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palgia
17th Aug 2004, 03:36
Hello everyone!

Quick question on the B1900. What are those fin-like structures that extend downwards and outwards from the rear part of the fuselage? What are they called? What purpose do they serve? (keel effect?) I saw them also on several bizjets but never bothered asking one of the pilots what they were for. (I felt stupid asking the question;))

Also, what are those fin-like structures that extend straight down from the horitontal stab of the B1900? What are they called? do they serve the same purpose of the fins that extend from the fuselage? Is the B1900 the only aircraft that has them? (if so, why?)


While we're on topic, I recently flew on an MD80, and i noticed a small thin and long fin that sticks out from the fuselage into the airstream about 1.5 meters below the rearmost cockpit window (just rear of the AOA probe). What is it? Is it there to provide an aerodynamic effect, or to provide sensor informatin for certain instruments?

Thank you in advance for your answers:ok:

palgia

Dr Illitout
17th Aug 2004, 06:39
On the MD80 the items you are referingg to are spray deflectors. They stop water spray from the nose wheels getting thrown up into the engines.
On the Beech 1900 I think they are just to keep the thing flying in a straight line!. I remember seeing pictures of the proto type and it didn't have them for the first flight. Looks a complete mess to me, mind you they have made more of them than I have so it must work!!
Rgds Dr I

Mad (Flt) Scientist
17th Aug 2004, 16:31
Quick question on the B1900. What are those fin-like structures that extend downwards and outwards from the rear part of the fuselage? What are they called? What purpose do they serve? (keel effect?) I saw them also on several bizjets but never bothered asking one of the pilots what they were for. (I felt stupid asking the question)

Also, what are those fin-like structures that extend straight down from the horitontal stab of the B1900? What are they called? do they serve the same purpose of the fins that extend from the fuselage? Is the B1900 the only aircraft that has them? (if so, why?)

For the two bolded bits:

You may mean the "delta fins" which are fitted to the rear fuselage of Learjets. These provided a nose-down pitching moment at high angles of attack, helping stall characteristics, and also have some beneficial effects on Dutch Roll behaviour.

The CL-415 Waterbomber has "fin like structures" on the horizontal stab, too. In fact, the CL-415 and the B1900 are probably neck-and-neck for the title of "aircraft with largest number and variety of aerodynamic 'fixes' added". When you see anything that inelegant (is that a word?) it usually means that either they missed something in the design and had to fix it with an add-on, or they knew about it during design but weren't allowed to fix it for $$$ reasons.

avioniker
17th Aug 2004, 20:57
Actually on the MD80 I think he's refering to the nose strakes. They're there to smooth the airflow across the wing. The spray deflector is mounted on the nose tires.

From the MD80 AMM:
(4) Aerodynamic Strakes - Aerodynamic strakes are installed, one
on each side of the fuselage, just below the flight compartment
clearview windows and compartment floor line. The
strakes are provided to improve vertical and directional
stability during high angle-of-attack aircraft conditions.

On the Beech the fins aid the stability and effectiveness of the empenage controls at lower airspeeds when there's a lack of airflow across the stab because of the T-Tail's characteristics. You don't get the airflow effectiveness generated by the engines that you'd have in a low mounted Horizontal Stabilizer.

Flight Detent
18th Aug 2004, 11:30
Hi all,
Also on the RAAFs newly developed B737IGW AEW&C aircraft, it has two ventral fins extending downwards from the rear fuselage.

They are there to assist with directional control (single engine) since the 'Tophat' radar antenna has deemed to had some effect on the effectiveness of the vertical stabilizer.

Cheers,

FD

Diamond 'katana' geezer
18th Aug 2004, 12:13
If i'm thinking about what your describing, then i think they are called 'Strakes'


Geez :cool:

TCXCadet
20th Aug 2004, 14:24
Hi

Just heard second-hand so not sure if totally true - as everyone else has said, the fins improve directional control. Also, the B1900 is allegedly the only twin turboprop that can still dispatch with the yaw-damper u/s because of the extra fins.

Mad (Flt) Scientist
20th Aug 2004, 14:41
Those ventral fins on the lower rear fuselage of the B737 AEW aircraft (or indeed on any aircraft) will improve directional stability but will not help directional control - the latter arises from the fin/rudder, and they won't be helping that unless there's some really weird cross-flow on the rear fuselage.

Now in single engine the yawing tendency induced by the engine is counteracted by the directional stability of the plane, which tends to reduce the sideslip the yaw can generate (which the fins do help with), and by the directional control (rudder) which is then used to further reduce the sideslip angle. So the fins may help with the Vmc case, but not with 'control'.

On "strakes" vs "fins":

I very much doubt they are actually strakes, but probably are (auxiliary) fins - the different between the two being that a strake is a flow control device, which doesn't necessarily generate any large direct forces but instead does something to the air flow which helps the characteristics indirectly, whereas a fin is usually a direct acting surface, generating a load of some significance on itself.