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Re: P-51D stall and spin: make it milder at takeoff and land

Postby wkitty42 » Sat Apr 07, 2018 7:51 pm

yes... the question is "which input directory?"

there are two... one in the main FG installation tree and the other in your personal settings under %APPDATA%/flightgear.org/Input/Joysticks... the one you want to be editing is the 2nd one in %APPDATA%... not the master one in the installation tree...
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Re: P-51D stall and spin: make it milder at takeoff and land

Postby dany93 » Sat Apr 07, 2018 10:36 pm

OK-F wrote in Sat Apr 07, 2018 5:27 pm:....then landing oh dear sometimes the left wing drops several times i don't know maybe 40 degrees in a millisecond like the plane has no weight at all. Or the wing just drops right after I get the wheels off the tarmac stays there, with no reason.

You are experiencing the stall and spin. At stall, if the flight is not symmetric, the aircraft quickly spins, result of a single wing which stalls prior to the other one.

To avoid that:
- before takeoff (moderate, not full, and progressive throttle increase), wait for enough airspeed (tail lifted, horizontal), possibly by slightly pushing the stick, before carefully pulling it. Stall is all the more dangerous that you are in asymmetrical flight because of full throttle. And you have to anticipate the roll leftwards due to the propeller moment before taking off.
At start, rolling at low to moderate speed, steering is easier with the stick pulled back.
- at landing, do not hold the flare still at 2 - 3 meters height above the ground. By doing this, you will stall at that height. Rather in a first learning step, touch down with the main gear only, at a slightly higher speed. The ideal is a "three point touch", when the aircraft stalls at the right angle of attack just when razing the ground. But do not stall at 2 meters height. At best IRL, you damage or break landing gears, at worst, you go to a spin.

Many FG aircraft do not have this feature, but it exists IRL (and it results in deadly accidents).

Another thing (Joystick):
If you decrease your stick gain to make it easier, the risk is that you will no longer be able to reach the stall AoA at landing. Which means touching down at higher speed. Always compromises...
[EDIT] I realized later that it was for the rudder. Here, the risk is being no longer able to land with max crosswind.
Last edited by dany93 on Sun Apr 08, 2018 11:59 am, edited 4 times in total.
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Re: P-51D stall and spin: make it milder at takeoff and land

Postby OK-F » Sat Apr 07, 2018 11:03 pm

OK Dany will try it. What do you think about that the plane is pushing at idle

Anyway found this at APPDATA

</axis>
<axis n="4">
<desc type="string">Rudder</desc>
<binding>
<command type="string">property-scale</command>
<property type="string">/controls/flight/rudder</property>
<factor type="double">1</factor>
<offset type="double">0</offset>
</binding>
</axis>

and changed it to

</axis>
<axis n="4">
<desc type="string">Rudder</desc>
<binding>
<command type="string">property-scale</command>
<property type="string">/controls/flight/rudder</property>
<factor type="double">1</factor>
<offset type="double">0</offset>
<power type="double">3.0</power>
</binding>
</axis>

0.1 to 0.9 rudder fully extended on one side
more then 1.1 to 1.9 seems increasing the sensitivity.
2.0 seemed lowered the sensitivity. I set it to 3.0 and gonna to try it in the air
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Re: P-51D stall and spin: make it milder at takeoff and land

Postby swampthing » Sun Apr 08, 2018 6:07 am

Dany93 I will have to download it and look at the exact table again to see if it is still the same but 0 knots and 140 knots having the same value is not correct. sorry. Sure you have to work to take off, taxi and land a tail dragger but that plane does have some weight to it. I'm trying to tell you this as gentle as I can. Your comparing crab apples and apples to oranges here when you compare an ultra light or a Cessna to a P-51. ;)
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Re: P-51D stall and spin: make it milder at takeoff and land

Postby swampthing » Sun Apr 08, 2018 6:17 am

It is not your flight stick OK-F unless it is wore out. To the maintainer, I'm not trying to put anyone down here but I did have a pilot who trains people for a living and does fly some old war birds from time to time test it a few months ago and he said the same as me. It is twisting too much at take off. I'm only trying to help but I will say this. If it stays nearly impossible to take off people won't fly it and that would be a shame. I'm not saying gimp it down so its unrealistic, just to fix it is all. If you don't want help then OK and I'll say nothing more here.
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Re: P-51D stall and spin: make it milder at takeoff and land

