New Fulcrum PFM is very susceptible to PIO and excessive bounce. - Page 15 - ED Forums
 


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Old 06-22-2019, 06:07 PM   #141
HWasp
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I've done a little research, and this is what I have found:


Fuel: approx 800kg


1. Flying out of ground effect straight and level with gear and flaps and with speed of 280ish km/h AoA is approx. 10 degrees. For elevator deflection see the screenshots.


2. Flying IN ground effect straight and level with the same config with speed of 280 (274) km/h AoA is the sameat 10 degrees. Elevator deflection is very different, as you can see on the screenshot.


This was not during a flare, but straight and level flight at that altitude with time slowed to 1/4


What this shows me is that, since you can fly the same parameters with the same indicated AoA there (and pitch is the same), the is currently no inverse ground effect trying to suck me in the runway, but also not much the other way around (of course this whole thing is quite inacurrate on my part)


On the other hand, there really is a strong pitch down tendency there as you can see with the increased elevator input.
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Old 06-22-2019, 06:30 PM   #142
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"
When the first F-18 fighter […] was flight tested at Patuxent River, it became evident that the airplane would not rotate at the predicted speed. This made the field performance of the airplane unacceptable. The problem was traced to an error in the calculation of aerodynamic forces in ground effect. This is particularly severe in case of a low placed horizontal stabilizer. As a result there was insufficient down-load capability to effect early rotation during the takeoff ground roll.
The problem was fixed by toe-in of the rudders. A squat-switch on the main gear biasses the rudders to deflect inward while on the ground. This creates enough positive pressure over the aft fuselage to effect early rotation.
This fix, although impressive, came at a price. All flight control software had to be revalidated. Also, the squat-switches represented additional system complexity. "


https://aviation.stackexchange.com/q...ff/35867#35867


This is about the F-18, I know, but I think this is connected, the ground effect reducing elevator effectiveness, creating nose down moment.

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Old 06-22-2019, 06:48 PM   #143
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Also in slow motion (1/4) during a mil-thrust take off keeping the exact same elevator deflection, that is just enough to get me off the ground gently, and producing a near 1,0 G path in ground effect, produces around 1,7 G pitch up after the plane climbs above 20m AGL (pitch up starts to increase around 10m)

It is interesting to play around with it in slow-mo.
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Old 06-22-2019, 07:01 PM   #144
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Interesting observation since this is what I initially thought was going on in the F/A-18 as well (before doing the ground effect tests)

https://forums.eagle.ru/showpost.php...3&postcount=97
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Old 06-22-2019, 07:28 PM   #145
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Maybe these are separate issues, who knows...
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Old 06-22-2019, 09:50 PM   #146
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Quote:
Originally Posted by HWasp View Post
"
When the first F-18 fighter […] was flight tested at Patuxent River, it became evident that the airplane would not rotate at the predicted speed. This made the field performance of the airplane unacceptable. The problem was traced to an error in the calculation of aerodynamic forces in ground effect. This is particularly severe in case of a low placed horizontal stabilizer. As a result there was insufficient down-load capability to effect early rotation during the takeoff ground roll.
The problem was fixed by toe-in of the rudders. A squat-switch on the main gear biasses the rudders to deflect inward while on the ground. This creates enough positive pressure over the aft fuselage to effect early rotation.
This fix, although impressive, came at a price. All flight control software had to be revalidated. Also, the squat-switches represented additional system complexity. "


https://aviation.stackexchange.com/q...ff/35867#35867


This is about the F-18, I know, but I think this is connected, the ground effect reducing elevator effectiveness, creating nose down moment.

That was an interesting but of reading, thanks!
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Old 06-23-2019, 05:48 AM   #147
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Quote:
Originally Posted by Ironhand View Post
Is that Imperial or US?
Imperial Mirage IIIO/D
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