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What is this for on the MD-80? Is it a stabilizer?

enter image description here

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  • $\begingroup$ Aside - these are analogous in function to a Skeg or Bilge Keel under a ship intended for water. $\endgroup$
    – Criggie
    Apr 23 at 2:08

2 Answers 2

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TLDR:

They are there to improve directional stability. At high angles of attack, the wake from the fuselage covers the rudder, reducing yaw authority. Strakes mitigate this effect and thereby improve the directional stability.

More information:

This paper from NASA/Boeing contains more detailed information. An image from the paper indicates the difference clearly:

enter image description here

$C_{n_\beta}$ denotes the correcting moment yawing $C_n$ for when a given yaw angle $\beta$ is present.

As the image shows, it needs to be larger than zero for an aircraft to be stable. Strakes help to keep $C_{n_\beta}$ positive at high angles of attack.

$C_{n_\beta}$ is usually delivered by the rudder, which at large angles of attack gets blanketed by the wake coming off the fuselage, as shown in the image below:

enter image description here

Source

Once the rudder gets blanketed by low energy flow emanating from the fuselage, it becomes less effective and has greater difficulties straightening the aircraft into the wind. Strakes mitigate this effect and thereby improve the directional stability. The working principle is similar to vortex generators, see this question, and others on the same topic here on Aviation Stackexchange for more info on vortex generators.

The paper also shows a benefit in lateral stability (i.e. in roll), but this effect is minor (and does not change the aircraft from unstable to stable).

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    $\begingroup$ Interesting to see that Boeing published a study in 1998 to understand what Douglas had done in the early 70s. At thetime, Boeing just bought McDonnell Douglas and inherited the MD product line, but it lacked detailed engineering documentation that Boeing was used to. Boeing at the time was still very much an engineering company, while MD was run by accountants, financial controllers and other managers obsessed with cost savings. $\endgroup$
    – DeltaLima
    Apr 22 at 7:48
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    $\begingroup$ Unfortunately the MD management culture became dominant after the merger. Over the next 20 year there's been less budget for this kind of research, up to the point where Boeing aircraft (or pieces thereof) started falling out of sky . $\endgroup$
    – DeltaLima
    Apr 22 at 7:48
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    $\begingroup$ This work was done by Boeing for the STRATA-project (Strake Technology Research Application to Transport Aircraft) which was a NASA/Boeing collaboration. So it could also be that they recreated earlier findings to bring it in the public domain (for example by using non-proprietary geometry etc.) $\endgroup$
    – ROIMaison
    Apr 22 at 7:59
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The nose strakes on the DC9-50 and subsequent models are to energise the airflow around the fuselage at high angles of attack.

Without the strakes, at high angles of attack, the turbulent flow around the fuselage interacts with the vertical stabiliser, reducing the directional stability. The strakes introduce a vortex into the flow, ensuring the flow contains more energy which in turn improves the directional stability at high angles of attack.

The shorter models didn't need these strakes since the shorter fuselage doesn't cause so much turbulent flow over the vertical fin.

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  • $\begingroup$ I'm sure they also help to improve spin characteristics, but for airliners directional stability is clearly more important. +1 $\endgroup$ Apr 22 at 13:44
  • $\begingroup$ What is angle of attack? (And which direction is high/positive ?) (Or does high angle of attack mean the absolute value (of whatever angle of attack is) is large?) $\endgroup$
    – Fattie
    Apr 22 at 17:40
  • $\begingroup$ The question which arises is then... how do other airliners which do not have these strakes do without? $\endgroup$
    – jcaron
    Apr 23 at 10:44
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    $\begingroup$ @jcaron, these devices usually get added as late-stage development mitigation device. For example, when during certification flights a deficiency gets discovered. In this case, the aircraft was a derivative (a lengthened version of the DC-9-10), and the strakes were a quick fix to prevent a more radical redesign. $\endgroup$
    – ROIMaison
    Apr 23 at 11:39
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    $\begingroup$ @jcaron also as @ DeltaLima said: "The shorter models didn't need these strakes since the shorter fuselage doesn't cause so much turbulent flow over the vertical fin." The wings and engine are very to the back and is a long aircraft. I also noticed these strake on the Concorde which is similar case(wings and engine to the very rear of the aircraft) $\endgroup$
    – Gabe
    Apr 24 at 1:50

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