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Is aerobraking really that hard?


Nealius

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After watching a few real life videos of F-5s landing, some planting pretty hard and others aerobraking, I can't help but feel that the DCS F-5 has sticky tires.

 

Here's a Taiwanese F-5 aerobraking for a good 10 seconds, if not more:

 

Now, with the same payload of a center tank and only 1,000lbs of fuel, I can barely manage to keep the nose up more than 5-6 seconds, and that's with almost max physical stick deflection on my Warthog.

 

Is this accurate?

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Trim doesn't make any difference with the stick fully aft.

 

Landing without any external loads I can keep the nose up until approx 115kts.

 

Just tried with a full center tank (which strangly moves the CG aft) I can keep the nose up until 105kts. At 160kts approach speed that's ~15sec of aerobraking.

 

An important item concering aerobraking is the touchdown speed. Most pilots don't fly on speed but slightly fast, so that the doughnut and the chevron are visible.

 

But the DCS F-5 has a strong tendency to drop the nose at mainwheel touchdown, something that isn't the case in the real F-5. Since the center tank moves the CG aft, it might be possible that the base CG is a tad too far forward.


Edited by bbrz

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A lot of this depends on how you're flying the approach. If you're doing a 3-degree descent at on-speed AOA, then you don't have a whole lot of extra energy to hold the nose up. That said, it's not an F/A-18, nor is it meant to be flown like one.

 

Mover, in his recent F-5 video, says the final approach is shallower than what most aircraft fly, and flies almost the entire pattern at amber chevron (fast), without so much as the green donut showing until the crack-shift-idle-flare at the threshold.

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I fly my approaches at around 160kts, showing donut+amber chevron on my AoA indexer. Flare a little to touch-down with the AoA indexer indicating on-speed, sometimes donut+red chevron right before the rubber hits. Full aft stick, nose starts to drop around 115-120kts and hits the tarmac around 100kts, giving me roughly 6 seconds of aerobrake time.

 

Weird thing is that my AoA indexer is showing on-speed everywhere between 130kts and 155kts or so...that's quite a wide margin. Am I just touching down too slow?

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If you want to fly exactly on speed, you should refer to the AoA gauge since it has a much finer resolution.

 

Using the AoA gauge I just found out that the AoA indexer gauge is incorrectly calibrated. The 'on speed' indication is 4-5kts too slow which is considerable, especially in this low speed regime.

 

With the center tank and 10% internal fuel (12200lbs), 'on speed' is correctly 142-152kts on the AoA gauge, but only 137-148 on the indexer.

 

Since the ground effect on the DCS F-5 is rather pronounced, it's easy to float and hence touch down too slow. (Too slow only in relationship to your aerobraking timing).

 

According to the -1 with an estimated 18% CG, 'on speed' is 148kts and touchdown speed 138kts.

 

Personally I love the ground effect and to hold her off as long as possible.

 

The wide 'on speed' margin you are seeing is related to the G-load. As soon as you apply a very small amount on aft stick, G load, AoA and hence 'on speed' increase. If you relax the stick pressure, the opposite occurs.


Edited by bbrz

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Trim doesn't make any difference with the stick fully aft.

 

... (which strangly moves the CG aft)...Since the center tank moves the CG aft...

 

An important item concering aerobraking is the touchdown speed. Most pilots don't fly on speed but slightly fast, so that the doughnut and the chevron are visible.

 

But the DCS F-5 has a strong tendency to drop the nose at mainwheel touchdown,

 

Of course trim does not change full aft stick position. That is not the point. The point is what you mentioned after, touchdown speed. So, trim on-speed. If you come in fast the nose drop will be more noticeable. It is much easier to keep the nose up if you are trimmed on-speed.

 

Curious why you insist the centerline tank brings the CG aft? Is the game bugged in that respect?

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Curious why you insist the centerline tank brings the CG aft? Is the game bugged in that respect?

I don't insist, it was just an observation. When I tested without the center tank (1100lbs lower grossweight), I couldn't keep the nose up as long as with the center tank, hence the CG seems to move aft.

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I have my own observations now about the flight modeling or the input modeling in various aircrafts when you are in the landing phase.

 

There was a thread about Mig-29 landing and observation how in real aircraft the pilot is waving the stick around a lot with big inputs and aircraft didn't perform much movement at all, meaning that in landing you need to do big deflections to get the aircraft react to them. Similar thing can be seen in Su-27 etc where big movements can be done and the aircraft doesn't react to them with exact movements but is more like a cruise ship that you are flying, meaning you start to perform the control and aircraft will react to it much later.

 

Now with VKB extension length of stick and no curves etc (that you shouldn't be required to have at all!) the aircrafts react to all inputs extremely quickly and it makes the aircrafts feel more like a papercrafts.

 

It is just very difficult, if not nearly impossible to really explain that feeling but in many ways it just feels that there is these inertial movements lacking or the air flow effect being somewhat off.

 

Sometimes it is just extremely difficult to get any aircraft to stop even with air brakes, engine in idle etc while coming to final, and sometimes it is like you need to give more throttle to get it keep speed up.

 

And when it comes these landings with front wheel up, I think I have never managed to get a such controlled nose as in the videos is visible. It usually ends up either slamming nose on ground or then lose lift too quickly.

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Don't know how long the extension is, but note that IRL you need considerable force to move the stick in any direction, plus there's an artificial feel unit.

Furthermore the F-5 is a paper airplane when compared to a MiG-29, let alone a Su-27!

No problem with landing the nosewheel after aerobraking.

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Now with VKB extension length of stick and no curves etc (that you shouldn't be required to have at all!)

 

For my own setup (Gunfighter w/ 200mm extension and MCG Pro grip) I *still* use curves in order to get a more realistic feel of the stick response.

