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Only 5deg Rotation on Takeoff - Anything more results in ground stall

[MVP] Mugga

By [MVP] Mugga
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[MVP] Mugga

[MVP] Mugga

Posted
Posted

Not sure if this has been brought up in any earlier reports, but the flight model during take-off has a glaring error that would be great if it could be fixed up.  The DCS F86 can be easily over-rotated during the takeoff roll with no corresponding increase in drag and subsequent decrease in airspeed.  Instead, the DCS F86F Sabre can rotate all the way to tail-strike and continue to accelerate to take off speed.  This does not happen in the real-world.

The real-world F86 Sabre was extremely sensitive to deck angle at rotation and pilots were trained to be aware of over-rotating the aircraft during the ground roll, given the swept wing aerodynamics etc.  In addition, the Sabre has a powerful all flying tail-plane that gives the aircraft excellent control authority at low speeds and a hydraulic flight control system that provides limited force feedback.  The Sabre did not have engine sufficient power to overcome the induced drag of excessive rotation angles during ground run, and would squat on it's main gear with it's nose in the air refusing to accelerate.

At normal rotation angle of approx 5 deg the speed is around 110knots and takeoff speed is around 130kts, at which point the aircraft is flying and can be pitched up to climb away accelerating.  However, if the aircraft deck angle exceeds 5deg, then induced drag loads rapidly overcome the thrust available and airspeed either stops increasing or starts decreasing as deck angle increases to maximum available (tail-strike).

The F86F Sabre Flight Manual informs that if the rotation angle is too high, the airspeed will begin to decrease with pilots trained to lower the nose to the three point attitude to regain acceleration.  The flight manual affirms that once flying speed has been attained, the pilot can pitch the aircraft up.

There was a famous F86 Sabre (Canadian Canadair Sabre Mk5) accident in Sacramento, where the pilot over rotated during the ground roll, resulting in a failed takeoff and the Sabre departing the end of the runway and spearing through a restaurant and killing quite a few children and adults.  The NTSB report details the over-rotation analysis.

F86F Sabre Flight Manual + Performance Data
https://drive.google.com/file/d/1-JV9l6z-Y6V9rS2W7e6DiC8NBwMx-ZEU/view?usp=sharing

NTSB Report into the Sabre Accident:
https://www.ntsb.gov/investigations/AccidentReports/Reports/AAR7306.pdf

 

  

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Flappie

Flappie

Posted
Posted

I could not find any internal report about this matter, and I agree with you: something needs to be done. I was able to take off with an AoA of 20° (I pulled to the max for a long time).

I read most of the manual takeoff section and part of the appendix. I could not find the recommended nose-high angle of attack for takeoff. It seems the manual only deals with speeds and distances. The NTSB report does not mention the recommended AoA for takeoff either, since its main reference is... the Sabre manual..

The NTSB report mentions a normal takeoff test that was made with an AoA of 5°, but does not mention whether it's the allowed maximum AoA for takeoff. About the accident, the report tells that the Sabre speed started to sink once the AoA reached 15.5°. Beforehand, the speed would keep rising even though the pilot took off with an AoA of 11°.

Finally, the Sabre Mk5 weight and thrust seem to be comparable to that of the F-86F, so I guess the Sacramento experience can be used as a reference for our module.

If you have a public, Rule 1.16-compatible, document stating the maximum AoA of Sabre during takeoff, please provide it. It will help me convince devs to tweak the module.

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[MVP] Mugga

[MVP] Mugga

Posted
  • Author
Posted (edited)
13 hours ago, Flappie said:

I could not find any internal report about this matter, and I agree with you: something needs to be done. I was able to take off with an AoA of 20° (I pulled to the max for a long time).

I read most of the manual takeoff section and part of the appendix. I could not find the recommended nose-high angle of attack for takeoff. It seems the manual only deals with speeds and distances. The NTSB report does not mention the recommended AoA for takeoff either, since its main reference is... the Sabre manual..

The NTSB report mentions a normal takeoff test that was made with an AoA of 5°, but does not mention whether it's the allowed maximum AoA for takeoff. About the accident, the report tells that the Sabre speed started to sink once the AoA reached 15.5°. Beforehand, the speed would keep rising even though the pilot took off with an AoA of 11°.

Finally, the Sabre Mk5 weight and thrust seem to be comparable to that of the F-86F, so I guess the Sacramento experience can be used as a reference for our module.

If you have a public, Rule 1.16-compatible, document stating the maximum AoA of Sabre during takeoff, please provide it. It will help me convince devs to tweak the module.

The Flight Manual states that a pilot should rotate and keep the nosewheel just off the ground, and maintain that attitude until takeoff speed is attained, after which the pilot can pitch the aircraft up accelerating away.  The NTSB report mentions that the deck angle was approx 5deg, which would equate to a nosewheel height just above the runway surface.  And yes... the Canadair is comparable to the F86F.

Edited by [MVP] Mugga
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MVP | Community Founder
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Flappie

Flappie

Posted
Posted (edited)

I see. I've just found two other documented crashes on takeoff which clearly point to a too high AoA : 1954 and 1955 (Sacramento is here: 1972).

I'll see if I can learn more about these:

Edited by Flappie

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Flappie

Flappie

Posted
Posted (edited)

Standby, I need to redo some tests. It just occured to me that the Sacramento plane was carrying two external fuel tanks (probably full). It was the same for the 1954 crash (drop tanks are mentioned), and probably the same in the 1955 crash.

EDIT: it does not change much with the 2 drop tanks. Issue reported.

Edited by Flappie

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[MVP] Mugga

[MVP] Mugga

Posted
  • Author
Posted

We spent some time flying the F86 Sabre last night and with tanks it is the same issue.  At higher gross weights an excessive rotation angle should result in the same ground stall situation, and would be far more amplified than at lighter loads.

MVP | Community Founder
Email:  muggavirtualpilot@gmail.com

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  • 6 months later...
Tiger-II

Tiger-II

Posted
Posted

Is this fixed yet? I tried it last night and was able to takeoff, complete with stall roll, but she still flew.

Motorola 68000 | 1 Mb | Debug port

"When performing a forced landing, fly the aircraft as far into the crash as possible." - Bob Hoover.

The JF-17 is not better than the F-16; it's different. It's how you fly that counts.

"An average aircraft with a skilled pilot, will out-perform the superior aircraft with an average pilot."

  • 1 month later...
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