Question on forces acting on an aircraft in climb

[Wizofoz]

PBL,

Interesting that you would say this:-

The view of science that quantities which you cannot discriminate with instruments are the same thing is about the only philosophy of physics which has been definitively discredited in the last hundred years.

Followed by this:-

Repeating an assertion doesn't make it any more persuasive

Kindly site your source for this "Philosophy"

Meanwhile, I am discussing Physics with people willing to put clear, easily understood principles in writing for discussion. You seem to think philosophy somehow contravenes the universes physical laws.

Good luck with that.

In any case, angular velocity is absolute. If you understand that, great. But if that is the case, why did you ridicule me for correcting a statement you made which apparently YOU knew to be incorrect?

[italia458]

Wiz...

My point on weight remains this- What instrument would you use to determine whether weight is "Real" or "Apparent"? If you can't differentiate between the two, are they not the same thing?

Instrument:
I believe the result you get from "measuring" or "observing" something is actually based on the instrument you use. This is particularly important when analyzing quantum mechanics and particles at the atomic level. I still believe it is relavent to the "normal" world tho. Take light for example, when observing it to see how it travels in waves, the instrument will indicate that it does indeed travel in waves, but when observing how it travels in a straight line, the instrument will indicated without a doubt that it does travel in a straight line. So if our instrument is not giving us an accurate "view" of what we are observing, how can having no difference in observation from a different instrument, prove that two different objects are actually the same.

Another example, it's like looking at a real apple and a fake apple sitting on the moon through a telescope and since you are detecting no change, saying that both apples are real based on only visual observation, through one instrument, and then through our eyes, which inherently aren't perfect. However, if we were actually on the moon right beside the apples, we could probably tell which one was fake just by observation. So it depends on how "accurate" the instrument is and also if it's giving us all the information that we need, feel of the apple, weight of the apple would definitely help determine if one was fake.

Frame of reference:
"Real" and "apparent" weight are based on your frame of reference and can be measured using the same instrument. "Real" shouldn't even be used cause it infers that "apparent" isn't a real weight, yet it is, in a different frame of reference.

Put a scale under your butt when you're sitting in your airplane on the ground. Say it says 200lbs, that's your weight while observing from a non accelerating frame of reference (inertial FoR). If you were flying level at 3000' with no acceleration, the frame of reference would still be the same (inertial FoR) and your weight would be the same. Also, in any other inertial FoR (for example standing on earth), observing you would reveal that you appear to weight 200lbs also. Now in a turn (observing from earth), you would still appear to weigh 200lbs, the observer hasn't seen you eat any Big Mac or grab onto a 20lb weight or something. To him, you're still 200lbs. But to you, in the aircraft in the turn, you are accelerating and you FEEL that extra force and sure enough when you look down at the scale in a 60 degree bank at 2G you indicate your weight at 400lbs. You're in a non-inertial FoR because you are accelerating. This does a pretty good explanation of non-inertial FoR (Non-inertial reference frame - Wikipedia, the free encyclopedia)

This is inertial FoR (Inertial frame of reference - Wikipedia, the free encyclopedia)

You earlier brought up the idea that an observer on the Earths surface could determine which part of the aircrafts weight was due to Gravity, and which to acceleration. True, but this gives the Earths surface a privileged frame of reference.

I'm not sure what he meant but maybe that observer on the earth has a weight scale inside the aircraft and is getting data sent down to him? The privileged frame of reference the earth is in is called a rotating reference frame.

Take an observer in freefall- from HIS POV, ALL the force on the aircraft would be due to it's acceleration, NONE due to Gravity (as from his frame of reference, there isn't any). The Aircrafts acceleration is absolute. Which part is "Real" and which is "Apparent" is relative.

Exactly! I think you might be meaning the same thing I am but just saying it differently. "Real" and "apparent" are relative to your FoR. Does any of this make sense?

