Flight Dynamics
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Hello, I recently purchased the VC10 Jetliner for FSX from the SimShack website, I appreciate that this is an older product and FSX is somewhat old hat now but it suits my needs at the moment. I was quite disappointed to find that on flying the model the flight dynamics were quite frankly atrocious, when taking off (full fuel load and loaded as per the tutorial flights) with the trim set just into the white band and flaps at 20 degrees the aircraft lifted of the runway without any control input before reaching Vr and the response to trim changes was so sensitive that it made the aircraft virtually un-flyable, the slightest change of trim would cause the aircraft to pitch up or down wildly and attempting to trim for straight and level was impossible. Whilst I have never flown a VC10 I am a real world pilot and can be fairly sure that this is not how a VC10 would handle!
A quick look into the aircraft.cfg file soon found the reasons for this behavior, the value for empty_weight_pitch_MOI which controls the pitch inertia of the aircraft was set to a value of 19115.4, for an aircraft the size and weight of a VC10 this parameter should be in the region of 4000000 - 5000000 (for reference the default 737 has a value of 1600655 and the 747 is 24223159). In addition the value for elevator_trim_effectiveness was set to 7.0, I have never known this value to be set to anything other than 1.0 or very close to 1.0 for any aircraft and this was clearly the cause of the over sensitive pitch trim. Setting these parameters to sensible values and moving the CG position slightly has now resulted in a flight model that flies quite nicely and behaves as I would expect a VC10 to.
I find it somewhat strange that a commercial product could have been released with such obvious faults and would be interested to know what other peoples views are on the flight dynamics of this product.
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@pauld firstly, do not use default aircraft as reference. The figures used in them bear no relevance to actual, calculated aerodynamic coefficients. The default 747, for example, has a pitch damping level a few hundred times higher than the calculated figure and therefore it also has a pitch moment and MOI similarly distorted from reality in order to balance it. The VC10 MOIs were calculated using a real-world aerodynamic tool.
The elevator has had to be created very differently, as it is an all-moving tailplane. The simulator does not have any way to deal with this, it expects a horizontal tail and a separate elevator. By overcoming this obstacle in the sim's core flight model, it introduced further issues which had to be dealt with, such as the lower deflection angles of the 'elevator', which is where I suspect the trim effectiveness was adjusted to compensate (though it is several years and multiple aircraft and sims since I worked on this).
We had a recently-current VC10 pilot on the test team, and he was quite specific about many aspects of the pitch (especially the slat / flap behaviour) so I would be very surprised if he passed over anything as bad as you make it out to be. I do wonder if it is an FSX v P3D thing, as the latter was where this was primarily tested. Unfortunately, I no longer have FSX on my system.
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@delta558 As the product is sold as being for FSX and P3D I would expect that it should have been fully tested on both platforms before being released for sale. I am well aware that the default aircraft are less than perfect but as a former airline pilot I can assure you that their behaviour is far closer to how a real heavy aircraft handles than this VC10 model.
The MOI values that flight simulator uses are based on the weight and dimensions of the aircraft and the Microsoft SDK includes a formula that can be used to calculate ball park values, again it may not be perfect but will be somewhere near what the value should be. Using this formula on VC10 weights and dimensions results in a figure of around 4.3 million (slugs per square foot) for the pitch MOI so how you have arrived at figure of around 19,000 is somewhat strange, a figure in this range would probably be appropriate for something like a twin otter. I also noticed that there are some commented out values in the config file, presumably these were the the original calculated values but all of these are wrong by a factor of 10, presumably someone got the decimal points in the wrong place!
The bigger issue with the flight dynamics (although probably related to the MOI) was the pitch trim sensitivity with small changes in trim resulting in large and rapid changes in attitude. The use of a value of 7 for elevator_trim_effectiveness is somewhat beyond my comprehension, I have never seen values outside the 0.8 - 1.2 range used for this.
I can only assume that your VC10 pilot did not hand fly a take off and climb in FSX with this aircraft as I cannot believe he would have endorsed it if he did. The VC10 was well received by customers when it entered service due to it's quiet and comfortable ride, if the real aircraft had flown as this one does I think the passengers would likely have arrived at their destination wearing their in-flight meals and with several broken limbs!
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@pauld said in Flight Dynamics:
The VC10 was well received by customers when it entered service
As was this simulated one after a lot of testing and various updates. It has been out for some years now, so to describe it as 'atrocious' seems grossly inaccurate.
