Turns at low IAS

[old man emu]

1 hour ago, Bruce Tuncks said:

We should however always state the frame of reference we are using.

That is indeed the source of confusion in this debate. What you have to do is think of the aircraft occupying a bubble of air and that bubble is floating in the general airstream. It is so hard to create an analogy for this frame of reference especially when people are so used to seeing this diagram

 

 

CONCERNING MOMENTUM

 

According the Newton's First - an aircraft flying straight and level will continue to do so until an external force is applied to it. Because of its mass and velocity, the aircraft has an amount of momentum, determined by (m x v). If you measure the momentum at an initial time, T1, and again later at T2 and compare them, then you will find that since they are the same, the change in momentum is zero.

 

If you apply an external force to the aircraft you will alter its momentum. You could apply the force "instantaneously", say by flying through an air pocket which would alter the velocity of the aircraft - not going into the complex details of that now. Otherwise you could apply a force over a longer time to change the direction of the aircraft by making a turn. The application of a force over a period of time is called "Impulse", represented in physics shorthand by the letter J

J = F x (T2 - T1)

 

When you turn an aircraft, the movement of control surfaces produce forces which eventually result in one force called Centripetal Force. This is the force that you generate in your arm when you swing a weight on a string around your head. While you are swinging the weight on a constant length of string, you are pulling it towards you. Centripetal force depends on mass, velocity and radius of turn.

F = m(v^2) r

If we substitute centripetal force into the impulse equation we get

J = m(v^2) r x (T2 - T1)

 

If the mass and velocity of the aircraft do not change during the turn, then it is acceptable practice to recognise their involvement, but to give them each the numerical value of one (1). This makes the impulse equation

J = 1 x 1 x r x (T2 - T1)

which tells you that the Impulse is inversely proportional to the radius of the turn and/or the time taken to complete the turn. You can feel the impulse in your body. A wide, slow turn doesn't push you into your seat as much as a sharp, quick turn. 

 

Just to clear up what seems to be a logical result of Newton's Third - equal and opposite forces, but is in fact a myth. We have Centripetal Force, which is a force tending to move a mass to the centre of its circle of rotation. We expect there to be an equal and opposite force, and have given it the name, Centrifugal (centre fleeing) Force. No such force exists. What our logical thought has invented in order to seemingly comply with Newton's Third, is called an Imaginary Force. 

 

 

[old man emu]

4 minutes ago, aro said:

The source of the force that makes the change is the wing.

 

We bank the aircraft, which means that there is a sideways component to the lift force. This sideways force turns the aircraft.

Well, yes. But as I intimated, I was talking then about momentum in general terms and trying to list factors that could alter the momentum of an aircraft. Actually, in straight and level flight in nil wind the momentum of the aircraft is continually changing. "How?, you ask.

 

Because you are burning off mass at the rough rate of ( 0.75 x fuel burn rate)/3600 kg per second.

 

Bloody Reality. Nothing ever stays the same!

 

[aro]

12 hours ago, old man emu said:

Making a turn is, in physics, a change in the vector value of velocity, so we say that the aircraft's momentum has changed. It's a bit hard to spot the source of the Force that makes the change

You specifically said making a turn.

 

A change in the mass due to fuel burn is one of the changes it is simpler to ignore. But the force resulting in THAT momentum change is drag.

[old man emu]

3 hours ago, aro said:

You specifically said making a turn

No I didn't. I said,

So if an aircraft of mass m, is flying along, straight and level, at velocity, v, then it has momentum of mv. If anything happens to change v, then the momentum of the aircraft changes. Making a turn is, in physics, a change in the vector value of velocity, so we say that the aircraft's momentum has changed. 

 

Please don't quote me out of context. 

 

Also, 

3 hours ago, aro said:

A change in the mass due to fuel burn is one of the changes it is simpler to ignore.

Agreed. That's why you've probably never considered it in this discussion of momentum. Actually, the realisation that fuel burn would affect aircraft mass in an ever changing way simply popped into my head as I was writing.

 

3 hours ago, aro said:

But the force resulting in THAT momentum change is drag.

 

Whoa there, cowboy! If fuel is being consumed in order for the engine to produce a constant amount of thrust, the fuel of itself is not creating any force. It is simply being introduced to an engine where one unit of fuel is combusting in approximately 14 units of air, resulting in the conversion of a mixture of long chain carbon molecules and oxygen into a mixture of very short chain oxides of carbon, nitrogen and some trace elements. 

 

It's the same as if there was a leak in the fuel lines, or the fuel caps came off wing tanks. The mass of the fuel is removed from the overall mass of the aircraft, but no force is created by that action.