[Pugilistic Animus]

well at the least, we can say definitively that:

the product of uncertainty in position and momentum, must be greater than or equal to Plank's constant divided by 2pi

for airplanes Stick and Rudder and the FAA Airplane Flying Handbook answers best the original question....
So, far everytime I wrote anything about climb performanceThe British said "wot are you on about"
...so

also, for those who really can't get enough You have Hurt's text

and beyond that, you'd really better know engineering math and physics

[Wizofoz]

Hi Italis,

We are on the same page if not the same verse.

Another example, it's like looking at a real apple and a fake apple sitting on the moon through a telescope and since you are detecting no change, saying that both apples are real based on only visual observation, through one instrument, and then through our eyes, which inherently aren't perfect. However, if we were actually on the moon right beside the apples, we could probably tell which one was fake just by observation. So it depends on how "accurate" the instrument is and also if it's giving us all the information that we need, feel of the apple, weight of the apple would definitely help determine if one was fake.

Yes, but there is NO experiment, even theorecically, which can differentiate between "Real" and "Apparent" weight. They will both make a mass accelerate in the normal direction in the relevent FofR. They will both make a mechanism distort and give the same reading on a scale. Without perfect instrumentation, things appear to be the same. Without any WAY of differentiating between two things, they ARE the same.

I think you might be meaning the same thing I am but just saying it differently. "Real" and "apparent" are relative to your FoR. Does any of this make sense?

Exactly.

An observer on the ground would observe the aircraft in a 2g turn, whilst ALSO feeling the force of his own mass times gravtiy, evidenced by the scales HE is standing on. He would assume the Pilot he is observing has 200lbs "Real" weight, and another 200lbs "Apparent" weight.

Now take an observer in a uniform acceleration due to gravtiy (yes, yes, gravity gradient, but close enough). The scales HE is standing on read zero. He observes the aircraft not only turning, but also in a uniform vertical acceleration.

He ALSO concludes the pilots scales reads 400lbs, but all of it "Apparent".

From the Plots POV, it's ALL "Real".

As such, it seems the whole concept of "Apparent' weight is relative to the observer, and the absolute term is, well, just "Weight"!

ETA do YOU have any idea what PBL is trying to say?

[Pugilistic Animus]

A scale measures weight and a balance measures mass

a scale for a 200 lb man would indicate 400 at a 60 bank angle in steady coordinated flight...a balance would remain at 200 and not move

[Wizofoz]

Correct, Pug. No-one is suggesting Mass changes.

[SomeGuyOnTheDeck]

No-one is suggesting Mass changes.

Except possibly Albert Einstein.

Invariant mass - Wikipedia, the free encyclopedia

[PBL]

Originally Posted by Wizofoz

Kindly site your source for this "Philosophy"

For example, Operationalism in the Stanford Encyclopedia. Notice, in the first paragraph, that:

operationalism is nowadays commonly regarded as an extreme and outmoded position

BTW, I am here to discuss things which I find fun to discuss with people who like to discuss them. So if you want to continue schimpfing at me, Wizofoz, please do so privately and not burden others.

PBL

[Wizofoz]

PBL,

To complete the statement you quote-mined out of context.

Both in philosophy and in psychology operationalism is nowadays commonly regarded as an extreme and outmoded position, but that is not to say that the potential of Bridgman's original ideas has been exhausted.

We are not talking about either Philosophy ao Psychology, but Physics.

Physical sciences do indeed depend on what can be demonstrated and measured. How else do you propose we can differentiate between things if we cannot measure the difference?

You appear to have the hump with me because you made a plainly wrong statement about a physical reality and I corrected it. You will now not even show the common courtesy to state whether you actually agree with what you wrote, or intentionally made an incorrect statement.

Do so and the "schimpfing" (???) will end.

[italia458]

Wiz...

regarding this...

Yes, but there is NO experiment, even theorecically, which can differentiate between "Real" and "Apparent" weight. They will both make a mass accelerate in the normal direction in the relevent FofR. They will both make a mechanism distort and give the same reading on a scale. Without perfect instrumentation, things appear to be the same. Without any WAY of differentiating between two things, they ARE the same.

