Engine Boost Control + Ground Handling

[NightRush]

Hi!

 

I have several bugs here I would like to report with the Spitfire! First thing, beautiful aircraft the work that has been done is fantastic.

 

Engine

The problem is the in game engine model does not have any Automatic Boost Control. How this behaves in game, is that after setting a given Boost setting with the throttle and the aircraft either climbs, descends or RPM changes the boost is either rising or falling.

 

In the real aircraft, the Automatic Boost controller is maintaining any given boost input through a operational range. So in practice what this means, that if in a climb I set 8" boost at 2600RPM the controller will maintain 8" of boost until the engine reaches its "boost critical altitude". Where the set RPM and air density is not sufficient for the supercharger to provide the set boost. From here the throttle will be wide open, and boost will continue to drop until Supercharger Hi kicks in or critical altitude is met.

 

Now the solution to this, is probably only going to come with a educated guess. I looked through all my manuals and I can not find a Automatic Boost operating range. Not being a DCS developer I cant really help too much.

 

However my idea would be to use a time to climb chart for the Merlin 66, find the transition point from where the engine no longer can hold 12" boost before Supercharger Hi and work back from there. For any subsequent lower RPM setting the delta between air density would be reduced.

 

Anyway, snipped from the appropriate maintenance manuals. The information can also be found in the Rolls-Royce Two Stage, Two Speed Engine Maintenance Manual; Publication No TSD 94

 

 

 

Ground Handling

 

Tail Wheel Castering

Tail wheel, currently in game the aircraft's tail wheel behaves like a shopping cart; in that is has reduced or no centering force to bring the tail-wheel aligned under the fuselage centerline. So what is happening in sim, the aircrafts tail refuses to stay behind the main wheels even when centered, and no external force is added (Power applications or wind)

 

In real life, while it is true the tail wheel is free-castering that is not the full story. The tailwheel design has a built in weight and geometry centering. I will do my best to describe it here with images and words.

 

The first is the weight/geometry holding the tailwheel centered. The axis that the tail wheel rotates through horizontally is on a tilted plane. Unlike say a shopping cart where the wheel rotates in a plane parallel with the ground, the wheel has a high point and a low point in its travel.

 

What this means is for the tail wheel to move from its centered position, there needs to be enough side load to compress the tail wheel oleo strut to effectively raise the tail. As the contact patch of the tail wheel rotates to say 180 degrees to centreline, the angled plane mean the wheel is in a "lower" position that when it started from its trail position. (See next picture) Please note that the image is showing the tail wheel oleo uncompressed, ie no weight on the wheel (and also those sweet sweet photoshop skills)

 

This means the tail wheel has oleo compression force centering the tailwheel once there is forward movement. Ontop of which any elevator and prop blast forces needed to be added to this as well.

 

 

The other centering force thats acting on the tail wheel is simply the wheels axis is behind the yawing axis/horizontal plane. This trail force naturally makes the tail wheel want to be behind the rotating (castering) axis due to friction.

 

What this means in practice with the above two scenarios, is as the aircraft starts rolling forward the tail wheel naturally wants to allign itself with the center line of the fueslage. However in game, we currently have a tailwheel that wonders around aimlessly. It however does feel like it has some centering force as ground speed increases and elevator loads hold the tail wheel to the ground. But its at the slow speeds where the tail wheel should have a little bit more natural centering force. Once the aircraft is at a walking pace, brakes should only be required to turn the aircraft, not keep it straight (in a zero wind scenario)

 

 

Main Gear Struts

 

After a fair bit of testing, as much as I can in 3 days. The main undercarriage oleos are just slightly out in regards to dampening/spring rate and oleo compression. This shows itself initially when starting the aircraft as it rocks back and forward, its hard to make a definitive point from an animation so Ill use handling instead.

 

Currently as side load is added to the gear when taxiing or landing, the aircraft simply falls over on a wing. There is little or no progression/takeup load on the outside undercarriage oleo of the turn.

 

My theory is based in reality, that the oleo part of the undercarriage absorbs some side load (ground loop force) as it compresses. In game it feels like this is not happening, that simply the side loads on the main gear is being directly sent to the fuselage, with very little absorption though the gear legs.

 

I think the solution would be slightly decreasing the dampening/spring rate (I know there is no spring, but its the force we are talking about here). its probably just worth mentioning the spring rate should be progressive and not linear due to nitrogen being the "spring". This would allow more progressive uptake of torque and side loads on the fuselage/undercarriage. Also this would in turn allow the oleos to have slightly more travel in compression.

 

Its only a very small thing (The main gear thing) But if the physics within the sim are good, it will greatly enhance the model of the aircraft.

 

The end

 

Thanks, I really hope this helps with a beautiful plane.

 

Regards

 

Rush

Edited December 18, 2016 by NightRush
Getting Pics working

[dali]

Hi!

