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Re: JSBSim Piper J-3 Cub

Postby wlbragg » Sat Jan 02, 2016 8:27 am

I added a "placeholder" for contact unit[3] which moved wings to unit[4] and [5]. I don't think anything was set to use wings at 3 & 4 but I need them at 4 and 5 to make my conversion and effects additions easier so I added a tail_skid to 3. I did not verify if anything was using wings at 3 and 4 though.

I also added the first part of the gear configurations (bush wheels). This is jutt the initial step, I don't even know for sure what size they were or if I am going to change them to a different size. I need to research and come up with a commonly used size. Unlike the c172p I plan to use only one bush tire size and not two.

There are many, many pieces to add the gear configurations including FDM weight and drag data points. Those data points are not included in the existing FDM structure yet. They will need to be verified and adjusted to match the Cub's version of weights and drag. For example, in Systems/bushkit.xml the data variable produced when switching to the bush gear (currently called 36" even though I don't know that that was the size I made them, I just copied over what i had in the bush version of the Cub) is gear/drag-coef[1]. This additional drag still needs to be added to the FDM at the appropriate place.

This is an example of how we added it in the c172p

Code: Select all
        <axis name="DRAG">
            <function name="aero/coefficient/CDo">
                <description>Drag_at_zero_lift</description>
                <product>
                    <property>aero/qbar-psf</property>
                    <property>metrics/Sw-sqft</property>
                    <value>0.027</value>
                    <property>gear/drag-coef[1]</property> <!-- bush wheels 26" -->
                    <property>gear/drag-coef[2]</property> <!-- bush wheels 36" -->
                    <property>gear/drag-coef[3]</property> <!-- Floats -->
                    <property>gear/drag-coef[4]</property> <!-- Amphibious -->
                    <property>gear/drag-coef[5]</property> <!-- Ski -->
                </product>
            </function>


Note: In the J3Cub there will only be
Code: Select all
<property>gear/drag-coef[1]</property> <!-- bush wheels 36" -->
<property>gear/drag-coef[2]</property> <!-- Floats -->
<property>gear/drag-coef[3]</property> <!-- Amphibious -->
<property>gear/drag-coef[4]</property> <!-- Ski -->


The weight for each wheel is also produced in Systems/bushkit.xml.
Code: Select all
<channel name="extra-weight">

        <switch name="extra-weight-left-36-in">
            <default value="0"/>
            <test logic="AND" value="15">
                bushkit EQ 1
            </test>
            <output>/fdm/jsbsim/inertia/pointmass-weight-lbs[5]</output>
        </switch>
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Re: JSBSim Piper J-3 Cub

Postby erik » Sat Jan 02, 2016 8:49 am

wlbragg wrote in Sat Jan 02, 2016 6:00 am:@Eric,

In J3Cub-set.xml can we get rid of the reference to "yasim" in the following code blocks. As far as I know, the Yasim version should be totally isolated from the JSBSim version at this point.

When I say "we", I mean "you", as I am not sure what needs to be done here?

Normally you would create three -set files in this case.
Start out by putting everything which is shared an a J3Cub-shared.xml
Then move the YASim code to a separate file and include J3Cub-shared.
And the same goes for the JSBSim version.

Erik
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Re: JSBSim Piper J-3 Cub

Postby erik » Sat Jan 02, 2016 8:55 am

Bomber wrote in Thu Dec 31, 2015 11:05 am:<function name="aero/function/kCLge">
<function name="aero/function/kClge">

my mistake in that one... it's ok..

I had trouble with the ground handling and wanted a way to reduce the 'stall effects' ti make sure it wasn't causing this. The values probably should get altered, or removed all together.

I would however get an explanation as to the origin of the ground effect on drag.... I believe ground effect on drag effects cdi (induced drag) only... So as you're adding these values into the general drag [as below], you're reducing the drag by too much..

Normally I don't answer to people who demand explanations but you are correct.

As I see you're quite busy with the Cub, you could take over development as I've got many other things to work on and the coefficients should be ok now (typo's and such not withstanding).

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Re: JSBSim Piper J-3 Cub

Postby erik » Sat Jan 02, 2016 10:08 am

erik wrote in Wed Dec 30, 2015 9:44 pm:
Bomber wrote in Wed Dec 30, 2015 8:24 pm:1) Check the function names...

Ahh yes, the first is for Lift the second for rolling moment. Looks strange but is actually correct.
2) 1 slug ft3 = 32 lbs ft3

Hmm, good point, I don't know from the top of my head which one should be used.
But if slugs is wrong then the c172p also got it wrong.
3) the equation uses a sum not product.

