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Re: JSBSim Piper J-3 Cub

Postby Thorsten » Sun Aug 06, 2017 10:53 am

Alan has explained that there are only a handfull of planes in FG of the 400 fdms that use published data.


If Alan wants to be quoted with the number, that's his thing. I haven't looked into every FDM on the repository and I'm not sure Alan has. I can offhand count to the number quoted based on the types of planes I like to fly which would suggest there are quite a few more around which I do not know.

But since I didn't do an inventory of FDMs on the repository I refuse to discuss any number before evidence is produced that that number isn't a random guess (I don't think anyone even knows the number of JSBSim vs. YaSim planes offhand).
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Re: JSBSim Piper J-3 Cub

Postby Thorsten » Sun Aug 06, 2017 11:01 am

Sorry.... are you saying that only fdm's that are 100% correct are deemed 'ready' to be added into FGdata...


Just to clarify - no planes except the ufo and the C-172p are added to FGData, the repository for aircraft is FGAddon.

And of course FGAddon is open for content that is 'not ready' - but someone who laments the poor state of the FDMs in FGAddon and touting 'realism' and 'everyone else is doing it wrong' all over the place could perhaps be expected to not contribute to that poor state himself. Or to take responsibility and fix his mistakes.

Or not express surprise when I mention a problem you claim to have fixed in the mean time, as the problem ought to be known to you then.

You know.
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Re: JSBSim Piper J-3 Cub

Postby Bomber » Sun Aug 06, 2017 11:16 am

Now that's an example of sticking your head in the sand....

"Well I'm all right Jack I've chosen to model a plane that's we'll documented"

That doesn't give you the right to critise others that are trying to work out flight models for planes that aren't well documented and that usinf the 'conventional' aproach still don't fly to real life pilots documented descriptions.

You have no experience of doing such work....

So once a person has extrapolated his fdm from handfull of well documented planes and ends up with a complex flight model that doesn't reflect real life.....what's he to do ?

He's to come on here and discuss with other flight modellers because that's what a development forum is all about, not for Thorsten to belittle at every turn the ideas and possibilities of doing something different with his subtle 'red herrin' gags.

We look at the code and review it....

We shouldn't accept the 'you're not entitled because I've reviewed a 3 year old fdm and it's incorrect' or the 'you don't have a degree in aerodynamics'..

I've been modelling for that long I could have 4 degrees by now..... and aerodynamics is NOT flight modelling.

Those that work on planes that don't have comprehensive documented wind tunnel testing data, flight model....
"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 Aug 06, 2017 11:26 am

We shouldn't accept the 'you're not entitled because I've reviewed a 3 year old fdm and it's incorrect' or the 'you don't have a degree in aerodynamics'.


If you quote something, please make sure it actually has been said. Otherwise it's a red herring.

I never said 'you're not entitled to', I pointed out where you are wrong. And I pointed out that it smells like hypocrisy to express concern about the poor state of FDMs in FGAddon if you're partially responsible for the poor state of FDMs in FGAddon and could easily fix your part, but opt not to.

You have no experience of doing such work....


Quoting myself:

There would seem to be a relation though - studying the cases where there is detailed data give you insight into how certain effects play out. Having seen a couple of functions of elevator action against alpha allows you to see a pattern and will make your guess better if you have to guess.

First you need to understand something. Then you can extrapolate it. This thread didn't convince me you've done the first, neither did the ASK-13, sorry.
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Re: JSBSim Piper J-3 Cub

Postby dany93 » Sun Aug 06, 2017 12:27 pm

@erik,
I have the impression of some misunderstanding, at first view I partly disagree.

To be clear:
- When I speak about 'lift' in this context, it is not the wing lift or the wings + tail. It's only the tail contribution (hstab or hstab+elevator)
- Usually, the CG is before the aerodynamic focus, making the aircraft wanting to pitch down without the tail effect. Hence, the tail lift is negative in steady flight to counteract.

Now, let's imagine two steady flight situations at low airspeed, high alpha:

1 - Usual one:
The aircraft tends to pitch down with the elevator at or close to neutral. The pilot holds the stick pulled back to get a high AoA. The elevator (up deflection) makes negative lift. The hstab, seeing an incident wind from below (high alpha), makes positive lift. The sum of both is still negative to keep the steady equilibrium, steady pitch up moment (counteract the nose down tendency).
- The elevator tends to make the aircraft pitching up, with its own area and deflection,
- the hstab tends to make the aircraft pitching down (because of alpha) with its own hsatb area only.