 

[Bruce Tuncks]

The force impulse which changes the momentum comes from the bank angle used in the turn. If you were to simply and quickly rotate the helicopter 180 degrees, you would suddenly have a reverse airflow. In the example, with a 50 knot asi  and a 50 knot wind on the nose initially, you would now have a 50 knot wind from the tail and the asi would now show minus 50 knots but the helicopter would have zero groundspeed just as before.  Note that the rotor disk which was tilted into the wind has not changed so to do this swivel, the helicopter would now be nose-up and "flying backwards". Also note that the groundspeed is not relevant.

Of course, if you lowered the nose and applied more collective ( and power) you could accelerate downwind and the impulse of the new force would equal the momentum change. In this case the force would not come from the bank angle but from the tilted rotor.

I guess this stuff really is rocket science.

 

[Bruce Tuncks]

OME, while I liked your posting, I have to say that I have never heard of centrifugal and centripetal forces as "imaginary ". Gosh, there is 7000g's at a Jabiru's prop tip! That really can unpeel tape I can say for sure.

[Student Pilot]

Why do most threads these days turn into PP?

Edited January 17, 2021 by Student Pilot

[old man emu]

2 hours ago, Bruce Tuncks said:

I have never heard of centrifugal and centripetal forces as "imaginary ".

Centripetal force is a real force. Think of this. Put a weight on a length of string and toss it away from you (to set a starting position). All set? Now pull the weight towards you . The force you are applying is pulling the weight towards you. That's called the centripetal force - from modern Latin centripetus, from Latin centrum (see center) + -petus ‘seeking’ (from petere ‘seek’).

 

Next test. Put the weight away from you as before. Get a helper to pick it up and start walking in a straight line that is not towards you. The string will tighten and the helper will feel a continuous pull towards you. That's the centripetal force that you are exerting on the weight and the helper. The helper will not feel any force pushing the helper in the opposite direction along the outwards extension of the line of the string. If the helper feels the centripetal force, but no force sending the helper in the directly opposite direction, then there is no force.

 

Final test. Get out in an open area. Now start swinging the weight around your head, parallel to the ground. Look along the arm that is holding the string and pick a reference point that is at a right angle to the direction you are facing. When the weight approaches the line between you and the reference point, let go of the string. The weight will not fly off towards the reference point because there is no force acting on it to make it go that way. We think that because of Newton's Thirds Law - equal and opposite forces - there should be. So to satisfy ourselves we make up a force and call it Centrifugal Force - from modern Latin centrifugus, from Latin centrum (see center) + -fugus ‘fleeing’ (from fugere ‘flee’). These two words come into the English language in the early 1700s, after Newton had published his three laws.

 

When the weight is whirling around your head, it wants to keep moving in a straight line (Newton's First), but you are exerting a force on it to pull it towards you (centripetal force) This force is acting on the weight at every instant it is moving around over you head in a circle. Every instant its inertia wants it to go in a straight li line that is a tangent to the circle, but it can't because you are exerting on it. If you let go of the string, the centripetal force is gone and the weight continues along the straight line that is tangential to the point where the weight was when the centripetal force was removed.

 

 

[Jabiru7252]

All falls apart at the quantum level though...

Actually, that's not strictly true, but things do get weird...

Edited January 17, 2021 by Jabiru7252

[Garfly]

Falls apart at the WTF? level, too ... as a wise old bird once said:

"Does any average person know the physics of how their car manages to turn corners? Or the thermodynamics of the combustion process that produces the forces necessary to propel a car? NO. As long as the driver has the skills required to make the car go in the desired direction, and the desired velocity, then the practical operation of the car is achieved."

 

Source:

 

 

Edited January 17, 2021 by Garfly

[Jabiru7252]

I think a basic understanding of lift and how a plane gets airborne is essential, but the deep physics isn't needed and doesn't make you a better pilot. 

[RossK]

On 17/01/2021 at 10:01 AM, old man emu said:

While you are swinging the weight on a constant length of string, you are pulling it towards you. Centripetal force depends on mass, velocity and radius of turn.

F = m(v^2) r

If we substitute centripetal force into the impulse equation we get

J = m(v^2) r x (T2 - T1)

 

If the mass and velocity of the aircraft do not change during the turn, then it is acceptable practice to recognise their involvement, but to give them each the numerical value of one (1). This makes the impulse equation

 

 

 

This thread has too much of this^^

and not enough of this

[old man emu]

51 minutes ago, RossK said:

This thread has too much of this^^

and not enough of this

Isn't Bruce allowed to be provided with new information?

20 hours ago, Bruce Tuncks said:

I have to say that I have never heard of centrifugal and centripetal forces as "imaginary ".