 

 

After a fair bit of testing, as much as I can in 3 days. The main undercarriage oleos are just slightly out in regards to dampening/spring rate and oleo compression. This shows itself initially when starting the aircraft as it rocks back and forward, its hard to make a definitive point from an animation so Ill use handling instead.

 

 

I concur on that. Spit is jumping (rocking) from one wheel to the other when engine is starting. I would say not only slightly out, they are way too hard. The wings should be rocking, but the oleo compression should dampen the shaking

 

 

[Schwarzfeld]

Absolutely agree. I was trained to fly tailwheel in a Piper Cub by a crusty old racing pilot with a grass field earlier this year - and I feel like I'm a pretty decent stick with the taildraggers in DCS (because they are so well made they really do feel as real as it gets)

 

HOWEVER when I climbed into my DCS Spit on Friday, I bent up or wrecked two aircraft just trying to get off the ground, do a pattern and land safely. See my third try below, but you can see how ridiculously difficult the shopping cart tail wheel is making life - there's no way real pilots ever agreed to fly a plane multiple times a day (and in combat) with a tailwheel like this, its just not possible lol.

 

I've never logged time in anything close to a Spitfire and I'm no expert on it but I agree with these guys above, the tailwheel free castering physics are COMPLETELY WRONG, I can hold the stick back all day and it still spins around LoL

 

 

[YOUTUBE]Gcb2qlk2UM[/YOUTUBE]

Edited December 20, 2016 by Schwarzfeld

[Schwarzfeld]

Bump?

 

I've checked out in real life in taildraggers flying real airplanes and there's no way this tailwheel shopping cart physics thing is correct here... Bueller? LoL

[NightRush]

I probably should have mention that I am an aviation engineer, and have experience taxiing and operating Spitfires on the ground

[Reflected]

Brilliant post, NightRush!

[NineLine]

Once the aircraft is at a walking pace, brakes should only be required to turn the aircraft, not keep it straight (in a zero wind scenario)

 

I will digest the rest of your post later, on this comment, you don't really need the brakes to keep it straight?

 

[jocko417]

Brilliant post, NightRush!

 

Yes. Very interesting, thanks!

Edited January 14, 2017 by jocko417

[NightRush]

I will digest the rest of your post later, on this comment, you don't really need the brakes to keep it straight?

 

 

Hey Sith

 

Im not talking about in the takeoff run, but taxiing the aircraft. In the takeoff prop blast seems to be taking effect fine and loading up the tailwheel, and it behaves very reasonably.

 

The times when the tailwheel is acting like a shopping cart, is at taxi and slow manoeuvring speeds.

 

The other time is at landing when there is no power, instead of the tail-wheel wanting to centre at touchdown speeds it has a tendancy to caster instead of track the aircraft. Of course here the tailwheel should caster if there is any side load due to not landing straight and the tail starts to swing.

 

Example, imagine the tailwheel had been spun around in flight and is facing 90 degrees to the fuselage centreline. When it touches down the tail wheel should immediately centre to the direction of the aircrafts ground track. (In real life it will shimmy, and the tail wheel will eventually dampen out)

 

However what feels like is happening in DCS, is the tailwheel from this position would continue to divert and have no tendency to track the centreline/ground track of the aircraft at touch down

[Duck21]

+1

[Art-J]

About the boost control, it's an interesting subject, as it's been modelled the same way in the Mustang, obviously. Maybe one could search through post 2012 threads to find why it works the way it works in DCS. But it's certainly intended behaviour - here's recent Yo-Yo's take on it:

https://forums.eagle.ru/showpost.php?p=3007237&postcount=7

 

We do have an operational Auto Boost Control in DCS for full throttle, otherwise we would be blowing up engines at sea level all the time with 20+ PSI. Partial throttle operation seems to be more complicated matter, though. Later, I'll try to digest this nice write-up on the subject, to try understand it better.

http://www.enginehistory.org/Piston/Rolls-Royce/R-RmerlinABC/R-RmerlinABC.shtml

[9.JG27 DavidRed]

Hey Sith

 

Im not talking about in the takeoff run, but taxiing the aircraft. In the takeoff prop blast seems to be taking effect fine and loading up the tailwheel, and it behaves very reasonably.

 

The times when the tailwheel is acting like a shopping cart, is at taxi and slow manoeuvring speeds.

 

The other time is at landing when there is no power, instead of the tail-wheel wanting to centre at touchdown speeds it has a tendancy to caster instead of track the aircraft. Of course here the tailwheel should caster if there is any side load due to not landing straight and the tail starts to swing.

