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

Postby AndersG » Mon Apr 09, 2018 7:38 am

swampthing wrote in Mon Apr 09, 2018 3:35 am: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.


In this regard I think you have to appreciate that they were coming from training on a sequence of tail draggers before being let loose on a P-51D (having at least 100+ hours at that point and I think considerably more in the American training system) and that they were at war. Still, finding some statistics on combat v.s. accident losses could be interesting. Or just on types of accidents in peace time (I might be able to find something on the Swedish Airforce's use of the P-51D post-war). I suppose most high powered tail draggers would be death traps for the unwary (but some worse than others).
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Re: P-51D stall and spin: make it milder at takeoff and land

Postby swampthing » Mon Apr 09, 2018 9:25 am

Sure, but they are usually sure to complain about going from one plane to another and the veteran pilots would have flown other warbirds with different sometimes better or worse characteristics. Many of the pilots in the old Docs I watched didn't hold back much. I also watched a ton of Youtube videos of P-51's taking off in the last day or so and you saw the rudder being used more in some and less in some, this was obviously due to wind. Some you could really hear the wind in the camera and the pilot was into the rudder more but in none did I see you having to give full rudder and praying the left wing didn't hit the ground they all went up pretty level and some went right to climbing hard. Just like with pretty well all tail dragger’s you saw rudder movement, you have to use it but you also can't be too snappy for those with no experience. I don't think we should downplay that there is certainly an issue when you rotate with it as the FDM is set up now. To say otherwise is just to be in denial. One question. Is the manifold pressure gauge reading correctly inHG? Please see the manual I linked in a previous posted. If at 25-30 inHG is causing the rotation to the left so bad what would 61 do as the manual says to use on take off? And yes I know its a good idea to ease into it. I hope I'm not getting under anyone’s skin here, its not what I'm trying to do but I think it should be looked into and thought over. I know Dany93, you may not want to mess with it much since he has passed away but maybe he wasn't done tweaking it. Just a thought.
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Re: P-51D stall and spin: make it milder at takeoff and land

Postby dany93 » Mon Apr 09, 2018 9:47 am

swampthing wrote in Mon Apr 09, 2018 3:35 am: I would look at that and adjust it back from there. Dany93 can look at this and make a decision from there

Good thing if you participate to maintain and improve this aircraft.

But you grant me more skills than I have.... The P51D, with its very powerful engine, is very uncommon. It would be better (even essential) to find a RL pilot who has a knowledge, if not of the P51D (that would be ideal) but at least of this kind of aircraft.

Which doesn't mean that I will not look at it, test it and give my opinion. But for what it's worth... Like you, I only have the POH as a reference. My limited experience on ultralights can help (for general behavior) but the P51D is another beast, which can only be much harder.

I guess you are at least considering some refactoring of the engine.
It may lead to check and maybe recalculate the propeller.

At the end, when we agree (I hope, with enough other competent people) on a solution, you will have to propose your changes to the FGDevel list for merging it in FGAddon.
FGAddon procedure.
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Re: P-51D stall and spin: make it milder at takeoff and land

Postby swampthing » Mon Apr 09, 2018 10:14 am

I forgot to mention (not to nitpick to death) a change that drives me crazy trying to test it. I loaded up Windows which had a very slightly older version and it started with the parking brake on, good. The new one that I just got is not so pleasing. It starts with the parking brake off and wants to move away at idle. I have my brake bound to my stick, its convenient. When I hit the brake to stop it from rolling away it turns on the parking brake so that causes a bit of a fight. I think shift b as it was before was a better idea. I'm sure there were good intentions but for those of us with the brake bound to the stick its quite annoying. And its not the parking brake I have bound either, just like hitting b. If it were the parking brake it would stop me dead in my tracks in all of the planes I fly. Just throwing that out there. :)
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Re: P-51D stall and spin: make it milder at takeoff and land

