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Re: JSBSim Piper J-3 Cub

Postby wkitty42 » Sat Jul 29, 2017 7:14 pm

@bomber: calling you an "aerodynamics expert" is an insult??? interesting...
"You get more air close to the ground," said Angalo. "I read that in a book. You get lots of air low down, and not much when you go up."
"Why not?" said Gurder.
"Dunno. It's frightened of heights, I guess."
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Re: JSBSim Piper J-3 Cub

Postby Bomber » Sat Jul 29, 2017 7:56 pm

Calling you a lickspitle would be a compliment.
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Re: JSBSim Piper J-3 Cub

Postby Richard » Sat Jul 29, 2017 8:10 pm

Bomber wrote in Sat Jul 29, 2017 4:33 pm:Richard can you clear up what the axis and values are....
Looking at the graph with flaps up there doesn't seem to be any effect on the stabiliser untill 18degs.... which concurs with the table.... after this the values are different.


You are understanding it correctly - refer to the geometry picture at the bottom and it makes more sense as to why there is no effect until 18 degrees; because the stabiliser is mounted higher than the wings.

Y axis is the effectiveness; although it's hard to read the straight line part is 1.0 and it then reduces.

X axis is alpha w.d.p, which is Airplane angle of attack relative to the wing design plane (degrees); there is also alpha f.r.l - which is relative to the fuselage reference line.

Image
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Re: JSBSim Piper J-3 Cub

Postby Thorsten » Sun Jul 30, 2017 4:06 am

Simon, by your own words you want to have an FDM discussion. But see, a technical discussion carries its own conventions. We just need to look at the 18 deg issue to see where it derails.

You believe Dany's table is wrong - but we never learn why you believe that. (We actually never learn that you believe that either, it comes with a solid 'cannot be'.)

Whereas a technical discussion is about arguments. People are not interested what other people believe or think, they're interested in the arguments leading to the beliefs, and if there are not arguments, the beliefs are irrelevant.

Richard posts a graph which shows the same phenomenon. Your first reaction to that is not to question your own idea and inspect why things you thought 'can not be' actually can be under some circumstances - it is to question Richards as to what's seen on the graph.

Which isn't a bad thing, except Richard (who knows how a technical discussion works) has done his information duty to the letter and actually written what's on the graph right above it. So it doesn't make sense to ask for it again, it won't change (and the graph is actually labeled - hard to read, but it's there to verify).

Next comes an argument by myself that explains why what you think 'cannot be' can happen (aka, I state one of the foundation underlying my beliefs).

In a technical discussion, you'd be expected to come either with a counter argument, or, in the face of mounting evidence, simply say 'okay, I got that wrong'.

But rather than conceding the point, you reliably launch into the personal level by postulating the motives of the people who gave evidence where you are wrong. Which doesn't belong into a technical discusssion.

Read back, in this thread and others, and you'll find reliably the same pattern - if you can't counter an argument, you make it personal and break the discussion rather than admitting a mistake.

Try stop shooting the messenger when you got something wrong and simply concede points you can't defend - it's going to do miracles for both the discussion and your reputation.

In the case at hand, there's rather good evidence and explanations how there can be no effect up to 18 deg. Why don't you just admit that it can be possible after all, and we can all get on with the discussion.
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Re: JSBSim Piper J-3 Cub

Postby Bomber » Sun Jul 30, 2017 11:11 am

Richard, what do you make the right side axis 'flaps' values to be...
Up
5
20
1
30
10
25

This chart seems to shows the effect of front wing (flaps up) and varying degrees of flaps deflection on the H-stab effectiveness with varying front wing angle of attacks.

The question I have of you is this...

With this 747 arrangement if the main wing is seeing an angle of attack of 3 degs then we should expect that the h-stab also see's 3 degs.

With a h-stab that's in line with the main wing we should expect the downwash of the wing to effect the h-stab, maybe reducing the AoA that the h-stab see's by 2 degrees.

All of the above occurs with zero deflection of the elevators.. deflection of the elevators in effect increases/decreases the AoA that the h-stab see's. So if we consider the elevator is deflected by 25degs downwards giving an effective AoA on the h-stab of 8 degs... if we conjecture the stall angle of the h-stabiliser airfoil being 12degs then there's only 4 degrees of of AoA that the airplane can see before the h-stab stalls out and the nose drops.