 

Example, imagine the tailwheel had been spun around in flight and is facing 90 degrees to the fuselage centreline. When it touches down the tail wheel should immediately centre to the direction of the aircrafts ground track. (In real life it will shimmy, and the tail wheel will eventually dampen out)

 

However what feels like is happening in DCS, is the tailwheel from this position would continue to divert and have no tendency to track the centreline/ground track of the aircraft at touch down

 

yes i noticed that too...the tailwheel tends to act strange sometimes...would be worth to have a look at the 109 as well, as it behaves similar.

[Badger633]

+1 can taxi take off and land but the aircraft seems most eratic when power is back after landing at walking pace very easy to come off runway at walking pace.

[mkiii]

The FMOptions.lua for the Spit, 109 and Mustang (first 40 lines or so), bear some looking at, It seems clear that there has been some cutting and pasting of default figures. So I decided to have a fiddle,

 

The ones that look interesting to me are

tailGearYawDamperK (Not sure, but increasing this seemed to help with shimmy)

tailGearSpringForceFactor (I tried reducing this to 12,000,000 & seemed less bouncy on the tailwheel )

mainGearSpringForceFactor (I tried halving this to 31,000,000 & was less bouncy on the main gear, and less likey to veer off the runway. The original setting is the same for Spit and 109, but much lower on the P-51D)

 

TBH, I'm more convinced that the damping is off rather than the forces involved, not that I have any data to support that, but having worked on tail draggers, I never saw anything bounce as high as the tail of the Spit in DCS 3 times in a row. Maybe the Riggers have inflated the tyres and Oleos too much?

 

Anyone have a clue?

Edited May 14, 2017 by mkiii

[Xxx]

The FMOptions.lua for the Spit, 109 and Mustang (first 40 lines or so), bear some looking at, It seems clear that there has been some cutting and pasting of default figures. So I decided to have a fiddle,

 

The ones that look interesting to me are

tailGearYawDamperK (Not sure, but increasing this seemed to help with shimmy)

tailGearSpringForceFactor (I tried reducing this to 12000000 & was less bouncy)

mainGearSpringForceFactor (This is the same for Spit and 109, but much lower on the P-51D)

 

TBH, I'm more convinced that the damping is off rather than the forces involved, not that I have any data to support that, but having worked on tail draggers, I never saw anything bounce as high as the tail of the Spit in DCS. Maybe the Riggers have inflated the tyres and Oleos too much?

 

Anyone have a clue?

 

 

The castoring is definitely off in the spit and the me 109. Taxi in either aircraft is an awkward affair!

 

 

Looking at the Lua. The Mustang;

tailgear yawdamper=20

tailgearsprinforcefactor=9000000

maingearspringforcefactor=40000000 --5000000

 

 

109;

tailgear yawdamper=36

tailgearsprinforcefactor=24000000 --5590000

maingearspringforcefactor=6.2e6??? -- 109k

 

 

spit

tailgear yawdamper=36

tailgearsprinforcefactor=12000000

maingearspringforcefactor=6.2*1000000

 

 

FW190

tailgear yawdamper=20

tailgearsprinforcefactor=9000000

maingearspringforcefactor=4000000.....Looks like a Mustang???

 

 

Now just how to set these up I have no idea, given that each aircraft has different weights, CofG, power, etc...??

 

 

But anything which helps the taxi of the Spit and the 109 will help me suspend disbelief and help me fly the simulator!

 

 

I will have a conservative play.

 

 

Cheers.

[mkiii]

Having had a bit more of a play, These settings worked well for me, without completely removing twitchy behaviour. Taxying behaviour seems good, and landing/Takeoff involves a lot less bouncing from one wheel to another.

tailGearYawDamperK = 26.0 -- Original setting 36.0

tailGearSpringForceFactor = 10000000 -- Original 17000000

mainGearSpringForceFactor = 3.2 * 1000000 -- 6.2 * 1000000

mainGearBackDamperForceFactor = 70000 -- Original 10000.0

 

Lowering the MainGearSpringForceFactor seems to have the greatest affect, Allowing You to rev up & down on full brakes with gentle bobbing of the wings on the tyres rather than the wheels actually bouncing off the ground.

I have read a number of pilot accounts and test reports on Spits from the prototype to Griffon engine Marks, and none mentioned any digging in of the wings or massive ground looping on landing, in fact they say the opposite, and seemed to be implying that the handling was pretty tame, with only visibilty, and nosing over under harsh braking being noted as issues.

 

And I use these notes as a general guide:-

http://forum.keypublishing.com/showthread.php?72714-SUPERMARINE-SPITFIRE-Take-off-and-Landing-Guide

 

{EDIT2

tailGearStaticForce = 30000 -- Orig 3000 THIS Is the one that really does it for me. I tried this after reverting all the others back to their original settings. Note that this is 10x greater than the original, but still lower than the Mustang. Did a zero get left off this and the 109K?

/EDIT2}

Edited May 17, 2017 by mkiii
New info