Strange CG problem on low speed landings

[DaveRindner]

I wish I had saved a track from this issue, but here goes.

I was practicing short field landing in 10C, using those small strips in Georgia .

On one case, I had use up all of ammo, and had 60% fuel (internal). I approached the strip, and touch down at 105 knots, with high AOA of -20, and -5 VVS. Yes I know, I screwed up. On touch down, there was a small bounce, and loud thump. The aircraft remained at high AOA, and would not lower nosewheel, and wound up rolling dragging the tail, and came to rest in this attitude. Nose wheel up! How is this possible. That would mean that its CG had shifted aft. Did this happen because of empty ammunition drum? External stores at touchdown;

AIM9X2, ALQ-131, TGP.

[timc]

Sounds like you damaged your main landing gear. That'll cause the aircraft to sit back on its tail.

[roadrabbit]

On touch down, there was a small bounce, and loud thump. The aircraft remained at high AOA, and would not lower nosewheel, and wound up rolling dragging the tail, and came to rest in this attitude. Nose wheel up! How is this possible.

 

:cry: I think that if you had checked your landing gear you most likely had torn off both main gears - wheels and struts. The nose wheel was probably on the ground as normal. This would account for the nose high attitude, and discounts the suggestion of an aft shift of C of G. If there had been an aft shift of C of G to the extent you theorize, the aircraft would in all probability have been uncontrollable.

 

[Your approach path of 5 degs is really steep (as you know). Even the standard approach path of 3 degs would likely damage the landing gear if no flare was made. This latter approach angle would equate to a descent rate of 500 ft/min and the 5 deg path would equate to 840 ft/min, both at 105 kts airspeed. Another point to note is that a very late flare can momentarily increase the descent rate of the main gear due to the rotation to the flare attitude - this is known as "driving the wheels into the ground"!]

Edited August 23, 2013 by roadrabbit
Clarification

[IonicRipper]

Happened to me too once. Main landing gear was still there, nose high in the air with the front wheel off the tarmac. It dragged its tail to a halt. Im not sure what causes it but main gear damage seems likely.

[Golo]

Happened to me too, nose is indeed very light w/o any ammo so I trimm nose heavy very often. As for the speed you should be "On speed" - green donut shape symbol left od HUD - and VS less than 600 depending on weight.

 

I have been even bounced back up by the air itself during landing. I was very light with slightly more speed than optimal and after in ground effect kicked in I found myself actualy climbing past touch down point so I had to go around.

[timc]

If you land hard enough, the main gear kicks forward, ie the wheels end up further towards the nose than usual. You can see this in external view - the gear looks like its pointing forward. Normally the CG is in front of the position that the main gear touches the ground. The CG doesn't move but the rear wheels are then ahead of it after this issue which causes the pitch up. I've tried to find a photo but no joy.

[IonicRipper]

I wasn't able to reproduce the exact same thing but this screen should give you an idea what happens on hard landings...

[cichlidfan]

This sounds like normal 'hard-landing' damage to me. I must have a few dozen screens like that from when I first started flying the A-10.

[Purzel]

When you´re starting (with extreme weight/weaponload) and you "hop" on the ground after liftoff again while your gear is still retracting, you might have problems getting it out correctly when landing. Maybe that happens, did you hear a strange airflow-noise while flying?

If so, the covering of your gear was not closed completely and that would have caused the noise and your landing problem (gears broken and not able to extend...)

[-Pv-]

Yes, I have seen that same damage in the pic from poor landings myself. The F-16 in Falcon4 has a reputation for taxing landing skill, but this Warthog simulation is just a bit trickier. I watch the landing ball like a hawk and walk the throttle quite a bit

-Pv-

[kra961]

Someone correct me if I'm wrong here but at at 105 knots you're already in a stall and you compounded it by your high AOA, sounds to like you literately fell out of the sky and damaged your gear from the fall, that speed is doable but its has to be right at the moment of touch down flaring at the last moment before your wheels touch tricky at best.