I think there is a way to measure real vs apparent weight. If the aircraft is in a level turn you'll find that it has to produce enough "energy" to counteract the force of gravity. You could then calculate the energy that the aircraft system (majority from the wings) would produce downwards, which should equal the calculated energy required to keep the airplane in level flight.

Now enter into a turn and you require a lot more lift to stay level. In the 60 degree turn you're at 2G and therefore your apparent weight would be doubled from your real weight. So the whole aircraft system, (body, fuel, wings, people inside, etc) would now weight twice as much (apparent weight) which would require twice as much energy to counteract. This is a way to calculate real vs apparent weight.

[Wizofoz]

Italia,

But how do you know whether your "Real" weight was caused by gravity, or by acceleration?

[Capt Pit Bull]

Hi CBP,

My point on weight remains this- What instrument would you use to determine whether weight is "Real" or "Apparent"?

In general terms, you never perceive your own weight directly, only when something prevents you from accelerating in a gravitational field.

You can't measure your weight inside the aircraft UNLESS you specify that you are unaccelerated in relation to whatever is causing the weight, (i.e. the earths mass). In that condition only apparent weight happens to be equal and opposite to weight. Your other flight instruments will tell you whether the aircraft is unaccelerated.

If you can't differentiate between the two, are they not the same thing?

ummm.... sounds like more of a philosophy discussion to me, but as far as this goes....

They definitely aren't the same thing. Draw a freebody diagram of a pitbull sitting on a chair in an aircraft flying a constant speed turn at 60 degrees of bank. There are 2 forces applied to the pitbull; first is its weight, lets call that W Newtons, acting straight towards the Earths centre of mass. Second is a force of 2W acting from the chair to the backside of the pitbull, acting in the direction of the aircraft's Normal axis.

So we can see that (with the sole exception of the special case), in general weight and apparent weight have differnt magnituds and different directions.

How much more differnt would you like them to be ?

You earlier brought up the idea that an observer on the Earths surface could determine which part of the aircrafts weight was due to Gravity, and which to acceleration. True, but this gives the Earths surface a privileged frame of reference.

You say that like its a bad thing. The fact of that matter are that the OP was asking about forces in a climb. And in a climb, or any other analysis to do with the balance of forces on an aircraft to determine what its future flight path will be, the Earth is a pretty darn sensible frame of reference.

Take an observer in freefall- from HIS POV, ALL the force on the aircraft would be due to it's acceleration, NONE due to Gravity (as from his frame of reference, there isn't any). The Aircrafts acceleration is absolute. Which part is "Real" and which is "Apparent" is relative.

(Assumption: you are talking about actual free fall rather than the innacurate colloquial parachuting term describing terminal velocity)

I think I can see what you are trying to say. You'd have to have some dude up in space above the atmosphere looking down and saying "Hey! That aircraft is accelerating towards me....". Yes, on that basis, he could ascertain the apparent weight of the aircraft (assuming the mass of it was known)

But he would also look down and see a large planet accelerating towards him as well.

In fact this guy, by virtue of being external to the aircaft, would be in a position to see if the aircraft was accelerating towards him at the same rate as the planet (in which case he could deduce the weight of the aircraft in the planets frame of reference). But if he looked at the aircraft and saw it was accelerating at a different rate then he would known that the aircraft was accelerating relative to the planet. Thus the occupants would be experiencing an apparent weight not equal to their weight in the planets frame of reference.

In other words, nothing is different, the point of view of a free falling observer is basically a red herring. Its a useful exercise but doesn't really change anything. At least if I understood you correctly.

I understand what you are saying about Causality, but still think it doesn't apply- Take a meteor striking the Earth- It undergoes an acceleration. Has a force caused the mass to accelerate, or has it's acceleration caused a force? My answer is "It's the same thing, as they are EQUIVALENT

My answer...... Newton 1

It says words to the effect of "A body will continue in a state of uniform motion unless acted upon by an unbalanced force"

It does not say something like "from time to time a body will spontaneously accelerate and when it does so forces will be created"

You've expressed this viewpoint several times over the last year, and you are just plain wrong I'm afraid. As others have said in the thread, you can have forces without acceleration (if they are cancelled out by another force) but the presence of acceleration has to mean the existance of an unbalanced force causing that acceleration.