Postby dany93 » Mon Apr 09, 2018 10:39 am

I noticed this too (needed some time to understand). I think it was a change by onox. That's to see with him, why he thinks it's better (or can be discussed).
I also prefer the usual parking brake control (shift-b).
[EDIT] The parking brake logic fits the real one. See this message [/EDIT]
Last edited by dany93 on Sat Apr 14, 2018 3:40 pm, edited 1 time in total.
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Re: P-51D stall and spin: make it milder at takeoff and land

Postby swampthing » Mon Apr 09, 2018 10:58 am

I'll ask him later. We talk via Discord, as long as he hasn't read this and decided he doesn't want to talk to me now. :)
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Re: P-51D stall and spin: make it milder at takeoff and land

Postby OK-F » Mon Apr 09, 2018 10:08 pm

https://crgis.ndc.nasa.gov/historic/P-5 ... Collection Here can be find some relevant info
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Re: P-51D stall and spin: make it milder at takeoff and land

Postby OK-F » Mon Apr 09, 2018 11:11 pm

Tested with 46Hg and 2800rpm and still possible to take off without problems. The key of succesful take off with P 51 is to start pushing the stick continuously forward after we reach about 60knots and stop pushing it almost when the propeller is reaching the ground! The plane starts taking off by itself leveled with the ground and slowly gaining altitude. Otherwise the plane starts taking off too early and stall. For me it was lack of experience with this kind of wheel setting. Anyway for a new RL pilot this had to be a nightmare :D Front wheel setting is a different universe. For pilot who was used to the rear wheel setting skipping to a front wheel setting he could feel like he had switched on an autotakeoff setting :)
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Re: P-51D stall and spin: make it milder at takeoff and land

Postby swampthing » Tue Apr 10, 2018 1:13 am

OK-F what is your call sign? Be nice to fly with someone in an old bird. I look on MP map and it's usually airliners. Mine is swamp.
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Re: P-51D stall and spin: make it milder at takeoff and land

Postby OK-F » Tue Apr 10, 2018 3:01 am

OK-FLY... My base airport is LKPR
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Re: P-51D stall and spin: make it milder at takeoff and land

Postby swampthing » Tue Apr 10, 2018 4:33 am

Roger. Mine is KVIH and K07. No buildings but that’s where it is. ;)
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Re: P-51D stall and spin: make it milder at takeoff and land

Postby DFaber » Tue Apr 10, 2018 7:16 am

Personally, I think the p51d is a bit exaggerated.

According to the Training Handbook, the british Pilot Notes and Eric Browns Book it should be possible to take off in three point attitude. However, there are different views how to best take off in a warbird. Some prefer to gain speed with the tail raised, but that can be dangerous if the tail comes up too early. Others like to take off in three point to be sure the prop doesn't plow into earth.
The P-51d feels like the rear tank is full and the CG too far aft. The wings take off before the tail in threepoint, which isn't mentioned in any book I've read, unless the rear tank is used. Watching the "Kermie Cam" on the P51C ( https://www.youtube.com/watch?v=eOXxUApaaWo ) you can see how he works rudder and stick. It's a lot less hectic, than required with the p51d. Eric Brown describes it as "sweet and easy" and the training handbook says the torque can be completely controlled by trim.

A fundamental problem with PC flight simulation is the lack of recogniseable feeling when the aircrafts tail wanders out, or the ailerons and elevator heavy up when speed is built up. There is only visible feedback how the plane reacts, which leads to longer latencies in pilot actions than in a real one. There's no easy solutions to this, a PC Pilot just have to know what his plane does during take off.
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Re: P-51D stall and spin: make it milder at takeoff and land

