Exploring the NOTAR Design of MD520N Helicopters: A Case Study

[Cyrus]

We have to write a 10 page paper on any helicopter topic we want to. I decided to do mine on NOTAR design, NO TAil Rotor because the police fly over campus all the time. (We always have helicopters flying over campus constantly since were right next to andrews airforce base. If its not a helicopter, its Air force one, jets, C-130s, Hueys fly over every day at least 5-6 times. I called the guys up at the airport and asked them if I could talk to them about the helicopter for my paper. They said sure so I drove down and they let me snap some pics and talk to them.


Part of the course requirements for ENAE631 (Helicopter Aerodynamics) is to write a paper on any helicopter topic of your choosing. Insprired by the PG police MD520N that flies over campus every day at the college park airport, I decided to do my topic on the quietness of the NOTAR design. NOTAR standing for NO-TAIL-ROTOR.

http://en.wikipedia.org/wiki/Coandă_effect

Breif description. The NOTAR works on the Coanda effect. Basically, it states that a fluid (air, water, etc.) will follow the curvature of a surface as it passes over that surface provided the curve isn't too sharp.

The tail boom of the helicopter acts as that curved surface. The main wake (downwash) of the airflow from the main rotor flows past the specially designed tail boom. The design of the tailboom turns the flow from going downward to going side ways. This is because the air follows the gradual curve designed into the tail boom. It also has a fan that blows low pressure air inside the tail boom itself which gets vented out the side slots and a main vent in the rear.

http://photos-d.ak.fbcdn.net/photos-ak-snc1/v779/133/69/5736845/n5736845_41382547_2055.jpg​

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The air that bleeds out of the slide slots helps to promote the air to 'stick' to the tail boom as it curves around it. Without it, the air wouldn't be able to flow all the way around the boom on its own.

http://photos-d.ak.fbcdn.net/photos-ak-snc1/v779/133/69/5736845/n5736845_41382555_2423.jpg​

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70% of the anti-torque needed to keep the helicopter straight comes from this design. The other 30% comes from a rotating vent at the very rear of the tail boom. This is a directional vent that rotates left/right based on the pilots rudder pedals, much like a conventional airplane/helicopter design. At the same time, it also turns a conventional rudder on the pilots side of the aircraft (left side). The right rudder moves independently from computer inputs that via a gyro to sense the rotation. http://photos-f.ak.fbcdn.net/photos-ak-snc1/v779/133/69/5736845/n5736845_41382557_6781.jpg

http://photos-h.ak.fbcdn.net/photos-ak-snc1/v779/133/69/5736845/n5736845_41382551_951.jpg
Because the tail boom has no moving tail and transmissoin, its only held on by four bolts in the front and is very light weight.

The helicopter also features a high powered lantern, which is bright enough to set the grass on fire if the grass is dry on a hot summer day. They have to be careful when using the lantern close to the ground and around objects and people. http://photos-e.ak.fbcdn.net/photos-ak-snc1/v779/133/69/5736845/n5736845_41382580_4249.jpg

The helicopter has a very small interior. The back seats are made as light as possible. They are basically fold up lawn chairs with fabric over them. The pilots seats used to be the same, but they have recently upgraded them to foam with cloth. Every lbs counts when designing a helicopter. http://photos-h.ak.fbcdn.net/photos-ak-snc1/v779/133/69/5736845/n5736845_41382591_6323.jpg

The main rotor blades are quite small, only a few inches in chrod and about 20 feet long. The rotor hub is also quite small and has dampers on the lead/lag hinges, which are the black cylinders. http://photos-b.ak.fbcdn.net/photos-ak-snc1/v779/133/69/5736845/n5736845_41382593_1774.jpg

http://photos-e.ak.fbcdn.net/photos-ak-snc1/v779/133/69/5736845/n5736845_41382596_6969.jpg

The helicopter is VFR rated. The pilot sits in the left seat, like an airplane. The spotter sits on the right and operates the laptop computer that controls the lantern and various other police tracking programs.

In the summer time they take the doors off the hinges (which are just clipped on) and fly with the doors open.

http://photos-a.ak.fbcdn.net/photos-ak-snc1/v779/133/69/5736845/n5736845_41382616_2882.jpg


http://photos-b.ak.fbcdn.net/photos-ak-snc1/v779/133/69/5736845/n5736845_41382617_7698.jpg

Video of them launch to Crofton, MD.


Thanks to Cpl. Economes, Kelly, and El Rod for showing me around the helicopter.


(This formatting is terrible. It does not look like the format on my user note...awk)

I have pictures of the MD state trooper Dolphine helicopter too. I should post those. Thats another very pretty helicopter.

 

[WarPhalange]

Make sure to include that helicopters fly because the spinny things push air down really hard.

 

[Cyrus]

Do the pictures show up? I'm linking them from facebook so I am not sure if you just see a box with a red -x in it.

 

[Evo]

I can see the pictures.

 

[Proton Soup]

cute, it has its own litter box

 

[dlgoff]

Wow. Maybe next time you can get a ride.

 

[turbo]

For some tech stuff, does the exhaust from the tail boom have to have more or less control equipment than a tail rotor? Is the efficiency of that ported exhaust dependent on the RPM of the main drive, and can it be effected by the air-speed of the craft? (Is it as controllable at low-speed/hover as it is in forward flight?)

 

[physics girl phd]

This is very cool. I talk about helicopters in my intro class (physics 101 / How Things Work). That's for pointing this unique engineering out (I can see a short answer question for next year! ).

 

[Cyrus]

For some tech stuff, does the exhaust from the tail boom have to have more or less control equipment than a tail rotor? Is the efficiency of that ported exhaust dependent on the RPM of the main drive, and can it be effected by the air-speed of the craft? (Is it as controllable at low-speed/hover as it is in forward flight?)

The tail boom is simply a long hollow cylinder with a movable vent in the back. It's significantly lighter and simpler. No gear boxes, no shafts, no blades. There are only control cables that go to it and a few wires for the lights.

Hellicopters operate at constant RPM settings, unlike airplanes. The blades always rotate at the same speed. At high speeds, the main directional control comes from the twin rudders. Almost all helicopters have rudders in the back for high speed forward control.

http://video.google.com/videoplay?docid=-8346506091298356948&ei=3bYoSbL0DoruqALAlZCgDQ&q=notar+md+helicopter
Its a VERY VERY quiet helicopter. You won't hear it until its already passed you.