Postby swampthing » Sun Apr 08, 2018 6:38 am

Just for the record when I tested it I chose a strip with 0 knot wind from any direction and the reason I don't believe its the stick is because I have the same one nearly new still and I fly a lot of planes of all variants and have seen no sign of trouble from my stick. At take off it is like you get hit with a big side wind when there is none. I will send the line I am talking about in JSBsim when I get everything back on my system. I'm In the middle of a big download at the moment.
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Re: P-51D stall and spin: make it milder at takeoff and land

Postby dany93 » Sun Apr 08, 2018 8:48 am

@OK-F,

Yes, your
Code: Select all
<power type="double">3.0</power>
will enable your JS to keep full scale response from -1 to +1. Better than decreasing the gain (=factor).
It gives a non-linear response, softer near the neutral but more abrupt at the ends.
Last edited by dany93 on Sun Apr 08, 2018 3:18 pm, edited 1 time in total.
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Re: P-51D stall and spin: make it milder at takeoff and land

Postby dany93 » Sun Apr 08, 2018 9:45 am

swampthing wrote in Sun Apr 08, 2018 6:17 am: To the maintainer, I'm not trying to put anyone down here....

Please don't consider I am the maintainer. The maintainer was Hal Van Engel and, sadly, he passed away. After several years of work in the FDM, systems, etc,.... with a lot of rigour and documentation. We can assume that he had a good enough knowledge of this aircraft, much better than we have.

It may be that this aircraft is not perfect. If you carefully read this topic you will see that I've already made changes to have it less brutal, but my intention was not to redo it completely. It was mainly to keep it alive. When I started to touch it up (I didn't know yet that Hal was dead) the aircraft even did not launch. If you find someone who knows well the P51D, the aerodynamics and JSBSim, willing to spend much time on it to bring more realistic improvements (not make it easier for beginners), please go. The P51D and many people who like it will thank you.

As far as the twist due to the propeller moment, if I wished to do something I would be embarrassed because this moment is calculated by JSBSim. It's here, result of the prop diameter, prop pitch and the engine. And it might be true, a 1700 hp engine is very powerful. I've known a test pilot who said that, on some powerful aircraft, you could not control it if you abruptly pushed full throttle without having preset your aileron trim.

swampthing wrote:look at the exact table again to see if it is still the same but 0 knots and 140 knots having the same value is not correct

Which table? This one?
Code: Select all
            <function name="aero/coefficient/Cnspw">
                <description>Yaw_moment_due_to_spiraling_propwash</description>
                <product>
                    <property>metrics/Sw-sqft</property>
                    <property>metrics/bw-ft</property>
                    <property>propulsion/engine/thrust-lbs</property>
                    <value>-0.00002</value>
                    <table>
                        <independentVar lookup="row">/velocities/airspeed-kt</independentVar>
                        <tableData>
                            0    1
                            140  1
                            190  0
                        </tableData>
                    </table>                   
                </product>
            </function>

If you observe the property values carefully enough, you will see that the yaw moment due to spiralling propwash is not constant between 0 and 140 kt, for the simple reason that the thrust is not constant. Due to the curve of propeller-thrust vs airspeed (more accurately vs J = V/N.D).
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Re: P-51D stall and spin: make it milder at takeoff and land

Postby swampthing » Sun Apr 08, 2018 11:44 am

FGaddon is failing for me. I don't know if the one in Onox's repo is the same but as soon as I can get the plane I will send it to you via PM and you can test it. You'll find that you do have use rudder and other FCS according to the weather still to control the plane. I can tell you from experience that even I though I think JSBsim is the best we have for a realistic FDM it isn't always perfect. Anyway as soon as I get the model downloaded I will send the correction to you and I think you will be pleased. Hal Van Engel did a great job with this plane and I wouldn't want to ruin it but I would like to see more people flying it. I agree it is not a trainer and before I leave this world I would like to create a nice FDM for a trainer as learning to fly a tail dragger is a whole new world. You actually have to fly it. :) I hope I have not caused any hard feelings here I'm only trying to help.
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Re: P-51D stall and spin: make it milder at takeoff and land

Postby OK-F » Sun Apr 08, 2018 3:35 pm

Swampthing just don't worry I'm sure Dany and we all greatly appreciate that you interested in this plane and trying to help.