My point being for flight modelling we can't separate the h-stab and elevator effects as is done presently and explains why the elevator effectiveness can be too sharp when doing this.

Simon.
"If anyone ever tells you anything about an aeroplane which is so bloody complicated you can't understand it, take it from me - it's all balls" - R J Mitchel
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Re: JSBSim Piper J-3 Cub

Postby Thorsten » Sun Jul 30, 2017 12:13 pm

All of the above occurs with zero deflection of the elevators.. deflection of the elevators in effect increases/decreases the AoA that the h-stab see's.


Angle of Attack: Direction of the undisturbed airstream (aka, what hits the leading edge) relative to a reference axis.
Elevator location: trailing edge of the h-stab

You need to use AoA in a rather unusual sense (or fly backwards) for the above sentence to compute.

Edit: Unless you're talking about an all-moving tailplane - in which case the h-stab and the elevator are the same airfoil - in which case I have no idea who separates their effects in what way.

My point being for flight modelling we can't separate the h-stab and elevator effects as is done presently


I guess many people (including myself) have told you all along that it's a bad idea to separate flight dynamics into aerodynamics of separate airfoils which you sum later and that instead full-body measurements / CFD should be used to account for disturbances caused by parts upstream.

Nice that you now come to the same conclusion.
Last edited by Thorsten on Sun Jul 30, 2017 12:51 pm, edited 1 time in total.
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Re: JSBSim Piper J-3 Cub

Postby Bomber » Sun Jul 30, 2017 12:49 pm

Please I'm trying to talk to a fellow flight modeller.
"If anyone ever tells you anything about an aeroplane which is so bloody complicated you can't understand it, take it from me - it's all balls" - R J Mitchel
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Re: JSBSim Piper J-3 Cub

Postby Thorsten » Sun Jul 30, 2017 12:53 pm

Yeah, it'd be nice if you'd actually go on with doing this and respond to arguments - we could actually have a useful discussion.
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Re: JSBSim Piper J-3 Cub

Postby Bomber » Sun Jul 30, 2017 1:44 pm

Well it'd be nice if you stopped trying to score points and instead worked towards improving flight modelling by addressing individual issues that present flight modelling techniques suffer from instead of spouting your "I know better than you" rubbish.
"If anyone ever tells you anything about an aeroplane which is so bloody complicated you can't understand it, take it from me - it's all balls" - R J Mitchel
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Re: JSBSim Piper J-3 Cub

Postby Thorsten » Sun Jul 30, 2017 2:07 pm

Short recap:

You've been making more than one apodictic statement criticizing the work of other here - frequently without bringing good evidence for your position. Such as

Spiraling' propwash doesn't exist


or

This table says that the AoA has no effect on the value of the elevator moment up to 18degs... and only after this value does it start to reduce.

This cannot be correct.


Yet you act offended every time a flaw in your reasoning is pointed out - based on arguments and evidence. Surely you would agree that in the case someone does something wrong, pointing out that it is wrong improves flight modeling, yes? But, unfortunately, that also includes the case that your critique is wrong.

See, thread history it's actually you who chimes into threads, trying to score points 'I know better'. It's you who summarily declares other FDMs in FG bad. That's not me. I'm addressing issues in present flight modeling - you just don't like it if they occur in your flight modeling, that's all.
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Re: JSBSim Piper J-3 Cub

Postby Richard » Sun Jul 30, 2017 2:23 pm

Zooming in (on my copy) I get.

(Top to bottom)
UP, 5, 20, 1, 30, 10, 25

Bomber wrote in Sun Jul 30, 2017 11:11 am:This chart seems to shows the effect of front wing (flaps up) and varying degrees of flaps deflection on the H-stab effectiveness with varying front wing angle of attacks.


The alpha for table 4.0-19 (that I posted earlier) is the wing design plane alpha, taking into account flap angle. There are many other tables in the build up for pitching moment, including spoilers. ailerons, gear, sideslip, aerolasticity, ground effect and rudders.


Bomber wrote in Sun Jul 30, 2017 11:11 am:With a h-stab that's in line with the main wing we should expect the downwash of the wing to effect the h-stab, maybe reducing the AoA that the h-stab see's by 2 degrees.