 

Also, the attached guide helped on various DCS modules to improve the overall feeling on the controls (more heavy).

706938849_DCSrealisticstickcurves.thumb.jpg.43850750b958b98b98090bc8dec028ff.jpg

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VKBNA_LOGO_SM.png

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the green donut showing until the crack-shift-idle-flare at the threshold.

 

 

Off-topic Newb question:

 

 

I've heard crack-shift-idle-flare used before. Idle-flare I'm familiar with, I think, but for the sake of clarity, just what does all this mean?

 

 

We now return you to your regularly scheduled adult conversation.

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crack: reduce the throttles approximately 1" when 1000ft from the threshold.

shift: 750ft from the threshold shift the viewpoint to the far end of the runway.

idle: 500ft from the threshold.

 

This technique is being used e.g. on the T-38C and I don't think it can be used without modifying since the speed difference between approach and touchdown is considerable greater in the T-38C. 10kts in the F-5, 20kts in the T-38.

 

There are a number of techniques for the T-38, like e.g. the 'latch to threshold' technique where you start a continuous thrust reduction 750ft from the threshold to reach idle at the threshold.

'pull to idle' at 500ft from the threshold.

 

All these techniques must be adapted to the different glideslope angles (2.5°-3.25°)

 

Important note; All these techniques make use of the flight path marker on the HUD, something neither the F-5 nor the T-38A have.


Edited by bbrz

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I don’t really have anything to add to this convo other than to say aerobraking the F-5 is extremely satisfying. Not quite as satisfying as with the Mirage perhaps but still pretty good.

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One other observation:

 

From the cockpit perspective, controlling the nose looks very difficult and messy. For example the nose-down moment that needs to be corrected when the mains touch down, followed by the unavoidable overcorrection when you manage to keep the nose up.

 

But upon watching a replay track from an external view, the nose dip/overcorrection is not noticeable at all. Still unable to hold the nose up for more than 5-6 seconds, though.

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For example the nose-down moment that needs to be corrected when the mains touch down, followed by the unavoidable overcorrection when you manage to keep the nose up.

That's a bit of a problem but it's not that bad. According to the manual the nose should drop onto the runway within 3sec without any pitch input. It takes around 1-2sec for me.

The higher the pitch attitude on touchdown, the lower the pitch down moment.

Even more important, the higher the sink rate, the greater the pitch down moment. You really have to grease her on to avoid the ugly pitch oscillations.

 

If you pull the nose up to the 12° limit, (the lower edge of the AoA indexer with the default seat height) the nosewheel will hit the runway at 90kts.

Note that the stupid attitude indicator bug is still there, and after the nosewheel touchdown the indicated pitch attitude will be between -2.5° and -5° so it's defintely not reliable.

 

Last but not least the stick position is an important item.

If you happen to relax the back pressure just before or at touchdown the pitch down moment will be very noticeable and you need a lot of back pressure to correct it, with a very likely overcompensation.

 

The nose down moment will be minimal if you are still continuing to increase the back pressure as you touchdown.

 

Hope this helps. As Deano87 wrote, It's really satisfying so it would be great if it would work for you.


Edited by bbrz

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Don't they have Brake chutes? I never saw an F-5 land and use anything other than it's chute - no aerobraking at all that I can recall. These were NF-5s in the 80s admittedly, but the runway was not short (RAF Bruggen), so I assume they could have managed without chutes, but every one that landed needed a chute pack to be fitted.

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  • 4 months later...

I'm not so sure that full aft stabilator trim doesn't give you more slab movement. The trim moves the slab, not a trim tab.

 

That said, this flight model (accurate or not) is so nose heavy on landing that it does seem pointless to aerobrake for a short bit and then have the nose fall uncontrollably


Edited by Stratozombie
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I'm not so sure that full aft stabilator trim doesn't give you more slab movement.The trim moves the slab, not a trim tab

The stabilator has a fixed up/down limit which can only be utilized with the stick. With the trim you can move the stabilator only within this range.

A difference in elevator effectiveness due to trim occurs only with an adjustable stabilizer.

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I know there are limits to up and down stabilizer travel, but I wondered if the stick itself would still reach the full nose up limit if it were mis-trimmed to full nose down trim. Do the hydraulic inputs move along with the feel and centering unit or do they remain fixed. I think they must remain fixed or the stab would not trim with movement of the feel/centering unit.

 

The reported cause of a T-38 crash back in the mid 80's was that during a dive recovery either the student or the instructor had grabbed the stick in such as way to inadvertently run the trim all the way forward, causing the instructor to believe they had insufficient elevator control to recover from the dive, and they ejected at high speed. The investigation did determine that even with the mis-trim the aircraft was recoverable with higher than normal stick force, but I don't recall if it stated that stab travel to the limit was still available. (The instructor strongly disagreed with the findings, but of course the mishap ruined his reputation.)

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To answer the question, aerobraking is definitely a little bit of an art in this jet. If you look at the video I posted in the "Looking for critiques of my landing pattern" thread, you can see that it is entirely possible to aerobrake the F-5E until the nose comes down at around 100 knots.

 

https://www.youtube.com/watch?v=BplENaeyWfk

 

I've found that the key, other than a properly trimmed jet and transition to flare, is to not milk the flare searching for that "butter the bread" landing. If you get slower than 10 knots below final approach speed (whatever was holding on-speed on final), you'll be almost out of aft longitudinal control margin and unable to hold the nose off for more than maybe 1 second.

 

If you end up flaring too high or too long and want to aerobrake the jet, you may end up having to relax the stick to kill some lift and accept a harder landing. Or you can waveoff and try again.


Edited by Chuck_Henry
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