AJS37 and Acceleration

[outbaxx]

Did some testing with the acceleration of the AJS37,

Clean AC zone3

ISA H0 M0.55-M1.1 =45sec

ISA H6 M0.55-M1.5 (maxed @M1.46) =1min 30sec

 

ISA+10 H0 M0.55-M1.1 =55sec

ISA-30 H0 M0.55-M1.1 = 37sec

Full military

ISA+10 H0 M0.55-M0.84 = 3min

 

My tests were within seconds of what the actual charts show (a few seconds on the slow side,factor could be ME).

 

VERY GOOD JOB HERE HEATBLUR!!

 

Acceleration 0-M0.55 ISA clean AC was a bit slow though, 40sec

Maybe it has something to do with the strange take off behavior that is currently there:

-not possible to rotate before 250kmph

-when rotating I need to push forward when the FPM meet the horizon line or else it will over rotate and α18... it seems like there is a very narrow scope between α12 &α18, where the smallest input takes you fast between these values.

[RagnarDa]

Nice and thanks!

 

I believe the late rotation might have to be related to, again, the suspension. In DCS the suspension needs to be pretty stiff to handle the hard landings Viggen does. To rotate an aircraft with main landing gears behind the aerodynamic center the landing gears needs to be able to contract a little to create a little alpha while the aircraft still has all wheels on the ground. If not the aircraft needs to accelerate to enough high speed to be able to fly at 0 alpha. So again I believe it’s a tuning issue.

[outbaxx]

Yes it probably is, but I would take a look at the aerodynamics in the α12-α18 region as well.

The documentation indicate that the ac should be “free from disturbance” between α10-~α25, but I get some disturbance at α12 when approaching to land, it start to shake sort of.

Can’t say it’s wrong, just a bit surprised that it’s so sensitive in that regime.

[RagnarDa]

Are you sure? Iirc the docs I had buffets could start being felt at 10 deg. There are a lot of ways to check high alpha/low speed, for example isn’t there a landing conf chart?

[outbaxx]

Are you sure? Iirc the docs I had buffets could start being felt at 10 deg. There are a lot of ways to check high alpha/low speed, for example isn’t there a landing conf chart?

 

 

Sure and sure, no not sure :) but it says in the Aerodynamical compendium that there is no disturbance in length stability between α12-~α25, I see now that there is a mention about the shake that occur with increase in alpha, but it also say that’s too small to surely warn about high alpha.

And that’s why I thought the AJS is a bit itchy in the 12-18 regime, specially when taking off, the nose is doing a pitch up that I don’t expect.

It could be just me of course, but it seem to react very high to small inputs there.

I’ll do some more take offs and landings to see if I can pin point what I think is strange and I’ll get back here :)

[RagnarDa]

If you do, do me a favor and try disengaging the SPAK and see what happens then.

[outbaxx]

If you do, do me a favor and try disengaging the SPAK and see what happens then.

 

 

Ok, did some landings, both in SPAK and GSA.

I will write what I experienced but I can’t say this is right or wrong.

SPAK:

-@ α12 and throttle down, the ac started to pitch up when closing to α18.

- Below 350kmph with gears down, if I increase speed the ac attitude/pitch decrease.

-Above 350 with gears down, increase speed = attitude/pitch increase.

GSA:

-Below 350 with gears down, increase speed = increase attitude/pitch (opposite of SPAK).

 

I experienced that in GSA it was easier to control the α, it didn’t change as quick as in SPAK.

(Roll in GSA is another story, it didn’t make sense at all to me but I’m not a real pilot so it could just be me there :) )

 

Start in GSA needs heavy trimming nose up, the ac seems to be very very heavy on the nose.

 

When doing a normal take off It looks like that the suspension of the nose wheel is fully extended at about 200kmph but you still can’t rotate until 250kmph, but that’s perhaps what you talked about an issue with ground handling.

[outbaxx]

Probably just me “feeling” stuff, updated today and tested, couldn’t reproduce the “weird” when doing landing approaches.

 

Start issue is still there though ;)

 

One other thing I have noticed is when you pull G’s there is something between M0.7 & M0.6 that makes the G increase.

What I’ve done is this:

Accelerating to M1.0 and throttle down roll 90 deg and pull 6G, keeping the stick still. Around 0.9-ish there is an increase but that is normal due to the trim as I understand. Still keeping the stick at the same position the G’s decreases but there is an increase somewhere between 0.7&0.6 and then it continue to decrease again.

Why is there an increase in M0.7-0.6 (maybe just me here too..) ?

[TOViper]

Just a side note about performance.

Yesterday I did a lot of Viggen sorties, and what I noticed is this:

 

.) Take-off with mil-thrust is now easily possible

.) The aircraft accelerates better until reaching 300km/h on ground (and until 400km/h during lift-off)

.) It "feels" better during take-off rotation and gains speed after lift-off

.) It lifts off like real Viggens in the videos that are around on Youtube

 

Payload was 100% internal fuel + XT + 4*Rb75 at an elevation of 1000m and ISA+7.

 

Edit: ALL I WROTE IS WRONG. This all is gone with the newest update. Don't know what happend, investigating.

Edited December 21, 2019 by TOViper
after updating to 2.5.5.41371

[TOViper]

....Still keeping the stick at the same position the G’s decreases but there is an increase somewhere between 0.7&0.6 and then it continue to decrease again.

Why is there an increase in M0.7-0.6 (maybe just me here too..) ?

 

 

This caused me headaches too since months ... I experienced the same and asked myself: where the hell does this come from?

[Airhunter]

So the FM was changed, huh? I need to test this.

[TLTeo]

This caused me headaches too since months ... I experienced the same and asked myself: where the hell does this come from?

 

 

It sounds like it's related to the pitch gearing system?