Edited August 24, 2013 by kra961

[Golo]

Someone correct me if I'm wrong here but at at 105 knots you're already in a stall and you compounded it by your high AOA, sounds to like you literately fell out of the sky and damaged your gear from the fall, that speed is doable but its has to be right at the moment of touch down flaring at the last moment before your wheels touch tricky at best.

 

You may not be necessarily in a stall at 105kts depending on your weight and configuration. I use to do slow speed pass at 100-110kts only with MNV flaps without any stall.

But AOA is high and you would most likely be draging your tail on the runway if landing (happened to me even without any damage to the wheels) and afterwards its very difficult to push the nose down because in slow speed elevators are not effective.

[Evil.Bonsai]

Does slamming the wheel brakes not put the nose back down?

[Golo]

Does slamming the wheel brakes not put the nose back down?

 

I did not think of that at the time, but yea it should work.

[Hamblue]

I think it's a bug that doesn't show up very often.

I've had it happen twice in the past and posted it in the bug reports over 6 months ago. Last time it happened to me I bounced into the air and wound up floating at a pretty high altitude with no control.

 

I just make it a point to keep the speed over 120 and never had it happen again.

[pj]

I've had this happen also. Usually I notice the plane starts to pitch up after touchdown, pushing the stick forward immediately will put the nosewheel on the ground.

[Flamin_Squirrel]

The aircraft sitting on its tail is obviously caused by CG being aft of the wheels. However, landing in a nose high attitude will in itself result in CG rotating aft with respect to the wheels. I doubt this is a bug.

[roadrabbit]

The aircraft sitting on its tail is obviously caused by CG being aft of the wheels. However, landing in a nose high attitude will in itself result in CG rotating aft with respect to the wheels. I doubt this is a bug.

 

:huh: Er ..... not necessarily. If the main gears were wiped off without damage to the nose-wheel, the aircraft would still "sit on its tail".

 

To rotate the C of G aft with respect to the wheels at any point would require the aircraft to be in an incredibly nose high attitude - and any touchdown attitude (a/c attitude to runway, NOT to do with angle of attack) greater than 16 degrees would result in the tail striking the ground (actually the bottom of the tail fins) first. The main gear would also have the wheel struts pointing forward by the same angle, and as has been shown earlier, this could also result in the main gears collapsing forward. This high attitude would be unsustainable and therefore only momentary.

 

I have reproduced the 5 degree runway approach slope previously described, with gear down and full flaps, speedbrakes set to 40 degrees. The lowest stable speed attained was 105 knots. I had 25% fuel loaded and no weapons loaded. An earlier flare than normal resulted in a smooth touchdown with no bounce. At Sochi I stopped less than halfway down the runway.

 

Whilst all this is very interesting, wouldn't it be more practicable to just fly the aeroplane as intended? :music_whistling:

 

When in doubt - go around!

[Flamin_Squirrel]

:huh: Er ..... not necessarily. If the main gears were wiped off without damage to the nose-wheel, the aircraft would still "sit on its tail".

 

To rotate the C of G aft with respect to the wheels at any point would require the aircraft to be in an incredibly nose high attitude - and any touchdown attitude (a/c attitude to runway, NOT to do with angle of attack) greater than 16 degrees would result in the tail striking the ground (actually the bottom of the tail fins) first. The main gear would also have the wheel struts pointing forward by the same angle, and as has been shown earlier, this could also result in the main gears collapsing forward. This high attitude would be unsustainable and therefore only momentary.

 

I have reproduced the 5 degree runway approach slope previously described, with gear down and full flaps, speedbrakes set to 40 degrees. The lowest stable speed attained was 105 knots. I had 25% fuel loaded and no weapons loaded. An earlier flare than normal resulted in a smooth touchdown with no bounce. At Sochi I stopped less than halfway down the runway.

 

Whilst all this is very interesting, wouldn't it be more practicable to just fly the aeroplane as intended? :music_whistling:

 

When in doubt - go around!

 

What are you on about? The question was whether it was a bug if the plane was able to sit on its tail, gear intact. I made the point that if the pitch attitude is high on the ground, it may rotate the CG back, moving it aft of the wheels (as you say, not AoA, in relation to the runway).