2 - Unusual one in steady conditions, only to explain:
The CG is aft the aerodynamic wing focus, located to get the same velocity, attitude, alpha than example 1 with the elevator at neutral.
- The hstab+elevator exert a positive lift,
- The area to take into account for the moment vs alpha (if some alpha appears) is the [hstab+elevator] one.
This equilibrium with aft CG doesn't exist usually, I imagine such steady conditions only to explain. But they (high alpha, elevator at neutral) can dynamically be encountered, shortly, on an aircraft with the usual CG location.
Just to say that, in these conditions, the area which makes the pitch-alpha moment is [hstab+elevator]. I say it again: this is temporary a situation, shorter than in the usual example.

Many (if not most) aircraft in FG decouple the elevator-deflection and hsatb-vs-alpha contributions. In this simplified case, to give priority to the stall behavior, I think it is better to take only the hstab area into account for its moment vs alpha.

But,
Thorsten wrote in Sun Aug 06, 2017 4:07 am:........ it is quite possible to capture the effects of hstab and elevator in one deflection-angle independent term and one term that depends on alpha and deflection angle.
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Re: JSBSim Piper J-3 Cub

Postby Alant » Sun Aug 06, 2017 12:50 pm

Simon

In this thread you have asked for the dihedral of both the Shuttle and the Cub.

Thorsten has pointed you to Shuttle data.

The thesis (https://dspace.lib.cranfield.ac.uk/bits ... s_2011.pdf) includes Datcom data for the Cub, and the writer used zero degrees. This agrees with all of the drawings of the Cub that I have seen, and is more or less standard on high wing monoplanes..

You could have come to the same conclusion yourself using Google.

Alan
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Re: JSBSim Piper J-3 Cub

Postby dany93 » Sun Aug 06, 2017 12:53 pm

Bomber wrote in Sun Aug 06, 2017 10:45 am:Well can someone tell me what the dihedral for this plane the Piper J-3 Cub is ?

It's a simple question.

http://www.eaglesport.org/documents/cub ... manual.pdf
p. 16 (PDF 8/27).

which would make about 2.125" / 211.25" ~ 0.01 rad ~ 0.57 deg if my calculation is correct.
Last edited by dany93 on Sun Aug 06, 2017 1:11 pm, edited 1 time in total.
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Re: JSBSim Piper J-3 Cub

Postby Alant » Sun Aug 06, 2017 1:08 pm

Thanks. That gives a dihedral of 0.576 degrees.

Alan
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Re: JSBSim Piper J-3 Cub

Postby Bomber » Sun Aug 06, 2017 4:05 pm

Alan, of course I could have found it (and found it wrong) but it's not the point.... the point was to ask a simple question and receive a simple answer.

Alan..... can I ask you a simple question ?

Do wings with zero dihedral produce side forces ?

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 dany93 » Sun Aug 06, 2017 4:34 pm

Wings, probably not (theoretically and at first approximation), but fuselage and the rest, yes.
Why wings only?
the point was to ask a simple question and receive a simple answer.

Is it a joke?
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Re: JSBSim Piper J-3 Cub

Postby Thorsten » Sun Aug 06, 2017 4:49 pm

Is it a joke?


No, as I said above, it's playing games.
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Re: JSBSim Piper J-3 Cub

Postby Alant » Sun Aug 06, 2017 4:50 pm

So Dany and I both did your searches for you.

Another Google search bring this https://en.wikipedia.org/wiki/Dihedral_(aeronautics)#Other_factors_contributing_to_dihedral_effect Please read the section "Vertical position of the centre of mass" This is why high wing monoplanes typically have little or no dihedral.

What are you going to argue about next?

Alan
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Re: JSBSim Piper J-3 Cub

Postby Bomber » Sun Aug 06, 2017 4:59 pm

No its going somewhere..... an important aspect of flight modelling.

I asked the question of the author because that's the person that will know what data was used.. We can see that Alant from research has determined the dihedral for the Cub is zero, you another number, what I'd like to know is what number the author of the Cub used ?
Last edited by Bomber on Sun Aug 06, 2017 5:02 pm, edited 1 time in total.
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Re: JSBSim Piper J-3 Cub

Postby Bomber » Sun Aug 06, 2017 5:00 pm

Alant wrote in Sun Aug 06, 2017 4:50 pm:
What are you going to argue about next?

Alan


Alan, I've asked a question..... what point did it become arguing to ask a question ?
"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 Bomber » Sun Aug 06, 2017 5:05 pm

Alant wrote in Sun Aug 06, 2017 4:50 pm:So Dany and I both did your searches for you.

Another Google search bring this https://en.wikipedia.org/wiki/Dihedral_(aeronautics)#Other_factors_contributing_to_dihedral_effect

Alan


And your answer is ?

What is it you know about side forces with zero dihedral?
"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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