There will be effects from downwash; but this also depends on the htail arm, the incidence of the wing and htail, deflection of ailerons, flaps, spoilers, beta, and maybe other things I can't think of. However often downwash isn't directly considered because the geometry is fixed and therefore it can be in the base coefficient.

Bomber wrote in Sun Jul 30, 2017 11:11 am:All of the above occurs with zero deflection of the elevators.. deflection of the elevators in effect increases/decreases the AoA that the h-stab see's.


That is misleading, as all pitching moment may eventually affect affect the angle of attack, but not it does not effect it directly, and there isn't a need to adjust the AoA used for the lookup based on the deflection of the elevators.

Bomber wrote in Sun Jul 30, 2017 11:11 am:So if we consider the elevator is deflected by 25degs downwards giving an effective AoA on the h-stab of 8 degs... if we conjecture the stall angle of the h-stabiliser airfoil being 12degs then there's only 4 degrees of of AoA that the airplane can see before the h-stab stalls out and the nose drops.


That is wrong. The deflection doesn't change the AoA directly because AoA is measured at the leading edge, except in the case of an all moving surface.

Bomber wrote in Sun Jul 30, 2017 11:11 am:My point being for flight modelling we can't separate the h-stab and elevator effects as is done presently and explains why the elevator effectiveness can be too sharp when doing this.


The effectivity table is only used as a factor on the coefficients that relate to elevator positions.

Image

Also needs to be considered are the derivatives, pitch damping due to q dot (CMq), change in pitch moment due to alpha dot. Last time I looked at one of your models you didn't have any derivatives - and they're quite important. I don't exactly understand the mathematics behind these derivates, I think they're second order effects, but not certain. Maybe Thorsten can explain this as I can't.

A reasonable model (for pitch moment) can be made with just 4 tables. I made the ones below with OpenVSP.
Image
The overly on these graphs is F-15 wind tunnel data that I use as a reference - to allow a quick visual check of the OpenVSP values.
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Re: JSBSim Piper J-3 Cub

Postby Bomber » Sun Jul 30, 2017 2:59 pm

Richard....

The AoA is an angular measurement between an airfoils trailing edge and it's leading edge against the relative wind..

If you lower the trailing edge then you've altered the airfoils AoA.... increasing it coefficient and thus generating more lift for the same dynamic pressure.
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Re: JSBSim Piper J-3 Cub

Postby Thorsten » Sun Jul 30, 2017 4:57 pm

@Simon:

The AoA is an angular measurement between an airfoils trailing edge and it's leading edge against the relative wind..


No, it's not - look it up, it's what I said it is.

@Richard:

Maybe Thorsten can explain this as I can't.


I can try...

Generically they're transient effects, the airflow adapting to a change and generating forces in the process. For instance one driving effect for pitching moment due to alpha dot is delayed downwash.

Say at low alpha the tail gets some airstream, at a higher alpha the wing partially 'shadows' the airstream and reduces the effectivity of tail aerosurfaces.

Now, if you pitch up and alpha changes, the wing sees the airstream 'now', but the tail sees the 'shadowing' of the wing delayed by a split second - the time it takes for the airstream to get from wing to tail. So the 'shadowing' the tail sees is not the shadowing generated by the wing 'now' but 'a few moments ago' - and if you pitch up, the effect was less a few moments ago than it was now, and how much it is less is dictated by the change rate of alpha. So if you increase alpha rapidly, the tail is getting more lift than it would get in equilibrium, leading to a nose-down moment. As soon as alpha ceases to change, the 'shadowing' now is the same as the 'shadowing' a few moments ago, and the effect vanishes.

This is a factor that drives the coefficient.
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Re: JSBSim Piper J-3 Cub

Postby Bomber » Sun Jul 30, 2017 5:24 pm

"If anyone ever tells you anything about an aeroplane which is so bloody complicated you can't understand it, take it from me - it's all balls" - R J Mitchel
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Re: JSBSim Piper J-3 Cub

Postby dany93 » Sun Jul 30, 2017 5:26 pm

Richard wrote in Sun Jul 30, 2017 2:23 pm:....pitch damping due to q dot (CMq), change in pitch moment due to alpha dot.

In my simple mind and after Richard's phrase I firstly thought of mere damping, which happens even without a propeller. Effects proportional to the pitch or / and alpha derivatives (also for roll and yaw).
Aren't they the most important ?
They at least have the advantage of being easy to include in a calculation.
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