 

At any rate, I tested it, and it was as I suspected:

 

 

I used the gun to push me back then slammed on the brakes. Gear remains intact.

[roadrabbit]

What are you on about? The question was whether it was a bug if the plane was able to sit on its tail, gear intact. I made the point that if the pitch attitude is high on the ground, it may rotate the CG back, moving it aft of the wheels (as you say, not AoA, in relation to the runway).

 

Er ..... I'm not trying to cross swords, but isn't the situation you describe a dynamic one? You say you had the plane moving backwards and slammed the brakes on. It could then be inertia tipping the plane on to its tail. Once stopped did the plane stay on its tail, or did it tip back level?

 

As I said it would take an attitude change of 16 degrees nose up for the tail to touch the ground - even so it would be strange if the static C of G could position behind the wheels. IMHO it would probably be a bug if it didn't tip back level.

 

The rest of my previous was exploring the dynamic situation of the landing from a steep approach described by the starter of the thread. Sometimes replies do get detached from the questions posed :smilewink:

[Flamin_Squirrel]

I'm not trying to cross swords, but isn't the situation you describe a dynamic one? You say you had the plane moving backwards and slammed the brakes on. It could then be inertia tipping the plane on to its tail. Once stopped did the plane stay on its tail, or did it tip back level?

 

As I said it would take an attitude change of 16 degrees nose up for the tail to touch the ground - even so it would be strange if the static C of G could position behind the wheels. IMHO it would probably be a bug if it didn't tip back level.

 

The rest of my previous was exploring the dynamic situation of the landing from a steep approach described by the starter of the thread. Sometimes replies do get detached from the questions posed :smilewink:

 

No problem. Once it was on its tail it stayed there.

 

Come to think of it, I'm sure I've seen a picture of a real hog on its tail after some harsh weather. I'll have to see if I can find it.

[roadrabbit]

No problem. Once it was on its tail it stayed there.

 

Come to think of it, I'm sure I've seen a picture of a real hog on its tail after some harsh weather. I'll have to see if I can find it.

 

:thumbup: I would be very interested to see it! My aerodynamics is maybe a bit rusty but I recall that the C of G is always forward of the main wing centre of lift (except in some 'fly-by-wire computer controlled flight bodies such as unmanned missiles). This centre of lift is at about 1/3 of the main wing mean chord, so on a straight wing design like the Warthog you can think of it as 1/3 the distance back from the wing's leading edge. [Counter-intuitively the tail-plane provides negative lift. This arrangement provides stability in flight where a small displacement in pitch results in a pitch couple (rotating force) that counters the small displacement and restores the aircraft pitch attitude to that existing prior to displacement. If this was not so, and the C of G was behind the main wing centre of lift, the displacement would result in a pitch couple that increased that displacement. This would make such an aircraft impossible to fly by a human pilot.]

 

So, in the real world, the C of G for a Warthog should be forward of the 1/3 mean chord. Even if sitting static on its tail the C of G should therefore still be ahead of the wheels if the main gear was undamaged (from looking at side views of the aircraft).

 

In another life I remember a Boeing 707 freighter sitting on its tail as the groundcrew hadn't put the tail prop in place prior to unloading from the forward freight door. As pallets were unloaded the C of G moved backwards until aft of the main wheels and embarrassment all round. Because of this possibility crew were always very aware of C of G position! :megalol:

[NoCarrier]

The F-16 in Falcon4 has a reputation for taxing landing skill, but this Warthog simulation is just a bit trickier.

I can't think of any other (quasi-)modern jet sim out there with jets that are even easier to land than the F-16 in Falcon 4.0. Green donut on the AOA indexer, 3° glideslope, FPM on the touchdown point (and on the AOA staple), and don't forget to flare. Bob's your uncle.

 

Same goes for the A-10C, save that the HUD doesn't show an AOA staple when the gear's down.

[Davros23]

Green donut on the AOA indexer, 3° glideslope, FPM on the touchdown point (and on the AOA staple), and don't forget to flare.

 

2.5° glideslope I think you'll find :) </pedant>

[WildBillKelsoe]

you should try rearming and firing the gun in that stance! :D