In our day-to-day lives, where the knowledge of the intricacies of the Laws of Nature are not essential to our activities we use borrowed words to name, or explain things. I've used a machine called a centrifuge to separate solids from a mixture of solids and liquids. I use a centrifuge to get the majority of water out of my clothes before I hand them out to dry completely. As Humpty Dumpty said

or Shakespeare, 

 

However, when we enter the specific context of an area of study, we must give the words the meanings that they are meant to have in that context. If someone does not know the meaning that a word has in that context, then they are entitled to have it explained to them

[RossK]

6 minutes ago, old man emu said:

Isn't Bruce allowed to be provided with new information?

 

 

However, when we enter the specific context of an area of study, we must give the words the meanings that they are meant to have in that context. If someone does not know the meaning that a word has in that context, then they are entitled to have it explained to them

Yes, but of the 2 activities I posted above, I know which one makes me a better pilot.

I think this thread would be better with less of the deep theory and more of the practical realities of piloting an aeroplane in the circuit.

I gave an example earlier where I felt least comfortable in the circuit and got some great practical responses, without the maths to go with it.

 

[old man emu]

OK. No more "deep theory". No more providing explanations. No more Old Man Emu. 

[Garfly]

Lucky we still have:

 

 

[spacesailor]

YES

BUt

IF RAA,  ( sport aviation  ), continues to put civil aviation in their examination,s we will have to learn, 

" How to fly all aircraft " including that grounded MAX AIRLINER.

When talking about " Icing ",

I would have thought, it would be pertinent to Our little planes not jumbo,s ,Almost in space.

" what causes Icing " . Answer : moisture in the air : 

Even CASA has the "veturi efect " , down pat.

How much education should we need to just fly a circuit? .

After all, the best of us started with Less than they have now with experience! .

spacesailor

[Jabiru7252]

Just to fly a circuit? Who 'just flies circuits'? I wouldn't feel safe flying with a pilot who didn't understand icing conditions, didn't understand the effects of temperature and density height on take-off and landing distances and a whole bunch of other stuff. In my opinion RA-Aus is dumbed down compared to what I had to do as a private pilot. 

[Old Koreelah]

3 hours ago, Garfly said:

Lucky we still have:

 

 

Wow, it’s great that someone has put Prof JSM’s shows out there; I guess those two young blokes are now old farts like us, perhaps lurking on forums like this!

[RossK]

1 hour ago, Jabiru7252 said:

Just to fly a circuit? Who 'just flies circuits'? I wouldn't feel safe flying with a pilot who didn't understand icing conditions, didn't understand the effects of temperature and density height on take-off and landing distances and a whole bunch of other stuff. In my opinion RA-Aus is dumbed down compared to what I had to do as a private pilot. 

Opening post was about turns in the circuit, at low speed.

Your points are valid though, I've come across RAA pilots who have no concept of weight and balance, but that's for another thread.

[spacesailor]

My comments, were about RAA tests being more for GA aircraft than a mere circuit pilot.

All pilots do circuits, before updating to Cross country endorsement.

Even those Smart Alec pilots that go out to the Training area, Don,t do Straight in approaches

On returning.

spacesailor

[jackc]

There are lousy car drivers and lousy pilots, but I bet lousy pilots don’t last long.....

[facthunter]

There's stuff you must understand. and know. There's stuff that's nice to know and there's stuff you  don't HAVE to know at all. (Unless you are also designing planes)

    I've seen pilots baffled with science talking about WHICH of Newton's theories is the "first" one. How lift is actually generated by a wing can be illustrated by a wind tunnel with "pulsed" smoke trails and the wing A o A moved through the range of  useful angles .The span wise movement should be covered also to explain wing tip vortices. IF the wing don't push the air around you don't get any lift . . It's the reaction to moving the air that gives you lift..Nev

[Bruce Tuncks]

Why has this never been tried anywhere? Making a driver's license dependent on understanding physics like OME does?

I reckon that doing this would reduce the road toll by about 90%. Most fatalities are caused by those too stupid to ever understand elementary physics.

My guess is that the fat lady vegetarian lesbian politically correct lot have successfully destroyed good sense everywhere.

[Garfly]

Yeah, they'd be the one's to blame. ( If not ANTIFA 😉 

But that's where the accuracy ends; OME is the one who said Physics ain't necessary for drivers.

 

"Does any average person know the physics of how their car manages to turn corners? Or the thermodynamics of the combustion process that produces the forces necessary to propel a car? NO. As long as the driver has the skills required to make the car go in the desired direction, and the desired velocity, then the practical operation of the car is achieved."