[TOViper]

Did some test flights today with the current openbeta 2.5.5.41371.

 

After testing a lot now with the mission I have flown with the update before the last one, I come to the conclusion that all I said is now wrong.

It seems the aircraft is again very slow in acceleration on ground, and I have the impression it is even worse than before.

 

After testing 2.5.5.41318 I got the same results as in 41371. I don't know what is going on here on my machine. Yesterday night I was flying the same mission as always in PG, and did some Rb75 runs against tanks. At a certain point I noticed the Viggen accelerating like a rocket on ground, and I swear take-offs have never been so efficient and easy like last night.

 

HELP :surrender:

Edited December 21, 2019 by TOViper

[outbaxx]

I don’t know how the flight model is set up but if I disengage SPAK and fly In GS ( basic steering sort of, you steer the AC with the stick and rudder, the servos move according to your stick movement and there is no dampening) the aircraft make no sense to me, it is most clear when using roll, if I do stick right and let go the ac start to wiggle like it’s on a swing or something.

 

This is just my opinions of course, I can’t say it should be this or that but i don’t understand how the difference between SPAK and GS can be so big.

In SPAK you get dampening in the axis and the use of the “force sensor” in the stick to provide more precise stick input.

 

But if SPAK has to deal with what we have in GS it sort of makes sense why it behaves so weird in pitch @take off, in GS you need to trim a lot of nose up just to lift the nose, SPAK seems to counter this to a degree ( one can see this if you disengage SPAK in air, the ac will dive for some reason ) I don’t understand the difference in trim needed between the two?

 

Perhaps the problem is in GS ?

 

Can’t say it’s wrong, just doesn’t make sense to me that the difference in behavior is so big.

[Skyracer]

GS?

[outbaxx]

GS?

 

 

 

Yeah, the basic control system of the aircraft. Referenced as GSA in the manuals.

[Skyracer]

Vad heter det på svenska?

[outbaxx]

Vad heter det på svenska?

 

 

Grundstyrsystem or GSA both Swedish :)

[theOden]

Eller Grund Styr Automat

för oss som gillar sär skrivning.

 

 

For our swedish viewers I do recommend:

https://chefsingenjoren.blogspot.com/2011/02/i-kanten-av-envelopen.html

[outbaxx]

I feel the need to clarify where I think it’s something weird with GSA.

It’s between 250-400kmph with gear down.

You have great roll authority but with a weird swinging lag. If I do gear up it’s almost impossible to do a complete roll. But with gear down it’s very easy.

It’s also with gear down and below 400kmph where you notice the nose drop when disengaging SPAK.

 

-This is just my personal opinion and I can’t say it’s wrong.

 

-the thing that bothers me is the take off. The way the ac behave before rotation and just after rotation.

If I look in outside view I can see that the ac gets lift by looking at the front wheel. There’s no weight on the front wheel when accelerating @200kmph, but the ac won’t lift it’s nose even with stick input.

I can see it within the cockpit too, the nose raises and the “speedbar” in the HUD almost disappear as the nose lift.

And when it’s possible to rotate I find it very hard to do a controlled rotation.

It’s like it’s accumulated all the lif during acceleration and it’s releasing it when you rotate :)

[TOViper]

-the thing that bothers me is the take off. The way the ac behave before rotation and just after rotation. If I look in outside view I can see that the ac gets lift by looking at the front wheel. There’s no weight on the front wheel when accelerating @200kmph, but the ac won’t lift it’s nose even with stick input. I can see it within the cockpit too, the nose raises and the “speedbar” in the HUD almost disappear as the nose lift.

And when it’s possible to rotate I find it very hard to do a controlled rotation.

It’s like it’s accumulated all the lif during acceleration and it’s releasing it when you rotate :)

 

OK, I now got the point more clearly you are talking about. Thanks for clarification!

Following this, I have to admit that I was talking about another story.

 

Regarding your story I can add the following: For a good and solid rotation, I had to learn to really wait for the time-distance line in the HUD to reach its markers. Then everything goes well.

But take-offs aren't sometimes so easy: Hot & high conditions + heavy payload will require more speed, even in the area of Vmaxtyre (320 kmh). This relates to runways in Persian Gulf and Nevada especially.

So the time window for rotation is very small, and I found myself thus overrotating a few times, leading to high pitch warning.

 

 

My story is that the Viggen is still not reliable for take-offs with mil thrust.

I will cover that in a separate thread if I find time to do so.

Edited December 23, 2019 by TOViper

[outbaxx]

The thing is that even with max weight @30C, I should be able to rotate at 235kmph.

But my concern has to do with empty aircraft, 100% fuel, @15C I should be able to rotate at 180kmph.

Zone3

[TOViper]

The thing is that even with max weight @30C, I should be able to rotate at 235kmph.

But my concern has to do with empty aircraft, 100% fuel, @15C I should be able to rotate at 180kmph.

Zone3

 

OT:

Not meant to destroy your concerns (in fact I really understand), and all you did so far to bring light into darkness ... but ...

HB recently has implemented the pilot body ... c.g. ... mass ... maybe a little decimal shift somewhere in the code :D

[outbaxx]

OT:

Not meant to destroy your concerns (in fact I really understand), and all you did so far to bring light into darkness ... but ...

HB recently has implemented the pilot body ... c.g. ... mass ... maybe a little decimal shift somewhere in the code :D

 

 

 

No worries :)

Just give me a bigger “rotation window” :)

[TOViper]

No worries :)

Just give me a bigger “rotation window” :)

 

O(ff)T:

No problem, in this case you might wanna take a look here:

http://www.roundwindowcompany.com

 

O(n)T:

I will do some tests right now in LAR airbase at standard conditions.

This will take some hours. I will record some situations, and maybe there is something worth to share. :joystick: