question Brain teaser

[Thruster88]

Has this thread run longer than the never ending story?

 

Time to think up another brain teaser.

 

Since I have only been here for a relatively short time I am not sure if we have done the "down wind turn".

 

We are flying a thruster at 40kts indicated airspeed into a 30kts wind, GS is 10. We can complete a 45° banked 180° turn in about 4 seconds just above the stall, GS is now 70kts. 10 to 70 in 4 seconds, why do we feel only the normal turning acceleration and not the super car like acceleration ? Why doesn't the plane stall and fall ?

Edited April 5, 2020 by Thruster88

[facthunter]

Because at height you are moving in a uniform airmass and the existence of the ground has no relevance except optical illusions and windshear effects closer to the surface and orographic effects ( Mechanical turbulence). While your wheels still contact the surface you are subject to forces from both sources and for a while there is a transition happening which requires your attention and serious consideration.. (Take off and landing). Nev

[Student Pilot]

Stop it, there you go thinking again................................................................Just fly the aircraft

 

[facthunter]

THIS one is pretty important.. Nev

[Student Pilot]

Yes it is, but you don't need to know flame propagation and the physics of the drag coefficient of over 50 weight oil on cross hatching and ring scraping action to know how to increase throttle to get higher revs.

Do the complete training and know your aircraft.

Low level handling should be part of every-bodies training, RAA and GA, including downwind turns.

[facthunter]

So, all the "trainers" have this right, do they?

[Student Pilot]

No idea what you mean Nev? Do you mean all can train low level? Or all should?

[facthunter]

No plenty can't but I believe ALL pilots should. You fly low twice each flight and never know ahead of time when you might do it for a bit longer than you normally do and it's something you must do well or you might not survive.. Nev

[Norm559]

Who doesn't believe in practising this manoeuvre at altitude and using altimeter to realise height loss?

[Norm559]

Who doesn't believe in practising this manoeuvre at safe altitude and using altimeter to realise height loss?

[old man emu]

This is the question

why do we feel only the normal turning acceleration and not the super car like acceleration ?

 

and this is the answer

Because at height you are moving in a uniform air mass and the existence of the ground has no relevance

 

The stall speed of a wing is related to the speed of the wing relative to the air surrounding it. This is an Earthly example of relativity, the theory proposed in 1915 by Albert Einstein. The central point of the theory is the principle of general relativity, which states that all observers, regardless of their state of motion, will see the same laws of physics operating in the universe.

 

Let's use the "walking through a train carriage" example.

A train is travelling along at 50 kph, and a passenger can walk at 5 kph. If the passenger in a carriage walks towards the direction of travel of the train. then

1. Relative to the train carriage, the passenger is walking at 5 kpH.
   
2. Relative to an observer standing on a station platform, the passenger is walking at (50 + 5) kph.
   
3. If an observer is travelling on a road alongside the train at 50 kpH in the same direction as the train, then the passenger is moving at 5 kph.
   

So, consider the passenger to be the wing, and the carriage the air mass it is located in. In order to produce Lift, the wing must move relative to the air mass at a speed greater than stall speed. That is the relativity situation the pilot observes, and why the pilot does not feel acceleration forces as the movement of the air mass, relative to an observer on the ground, results in a speed, relative to the observer on the ground, increasing from the 10 kts into wind to 70 kts with the wind (tailwind).

 

It should be remembers that ground speed is another case of relativity. In that case, the pilot makes observations of the relationship between the aircraft and the Earth's surface below. Don't forget that, relative to the Sun, both Earth and plane are moving at 30-odd kilometres per second. You never include that figure in your flight planning, do you?

[APenNameAndThatA]

Stop it, there you go thinking again................................................................Just fly the aircraft

 

I'm with Nev on this one. Thinking this through is not overthinking at all. It is important so that you know not to use ground speed as a proxy for airspeed.

[IBob]

There is an everyday (non-flying) parallel to this riddle:

My car has tires approx 24" diameter.

Choose a point...say a segment of tread...on the outside of the tire.

If I drive at 100mph:

that point is travelling at 0mph when on the road, accelerating to 200mph as it passes over the top of the wheel, decelerating to 0mph as it meets the road again, and it's doing that 24 times a second.

From one point of view that is exactly what is going on.

But the tire itself would be subject to massive distortion if that were so.............

[red750]

Similar conundrum with rotorcraft - rotor tip speed of advancing and retreating blades.

[facthunter]

THAT situation is extremely straight forward and has nothing to do with an aircraft turning in a moving airmass as the blades are rotating about a fixed point at a fixed rotation speed (R's PER MIN) so they go fast/slow RELATIVE to the air mass. A plane flying in a moving airmass doesn't do that. It's airspeed doesn't vary. Nev

[antonts]

We can complete a 45° banked 180° turn in about 4 seconds just above the stall, GS is now 70kts. 10 to 70 in 4 seconds, why do we feel only the normal turning acceleration

 

first of all forget about wind, just static calculations

 

you cannot make 180 turn in 4 sec with "normal acceleration" - 80 knots difference = 40m/s - you must have constant (straight back, no change in direction or value) 1G acceleration (OVERload! not total with gravity!) for 4 sec only for this. When you turn level 45 banked on average light plane (where engine is just enough to keep it in air) you have 1.5G total, with rotating vector, and 180 turn takes about 15sec.

 

If you make it for 4 sec - you get not 1.5g but 4g (lets take a plane which can support 4G on 40kn, with huge engine which can force you into desired path without any wings or other silly stuff, like rocket), and this is more than enough for required velocity change.

 

Now add wind. You will not sense acceleration vector rotating not as before, on steady circle, but by other curve - giving the same side acceleration (total zero, as you had zero side speed before and got zero after turn) but different acceleration in direction of course/wind.

[Flightrite]

You guys are scaring me I don't think I'll get back in another plane, just too risky??

[Thruster88]

first of all forget about wind, just static calculations

 

you cannot make 180 turn in 4 sec with "normal acceleration" - 80 knots difference = 40m/s - you must have constant (straight back, no change in direction or value) 1G acceleration (OVERload! not total with gravity!) for 4 sec only for this. When you turn level 45 banked on average light plane (where engine is just enough to keep it in air) you have 1.5G total, with rotating vector, and 180 turn takes about 15sec.

 

If you make it for 4 sec - you get not 1.5g but 4g (lets take a plane which can support 4G on 40kn, with huge engine which can force you into desired path without any wings or other silly stuff, like rocket), and this is more than enough for required velocity change.

 

Now add wind. You will not sense acceleration vector rotating not as before, on steady circle, but by other curve - giving the same side acceleration (total zero, as you had zero side speed before and got zero after turn) but different acceleration in direction of course/wind.

 

Turn rate is dependent on airspeed and bank angle, I used 45° bank angle in my example but a thruster is capable of a sustained 60° bank angle but not under raa rules. I used an online calculator to make this little chart. Load factor for the 4.2 second 180 would be 2g

Bank Angle.....Speed kts.......Radius Nm.....Rate °/sec...?.Time for 180° seconds

......45..................120................0.21....................9......................20

......45....................90................0.11..................12.....................15

......45....................60.................0.05..................18.....................10

......45....................40................0.023.................27.....................6.6......(turn radius only 43 metre! )

......60....................45................0.017.................42.....................4.2...................................31

Turn rate and radius is good to know if you get caught in a box canyon. It is also one of the reasons slow aircraft are fun to fly.

 

Not sure about your last paragraph.

[antonts]

in level fligt with engine thrust only against drag everything determined by simple equation - weight=lift*cos A. It means if you are on 45 - you have 1.5G (roughly), 60 - 2g (exactly). Speed in this case does not matter, it affects time of turn and radius, but not the acceleration. So if you have to change speed to 80 kn (40m/s) and have only 0.5G (5m/s2) available acceleration it takes 8 sec even if you accelerate straight against speed vector, which is not true in case of usual bank turn.

 

Of course if you fly slow but able to upkeep the same load factor (I do not know machine which is able to upkeep 1.5g at 40 kn without huge thrust, only by wings lift - usually 40kn is stall speed even for light planes, means they are able to achive only 1G, but may be some ultralights and STOL with stall speed 30 can) you turn will be faster and narrower, as you have to change less speed with the same acceleration. Time will be N times less, radius is N2 less.

 

last paragraph is about the same level turn but if you see it from (imagine instead of wind) straight moving car. Plane makes the same halfcircle - but from the car you see only distorted path, not circle at all, so nothing surprising in visible change of speed.