Postby swampthing » Tue Apr 10, 2018 12:06 pm

Good points Dfaber. One of the biggest headaches in a tail dragger is weather. You need to understand where your flight control surfaces should be especially when you taxi, start to take off or even rotate if it's a lighter craft. I posted a link that teaches this, maybe not the best one but there are others if people search. So my advice to anyone is to read up and understand how to counter tail winds and side winds before trying to fly one. Check your Metar and look at the wind sock. Flying in live weather can be a pain but once you get a feel for it you can find that being able to successfully maneuver on the ground is quite rewarding. We go to bad weathermetar and find some rough places to fly. If you don't make it the first time at least we get some good laughs.I have the old Yak53 stored away if anyone wants to try something lighter as kind of a trainer, it has a pretty nice FDM with damage and you can even blow the engine if your not careful. Kind of like the old saying learn to crawl before you walk. It is tempting to go in head first and just run though.:)
Last edited by swampthing on Tue Apr 10, 2018 12:17 pm, edited 1 time in total.
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Re: P-51D stall and spin: make it milder at takeoff and land

Postby swampthing » Tue Apr 10, 2018 12:14 pm

Here is a link to my Warbirds server if anyone is interested. https://discord.gg/UqMChbg There are development channels, General chat and voice channels if we do a group flight. Not trying to hijack the thread but since we are on the topic I thought I would throw it out there.
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Re: P-51D stall and spin: make it milder at takeoff and land

Postby dany93 » Tue Apr 10, 2018 3:18 pm

Propeller torque issue:

I've checked the propeller torque in the sim. The torque values in JSBSim are consistent with RPM (propeller and engine) and power values. I didn't have much doubt on this because I had checked it many times on other aircraft. Not only they are consistent in JSBSim but I still think that this torque is realistic.

I've had a look in the engine file. I'm not used to these boosted engines, however I find nothing weird.

An Observation which may help for taking off and, mainly, climbing:
Procedure:
Throttle, 30 inHg stick full back, up to 50 - 60kt, then let the tail lift, go on rolling on the ground, close to flight attitude (unchanged from previously).
When about 130 kt are reached, you can push the throttle to 61 inHg (even higher to be provocative, although out of the books) without loosing roll control. The aileron efficiency is sufficient.
The 130 kt value can be slightly lowered or / and the 30 inHg slightly increased, depending on your skills and some anticipation on roll correction.
However, still better to do throttle increase progressively.

To make it easier (please, still to be tested more), you can change these values in the FDM as shown below.
(@OK-F, make a backup copy of your file a just in case)
With your learning, I hope you will not find it too easy!
:lol:

p51d-jsbsim.xml, lines 2021 - 2055 (it can depend on your version).

Under

Code: Select all
        <axis name="ROLL">


initially:
Code: Select all
            <function name="aero/coefficient/Clda">
                <description>Roll_moment_due_to_aileron</description>
                <product>
                    (a few lines....)
                    <!-- Asymmetric stall -->
                    <table>
                        <independentVar lookup="row">aero/alpha-deg</independentVar>
                        <tableData>
                        -35    0.3
                        -9     0.3
                        -8     1
                        10     1
                        11     0.3
                        35     0.3
                        </tableData>
                    </table>
                 </product>
            </function>


In the table, change 10 and 11 for 15 and 17:
Code: Select all
            <function name="aero/coefficient/Clda">
                <description>Roll_moment_due_to_aileron</description>
                <product>
                    (a few lines....)
                    <!-- Asymmetric stall -->
                    <table>
                        <independentVar lookup="row">aero/alpha-deg</independentVar>
                        <tableData>
                        -35    0.3
                        -9     0.3
                        -8     1
                        15     1
                        17     0.3
                        35     0.3
                        </tableData>
                    </table>
                 </product>
            </function>


It should give a better aileron efficiency at the critical step of taking off.
[EDIT] Confirmed.
It enables you to push the throttle at 61 inHg, roll and takeoff provided you:
1 - let the tail lift up (to flight attitude) at about 50 - 60 MPH,
2 - be prepared to counteract the roll by a rightwards stick movement at about 130 MPH.
Still tricky... but controllable. During a few takeoff tests, I even got some of them with a small roll angle. You have to keep an eye on several parameters at the same (short) time. Luck? Training?
[/EDIT]
I hope that it doesn't degrade something else (I don't think it does).
Last edited by dany93 on Thu Apr 12, 2018 8:55 am, edited 2 times in total.
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