In the case of your meteor example, attempting to place a rock and air molecules in the same space causes electrostatic replusion (very strong compared to gravity which is a pathetically weak force), which slows the merteor down and speeds the air up. Large amounts of work are done leading to the release of a lot of heat... boom etc....

anyway, got to go.

(RL has intruded, the above is not proof read and may include bollox)

pb

[Wizofoz]

CPB,

Yes, I'll give you the force/acceleration argument. thanks for patiently getting it through my skull.

I STILL don't agree with the "Apparent weight" bit.

For instance, when you say this:-

But he would also look down and see a large planet accelerating towards him as well.

Why would the relative acceleration of a third body effect the observation of our soon-to-be-strawberry-jam Free-Faller? From HIS POV, the aircraft is accelerating. he might DEDUCE the reason for the aircrafts acceleration from the rapidly oncoming spheroid, but he would have no other way of MEASURIBG it other than it's chang of relative position to him.

As to the aircraft, lets assume it is maintaining the same AofA at the same TAS, only THIS time it's in a terraformed Martian atmoshere which has the same ambient pressure as the Earths atmoshere. Will what the pilot feels and his G meter measures be any different to what it would have been on Earth?

[Capt Pit Bull]

Nice thought exercise... I like it! Kind of pushed for time but here goes.

(Mars surface gravity about 0.4 of Earth)
(Lets say 100kg man, approx 1000 Newton weight on Earth, 400 N on Mars)



In all cases, weight constant, but less than on Earth.

S+L apparent weight = weight.

In a level turn, apparent weight increases as 1/cos bank angle. So in a 60 degre banked turn, apparent weight on Earth = 2000N, on Mars 800N.

So the mans apparant weight in that turn is 0.4 x 2, not 0.4 + 1.

Of course this means that the lateral component of the lift vector (same speed and bank angle) is smaller than on Earth. Which means the radius of turn is much larger.

However, that's not the end of the story. Since gravity is less, to get the same ambient pressure our martian atmosphere would need to be much denser; 1/0.4 in fact, so density would be 2.5 ISA sea level.

Thus, production of a given amount of lift would require lower lift coefficient. If was assume the same aerofoil our martian aircraft would be flying around with much lower Alpha. Looking at it another way for the same TAS our martian aircraft has a much higher IAS.

Lets imagine our Earth aircraft was flying relatvely slow, and was at CLmax at 60 degrees AOB, sitting in the light buffet. At the same TAS, our martian aircraft would still have plenty of alpha to spare.

I've scibled down a few sums here, and I reckon if it increased it AOB to 77 degrees it would fly the same radius of turn as on the Earth. At that AOB the pilot's apparent weight would be 1778 Newtons; that's 4.44 higher than S+L (Mars) and 1.78 higher than Earth.

Note that aircraft could still 'pull' 2 G, so the pilot still has some Alpha in reserve. (AOB could be increased by a couple of degree).

In summary to follow the same radius of turn at the same TAS, compared to our 'nibbling the buffet' earth aircraft, our Martian pilot:
- experiences less G
- has higher IAS
- Uses higher bank angles but lower alpha
- Sill has some Alpha in reserve

How does that sound?

[Wizofoz]

Sorry, sounds like bollox!

(Well, what you say is correct, but you missed the point)

Note the question- Same AofA, same TAS in same ambient pressure= same Lift vector.

You are correct about the aircrafts weight from a FofR of the Martian surface, thus it will be producing more lift than weight and thus accelerating vertically.

Lets assume the aircrafts mass is 1000kg.

Lets assume it is maintaining the AofA to fly level on Earth, so it is producing 1000kg*9.8kg= 9800N of force.

On Mars, its weght is 400kg, so the force needed to fly level would be 3920N.