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@pauld Okay, first point - when we developed this, it WAS fully tested on both FSX and P3D, my comment was purely that I no longer have FSX on my system to be able to check if there are any handling differences between the two sims with the same files (which has happened on occasion).
Second, and the main, point:
I have just got home and opened up my VC10 installation in P3D.
In ALL cases (Pro, Super, C1K, K3,K4) elevator_trim_effectiveness = 1.0
Pitch MOI for the K3, K4 and Super is 5057867.1
Pitch MOI for the Pro is 3198905.4
Pitch MOI for the C1K is 4050015.9These figures are probably in the ballpark for where you think they should be, and are certainly nowhere near the figures you quote.
Apologies if my initial response seemed defensive, but without the actual files in front of me it was difficult to be precise. However, I can now say categorically that there is something wrong with your VC10 installation, I'd recommend a re-install and if that fails we'll look at other ways to fix your problem.
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@delta558 No apology necessary, no one likes to hear criticism of their products and clearly with the figures you state in place there would be no problem at all. It is strange that this has happened but to double check I have re-downloaded the installer and re-ran it and it is definitely installing with the incorrect values that I mentioned above.
The version that I have does not have all the different models that you list, it just has one model (the 1101) with several different liveries. It was purchased and downloaded from SimShack https://www.simshack.net/products/vc10-jetliner-fsx-p3d-1540 so possibly they have an incorrect or outdated installer on their website? The installer file is named "JustFlight_VC10_TP_140.exe" and has a version number of 7.1.100.1248 if this is of any help.
@Derek I can assure you that if you had attempted to fly the model I downloaded and installed you would find that atrocious is not in any way inaccurate.
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@pauld Likewise, I can assure you that if we sold a product that was that bad we'd have heard about it by now - as would Google. As Paul says, it sounds very much like you have a duff copy - although I guess other Simshack customers would also have the same issues?
We will check their build out and see if that can clarify things.
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Just to check something here, are you all comparing the same product?
The user is talking about VC10 Jetliner. The replies aren't referring and talking about the VC10 Professional are they?
Just checking...
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@voice-of-reason I can't remember that far back 😳, but I would be surprised if there was such a large scale change on both fronts mentioned. The professional was developed directly onwards from the original. Not saying it didn't happen, but that's quite a massive set of differences - if it is the case that that is the state of the jetliner file, I'd suggest it would be best to provide a very late update to it!
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@delta558 said in Flight Dynamics:
@derek Yep, which is partly why I'd be surprised if it was the original file . . .
As per my previous post the product was installed from the installer I purchased and downloaded from SimShack https://www.simshack.net/products/vc10-jetliner-fsx-p3d-1540
If this is not an "original file" I would be interested to know exactly what it is that I have been sold?
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As discussed earlier, it looks like you probably have the Jetliner version, not the Pro. There are most likely different flight models as we didn't do the development work on the Jetliner, that was from Aeroplane Heaven. If you get in touch with JF support they can probably sort you with our flight model to try - although, as I say, the Jetliner seemed to work fine if reviews and sales are anything to go by.
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I am fully aware of which version I have, I stated which version it was in my original post and linked to the website where it was purchased from which also shows which version it is. As the product was sold as a JustFlight product I assumed (apparently incorrectly) that it had been developed by yourselves and that the JustFlight forum would be the appropriate place to post about issues with the product.
There is no need for any technical support, I have solved the issue myself by putting the correct values into the aircraft.cfg file.
My reasons for posting about this issue were that I thought (again apparently incorrectly) that JustFlight might be interested to know about this and would be keen to look into the issue and correct it, after all it is a simple fix that involves changing a couple of lines in the config file. The replies I have received on this forum have almost all been defensive or disbelieving of what I was saying and it would appear that no one has actually taken the time to look at the product or check the config file to see if there is problem so clearly it has been a waste of my time posting here.
I have nothing more to add and will not waste any more of my time on this.
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OK - glad you have resolved it for yourself, but, as with all issues with our products (or those we publish and sell) your best bet is always to contact Just Flight support. The forum is really only there for discussion and we can't provide any proper technical support here.
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@pauld I've just looked at my L1011 aircraft.cfg and the empty_weight_pitch_MOI= 12000000.000 and the elevator_effectiveness=1.240 so it seems strange the figures you get. I never make changes like this so I don't screw everything up so they must be original from the install.