I'm not exactly sure why this is begin used for the c172p, but for now I'll thrust it.


Ok I've researched this and both are correct.
It's derived from blade elements theory.

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Re: JSBSim Piper J-3 Cub

Postby Bomber » Sat Jan 02, 2016 11:54 am

erik wrote in Sat Jan 02, 2016 8:55 am:
Normally I don't answer to people who demand explanations but you are correct.



I was just offering an opinion on the flight model.... I make no demands..

And I'm also busy on the sycamore seed challenge, I just thought I'd offer some insight.

I'll back off if you find it insulting..
"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 erik » Sat Jan 02, 2016 12:06 pm

There's very little I find insulting but it came to me like a demand. And I don't do demands very well.

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Re: JSBSim Piper J-3 Cub

Postby Bomber » Sat Jan 02, 2016 12:58 pm

if I used code written by other people I'd want to understand the in's and out's of the thought process, physics and maths behind the code.... this is why I wrote..

I would however get an explanation as to the origin of the ground effect on drag.... I believe ground effect on drag effects cdi (induced drag) only... So as you're adding these values into the general drag [as below], you're reducing the drag by too much..


I explained why I thought it was wrong, I didn't just say it's wrong.... and I made no demands of you to put it right

If you wish to pick over every sentance I write and look for the bad in it, I can't stop you.... but you don't do it with other people on here so please don't do it with me..

I had trouble with the ground handling and wanted a way to reduce the 'stall effects' ti make sure it wasn't causing this. The values probably should get altered, or removed all together.


my suggestion here is that what you're attempting to reduce is known as power on stall.... the typical 'go around' stall that kills pilots. It's a product of the the engine/prop torque and is a feature of single prop planes... a quick test test to show this is to remove the prop and then perform that same type of stall the behaiour should be very different, escpecailly as this plane also had a glider version.
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Re: JSBSim Piper J-3 Cub

Postby wlbragg » Sat Jan 02, 2016 5:24 pm

@Eric
I don't see anywhere where this output is added back in to the FDM (in the c172p), would you know if this property is automatically a defined property in the FDM or did I just miss its location?
Code: Select all
<switch name="extra-weight-left-36-in">
            <default value="0"/>
            <test logic="AND" value="15">
                bushkit EQ 1
            </test>
            <output>/fdm/jsbsim/inertia/pointmass-weight-lbs[5]</output>
        </switch>

I found it.
Last edited by wlbragg on Sat Jan 02, 2016 5:58 pm, edited 2 times in total.
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Re: JSBSim Piper J-3 Cub

Postby wlbragg » Sat Jan 02, 2016 5:34 pm

erik wrote in Sat Jan 02, 2016 8:49 am:
wlbragg wrote in Sat Jan 02, 2016 6:00 am:@Eric,

In J3Cub-set.xml can we get rid of the reference to "yasim" in the following code blocks. As far as I know, the Yasim version should be totally isolated from the JSBSim version at this point.

When I say "we", I mean "you", as I am not sure what needs to be done here?

Normally you would create three -set files in this case.
Start out by putting everything which is shared an a J3Cub-shared.xml
Then move the YASim code to a separate file and include J3Cub-shared.
And the same goes for the JSBSim version.

Erik

In this case J3Cub-set.xml is already completely isolated and is the JSBSim version of Cub-set.xml. These two FDM versions of aircraft files are going to be so divergent that I am in essence not even going to touch the Yasim version, thus why I made the "new" Models/J3Cub.xml file, which is the JSBSim version of Models/Cub.xml.

So what I guess I am really asking is, does the JSBSim FDM use this property or can it be removed from J3Cub-set.xml, keeping in mind the Yasim FDM has its own exclusive "-set.xml" file.
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Re: JSBSim Piper J-3 Cub

Postby wlbragg » Sat Jan 02, 2016 6:35 pm

While I think I am beginning to understand these measurements a little bit more than I did, I obviously don't know enough to be of much use to make even minor changes confidently.

So Eric, I deffer to your expertise like I asked for in the first place and you so graciously volunteered.

Should I set the tail gear contact point back to where you had it or leave it as I adjusted it.
I only adjusted it because in the off chance someone rotated the aircraft prior to elevating the tail off the ground it would technically rotate below the surface (slightly) and now it won't. I just need to have the position pretty much "fixed" (as in-place) before I go much farther with the other gear configurations.

Things are looking good though and I really appreciate all the work you have put into it to this point.

One other thing you may be able to shed some light on. While I have no real problem with the following observations, I want to know what your take on it is.