Thus it is producing an excess of 5880N.

This will produce an "Apparent weight" of 5880/9.8=...600kg.

Thus, the aircrafts "Total weight" is STILL 1000kg, and it's (Earth calibrated) G meter will still read 1.

This is what I'm saying- fly an aircraft at a particular AofA and you will measure a force. You can try and deduce which part of this force is "Real" and which is "Apparent", but it makes no difference to the aircraft.

[Capt Pit Bull]

Ah, I thought you meant a constant AoA not the same AoA as the earth aircraft, and presumed we were talking about turning, but now I'm not sure what our original Earth flight path was; I'm assuming straight and level.

You are missing the crucially important point that IF pressure is the same THEN density would have to be radically different.

But it doesn't hugely matter. If I may be so bold, I think you want standard earth density rather than pressure. I'll assume that to be the case:

Lets assume the aircrafts mass is 1000kg.

Lets assume it is maintaining the AofA to fly level on Earth, so it is producing 1000kg*9.8kg= 9800N of force.

On Mars, its weght is 400kg, so the force needed to fly level would be 3920N.

Thus it is producing an excess of 5880N.

Correct end result, but the second and third line are dodgy.

Second line: lets get the dimensions correct. You say "1000kg*9.8kg= 9800N of force." I say "1000kg*9.8 N/kg= 9800N of force."

Third line: You are muddling mass and weight concepts. Its mass remains 1000 kg, its weight is 400 N. Neither its weight nor it's mass is 400 Kg.

This will produce an "Apparent weight" of 5880/9.8=...600kg.

Again, weight and mass confused. Weight is a force, leave it as Newtons. But MUCH more crucially, the 5880 is NOT the Apparent weight, its the increase in apparent weight (above the 400N needed to maintain S+L). In otherwords it is the unbalanced force, and could be used to calculate the resulting flightpath change.

This is what I'm saying- fly an aircraft at a particular AofA and you will measure a force. You can try and deduce which part of this force is "Real" and which is "Apparent", but it makes no difference to the aircraft..

You mean appart from the fact that the flight path will be different? Call me picky, but I would consider that to be a difference.

Lets try and cut to the nub of the issue:

Do you understand that:
- in order to move you around during manouevres, a portion of the unbalanced force needs to act on you.
- the proportion is set by the relative mass. e.g. if you mass 100 kg, and the aircraft mass is 1900 kg, then your mass proportion is 0.05
- The sensation of that force being applied to you is how you perceive your weight.
(ignoring vertical thrust components, and assuming not doing extreme manouevres involving high angular rates or high alpha the unbalanced force is mostly about changes in lift. Drag is a nil factor assuming the manoeuvre is sustained as it is implicit that the thrust is increased to sustain the manouevre and maintain speed)
- In otherwords your apparent weight = LIFT x Your mass proportion.
- Therefore whenever lift changes your apparent weight changes.

Meawhile your attraction to the nearby planet due to gravity is constant (ignoring gravity gradient and curvature of the planet), so your actual weight is constant.


((You seem to be hung up on the use of a set of scales. Yes, it is a very important point that you can't tell whether you are accelerating or being held up against a gravity field (or a bit of both) IF you are sitting inside a sealed box with just a set of scales. BUT as soon as you have external reference it is entirely possible to figure out what's happening.))

pb

[Wizofoz]

CPB,

We are in oerfect accord with regard th physics of the situation, except I disagree with this bit:-

You mean appart from the fact that the flight path will be different? Call me picky, but I would consider that to be a difference.

The flight path is either different or the same depending on the frame of reference. Different relative to what?

But that is relatively minor.

The nub of my argument lies here:-

You seem to be hung up on the use of a set of scales. Yes, it is a very important point that you can't tell whether you are accelerating or being held up against a gravity field (or a bit of both) IF you are sitting inside a sealed box with just a set of scales. BUT as soon as you have external reference it is entirely possible to figure out what's happening.))

