unchanged because gravity is directly below us. 

so for these purposes, momentum can be a scalar.

 

yes- in real life  there will be force required to make that change  but I specifically said that was not  going to be taken into account. 

 

[aro]

3 minutes ago, RFguy said:

No, the quantity of momentum does not change. 

The speed and quantity of kinetic energy do not change. Momentum definitely does change. Ignoring that is ignoring the laws of physics.

 

The question is fundamentally about the force required to make the change - ignoring it makes no sense.

[RFguy]

OME you wanted some figures.

Brumby 610 will do .  I have flown that the most.  

 

Downwind usually around 85-95kts,

-toward the end of downwind leg, I reduce power  and trim  nose up to reduce airspeed to about 80 kts 

-going into the base leg turn, I hold the nose up in the turn so there is no altitude loss, and airspeed falls well into FLAP range )(Vfe = 80)

-on turn completion into base leg , flaps go out  (all or nothing in that aircraft) ,  power is increased , and aircraft trimmed for approx 500 fpm descent at 70 kts.

- turn into final complete at 500', power and trim for 65 kts

- 50'  nose up a bit, flare. land about 57 kts .

- any slower than 55 knots at low power and you can find yourself in trouble if the flare or float is extended, airspeed reduces quickly ...

above is for nil wind. 

aircraft usually 500-600kg, not much difference in landing . takeoff quite different at low weights. 

 

 

 

[RFguy]

7 minutes ago, aro said:

The speed and quantity of kinetic energy do not change. Momentum definitely does change. Ignoring that is ignoring the laws of physics.

 

The question is fundamentally about the force required to make the change - ignoring it makes no sense.

Yes. The speed and quantity of kinetic energy do not change, that's right -  the quantity of momentum does NOT change  , since we have the same forces acting on the airplane . We will agree to disagree-   We are responding to different questions...My point is that ignoring something intentionally (but being aware of its presence)  is done to simplify an analysis that could otherwise be too complex to take into account every possibility.

Edited January 16, 2021 by RFguy

[Bruce Tuncks]

Nev beat me to it, but the point is important: If you have a long nose plane, at circuit speed everything in front of you gets hidden. So you have no choice but to fly a curved base leg, you see much better this way.

Learning in a glider, the main thing drummed in was " safe speed near the ground" . This was 1.5 times stall speed as a minimum. Most of us gave it a few knots more, those big dive brakes are wonderful .  We never had a stall-spin in a glider.

On take-off behind the tug you have no control over the speed of course, except to let go if it gets too wrong. At the 70 knots tow speed, a glider can do a climbing 180 turn if needed. Sometimes they do an exercise where a tug failure is simulated.

Once, solo and  at 600 ft, the tug did a diving turn and I had to let go. It turned out that the elderly tuggie had forgotten which part of the tow he was doing ... this was the last day he ever towed. I never complained, and I don't think anybody else noticed. I think he just felt unwell and knew it was time to retire. I hope to be as smart one day.

[Yenn]

I don't know what you mean by "use your shoulder" but I always adjust the start of my turn from downwind according to the wind direction. A strong cross wind will either extend my downwind if it is going to be a tailwind on base, or reduce my downwind if it will be a headwind on base. If I am going like a rocket on downwind I will start the turn early, so that I don't undershoot.

RFguy I think you need to update your understanding of how a plane flies in a body of air. If it was possible for you to change direction 180 degrees instantaneously, it would also keep its forward speed, so there is no change of indicated air speed and no fear of stalling. Of course if you have a tail wind you can afford to lower the nose, because the sight line for a tailwind landing looks as if you are way too low, compared to a headwind landing.

Where I fly I will often elect to land downwind at up to more than 5 knots, this is because I ill have an uphill landing and to land into wind I tend to just float along parallel to the downhill slope and also i have poorer visibility.

[old man emu]

Methinks that some people need a course in the definitions of terms used in Physics.

 

[Garfly]

4 minutes ago, old man emu said:

Methinks that some people need a course in the definitions of terms used in Physics.

 

I'm sure I do, for one.

But I'm here to learn how to fly better/safer.

(And have fun chatting about it. )

Edited January 16, 2021 by Garfly