1) Starting the aircraft now takes more cranks than before, why is that?

2) The throttle up seems to take a long time to be realized and the power seems weaker than the Yasim version, you think the JSBSim version is the "more" true and correct behavior or may need more adjusting?


EDIT:
One other thing to keep in mind, I'm pretty sure we will need a second (more powerful) engine for the float and amphibious version of the aircraft. However, I haven't researched it yet. As I'm sure you know, the c172p is already setup to use two engines and I eventually intend to do the same with the Cub, it that is the what the research confirms.

While it still might be cool to have a more powerful engine it appears that the average HP for including a float kit is 65. There is even applications that use a 55hp. But if I had the extra money I think I would add the 90hp for the float and ski versions.
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Re: JSBSim Piper J-3 Cub

Postby Bomber » Sat Jan 02, 2016 7:36 pm

wlbragg wrote in Sat Jan 02, 2016 6:35 pm:1) Starting the aircraft now takes more cranks than before, why is that?

2) The throttle up seems to take a long time to be realized and the power seems weaker than the Yasim version, you think the JSBSim version is the "more" true and correct behavior or may need more adjusting?
.


Just took a look at the prop Ct and Cp tables in excel....

Now I don't want to be critical.... afterall no one says nice things of me and I'd like that every now and again...

So you stick with that prop and good luck...

Simon

I do hope it's not used on the cessna... :)
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Re: JSBSim Piper J-3 Cub

Postby wlbragg » Sat Jan 02, 2016 7:54 pm

Thanks Bomber I appreciate your knowledge and help.

For me to understand things it may require a little more of an explanation as you are talking to a complete amateur when it comes to the FDM.

Just took a look at the prop Ct and Cp tables in excel....

~~~~
So you stick with that prop and good luck...

That is a commonly used prop with the Continental A-65-8 engine according to the wiki. It even uses that configuration for a seaplane variant.

Could you please elaborate?
Last edited by wlbragg on Sat Jan 02, 2016 8:28 pm, edited 1 time in total.
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Re: JSBSim Piper J-3 Cub

Postby wlbragg » Sat Jan 02, 2016 8:27 pm

Eric, I added several pointmass data points for the gear configurations, pilot and passenger. I put the Payload at the bottom and zeroed it out for the time being as you had it set to max weight and I am now breaking that down. I think I understand this part well enough to do it but you may want to keep an eye on my commits.

However, the gear/drag-coef[?] are another story, I really have no idea where to add them in to the FDM. Beings you made it and have an idea of what all is there, could you please add in the following properties to what you consider the proper FDM data points?
Code: Select all
<property>gear/drag-coef[1]</property> <!-- bush wheels 36" -->
<property>gear/drag-coef[2]</property> <!-- Floats -->
<property>gear/drag-coef[3]</property> <!-- Amphibious -->
<property>gear/drag-coef[4]</property> <!-- Ski -->

I'm really not sure why we didn't use gear/drag-coef[0]. If there is no logical reason not to use it to start the sequence, then go ahead and I will change it in Systems/bushkit.xml
.
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Re: JSBSim Piper J-3 Cub

Postby Bomber » Sat Jan 02, 2016 8:29 pm

The propellor if a corkscrew that pulls the plane through the air...

Your prop is 75" diameter with an angle of incidence ay 75% of its length is 45 degrees (I used terms you'd understand)

Ok..... if a plane goes faster that the propellor can spin.... the propellor acts as a brake.. This is why some planes can dive faster than others..... (but you'd only know that if combat and understanding why certain planes dive away from others works)

The object of a prop is to turn rotating power into a directional power.... in doing this there's some losses...

Now your prop uses too much power from the engine and in doing so propuces too little thrust.... also while we're at it the interia of the prop is based on it's weight and diameter....we know it's diameter and the weight should be comparible to one of it's era, ie material used and manufacuring techneques...

McCauley 1C172/MDM 7652, 53, or 55 30 lb. (Fixed pitch) 5.14111875 kgm2 3.79 slugft2

now this prop same manufacturer is an inch bigger....and my maths brings the inertia at, compare it to yours.....

If people want to compare maths then I'm up for it...in fact I'd like it.
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Re: JSBSim Piper J-3 Cub

Postby wlbragg » Sat Jan 02, 2016 9:41 pm

Very interesting.

Why do you think it was so widely used then if there were better props available at the time, or were there?

Because now were getting into our dream configuration VS replicating something common at the time.

See what you've done, now you've given me the bug to make this aircraft have 3 different engine choices and a few different prop choices. 8)
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