Basically I'm saying this:-

You define weight as being due to gravity and "Apparent" weight as being due to acceleration.

But gravity IS an acceleration. ALL the force being felt by the aircraft is due to acceleration, and there is no need to divide the too into different "Types" of force.

[Capt Pit Bull]

You mean appart from the fact that the flight path will be different? Call me picky, but I would consider that to be a difference.

The flight path is either different or the same depending on the frame of reference. Different relative to what?

But that is relatively minor.

Look, I hate to keep banging on at the same point, but the whole point of the thread is how do you analyse the forces in different phases of flight.

Therefore the fact that the aircraft doesn't move relative to itself, although true, is totally useless!!!!

Since (for the OPs question) things like climbing and descending are by definition relative to the Earth its pretty obvious that the aircraft as a frame of reference is singularly unilluminating.

Basically I'm saying this:-

You define weight as being due to gravity and "Apparent" weight as being due to acceleration.

No. You're the guy that thinks acceleration causes forces, I'm the guy that thinks forces cause accelerations.

I say weight is caused by gravity, whether you are accelerating or not. You can not perceive your own weight directly because gravity it is a field effect.

You can only perceive contact forces acting upon you (because they are non uniform). This is your apparent weight.

They are fundamentally different types of force, one is gravitational, the other is electrostatic.

But gravity IS an acceleration. ALL the force being felt by the aircraft is due to acceleration,

To say gravity causes acceleration does not mean "gravity IS acceleration". You still seem to be struggling with cause and effect. We've already been over newton 1 and although you said you accepted my point its clear you haven't taken it on board. An acceleration tells us an unbalanced force exists, but its the force that causes the acceleration, not vice versa.

You also don't seem to want to address the point that objects that are in a gravitational field but not accelerating (becuase they are supported) still have weight. This clearly demonstrates that gravity is NOT synonomous with acceleration.

Fundamentally I have shown you have Weight and Apparent weight usually do not have the same orientation. We've seen that they do not have to be the same magnitude. I've just explained how one is a field effect and the other is a contact force. You yourself needed to distinguish between them in order to analyse your martian flight path And yet you say

"and there is no need to divide the too into different "Types" of force."

The bottom line is this:

• I say the aircraft's weight is constant.
• The fact that the aircraft weight is constant is the starting point for drawing a free body diagram of climb, descent or turning.
• Yet many pilots have perceive changes in their apparent weight due to manouevre; however this because LIFT has changed, not because WEIGHT has changed.
• But because the effect of accleration is so compelling (especially if you've been squashed under 8 G) pilots tend to really *believe* their weight has changed, and when they come to draw the forces acting on the aircraft a fair few of them are totally confused and really struggle as a result.

pb

[italia458]

Italia,

But how do you know whether your "Real" weight was caused by gravity, or by acceleration?

First off, gravity is "acceleration".

Your apparent weight is like an "addition" to your weight caused by gravity (real weight). So your apparent weight will include your real weight and the remaining weight will be caused by the other acceleration, ie. in a turn.

Think of it this way. If you're straight and level, you have a certain lift vector, say it's 17640N (1800kg * 9.8m/s2) (1.0G). When you enter a 60 degree level turn you will be feeling a load factor of 2.0G. So now my airplane has to increase it's lift to 35280N (2.0G) to stay level. The lift always acts perpendicular to the wing. So now this has a vertical and horizontal component, the latter of which will pull the aircraft into the turn. 35280N * cos 60 = 17640N which is our vertical component of lift which counters the weight of the aircraft caused by gravity.

So using the load factor in the turn, the cos of the angle of bank and the weight (in newtons) of aircraft on the ground, you can find out the apparent weight at any angle of bank. With that you can subtract the weight of the aircraft and find out specifically how much the acceleration, due to the turn, is adding on to your real weight to end up with your apparent weight.

Does this help?

[Pugilistic Animus]

Fly at Vmo and do a High g pull up I think the wings would feel the weight because apparently they wont be there anymore

W =mg

y component = mg*sin theta

x component mg*cos theta