The thing is I know that if I lift the plane too early it will stall, wing drops etc. The think is it still behaves unrealistically at take off and if possible it should be corrected. I've manage already few good landings but I think only one good take off in maybe 50 attempts. There actually is very narrow space to make good take off but it's hard to find :-D

Seriously the plane has tendency to slip off the rwy once it gained some speed and even full contra rudder doesn't help.

If I Manage to keep the plane within the rwy with enough speed when the plane actually taking off hapilly by itself most cases left wing drops rapidly
not necesarilly touching the rwy.

When I was talking about the wing dropping several times very quickly yes that was probably stall it just felt unrealistic quick :-) ofc I'm aware that this is just a sim and it never be 100% realistic.

The rudder is better now - 2.0 (power type) value do the trick
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Re: P-51D stall and spin: make it milder at takeoff and land

Postby dany93 » Sun Apr 08, 2018 4:18 pm

An auto takeoff has been implemented. Start the engine and press F8.
For learning, observe the control/flight properties (aileron, elevator, rudder). Notice the elevator full up at -1 just after start (to make tail steering more efficient), then the aileron which quickly goes up to 0.6... 0.8 to counteract the propeller moment.

For us and manually, increasing the throttle very progressively makes things easier. If the runway is long enough, moderate throttle (25-30 in Hg at "MAN", not more than 30) is enough. It makes a moderate roll moment, 27 is almost easy, we can feel the roll happening. Higher throttle (40 -46 in Hg and more) is only for climbing and, above all, fighting emergency. Or taking off on a short runway, when you are skilled enough :D .
Last edited by dany93 on Sun Apr 08, 2018 5:43 pm, edited 2 times in total.
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Re: P-51D stall and spin: make it milder at takeoff and land

Postby OK-F » Sun Apr 08, 2018 5:25 pm

Yes I think I am adding too much throttle. I was studying real P51's taking off and the guy added full throttle but he was on the grass and as I see it makes a difference even in FG with this plane.
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Re: P-51D stall and spin: make it milder at takeoff and land

Postby swampthing » Sun Apr 08, 2018 10:27 pm

I know Onox made the auto take off due to him using a mouse,m it makes it pretty impossible to take off, taxi a tail dragger but I personally would have kept that code to my on plane no offence to Onox. You'll never learn doing it that way. Many things can cause you some troubles in a tail dragger, tail winds, side winds etc. you have to know how to compensate for this when taxing, taking off and landing. Also don't forget < and > your brakes are your friend too. Here is some information everyone should know before taking a flight in a tail dragger if we want to fly in a real simulation experience. https://www.guthrieaviation.com/index.p ... il-dragger If you already know this good but man think they can move around like a ttricycle plane and you will be in for a rude awakening unless the plane has been gimped down so anyone can just jump in and fly. Just like Flying helicopters in Flightgear I think we need to keep it as realistic as possible. It took me quite some time to fly a chopper. Getting off the ground was one thing landing and hovering is a whole different story. Its not easy to learn in real life. Why should it be here? Hence the word simulator. I understand some people only fly with a mouse but maybe pinch some pennies and get a proper flight stick. I'm sure pilots in WWII would have loved auto take off but I tend to lean against that option in the P-51D. That's just my opinion. Maybe I'm just mean and want people to actually put some effort into it. :)
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Re: P-51D stall and spin: make it milder at takeoff and land

Postby OK-F » Mon Apr 09, 2018 2:38 am

Yes looks like everyone wants to keep the plane realistic that is good! :-D Anyway I must admit that nice take off and landing is entirely possible with this plane. After several hours of trying today I finally learned it. So guys it needs just a bit of effort but the reward is nice.
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Re: P-51D stall and spin: make it milder at takeoff and land

Postby swampthing » Mon Apr 09, 2018 3:35 am

http://www.wwiiaircraftperformance.org/ ... pril44.pdf If you can take off with manifold pressure where the manual say to then you know its not too much. I've gimped it down to where I had it in the past testing and if I reach the manifold pressure where it should be per the manual it ends with a bad day. I would look at that and adjust it back from there. Dany93 can look at this and make a decision from there. Also in all of the man many WWII aircraft documentaries I've watched I don't remember pilots complaining that they were like death traps taking off. There really were not a lot of complaints other than don't get hit too many times. Not has heavily armored as some of the other planes but thats part what helped give them longer legs. Basically half the weight of the Jugg but the Jugg was like a flying tank.
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