Jump to content
This site uses cookies! Learn More

This site uses cookies!

You can find a list of those cookies here: mysite.com/cookies

By continuing to use this site, you agree to allow us to store cookies on your computer. :)

Sign in to follow this  

No NOTAR? Why not?

Recommended Posts

If the tail rotor on a helicopter is mechanically complex, potentially vulnerable and, if part of the system fails has the potential for a catastrophic accident (as it seems), why isn't the deflected jet exhaust (NOTAR) solution more popular? Is it because MD have patented it so no one else can use it?




Share this post

Link to post
Share on other sites

Here's some stuff




Basically, it's more complicated than a tail rotor, less effective at speed, and heavier. It's also 'different', and aviation is a fairly conservative field, so they're a bit leery of change.



Share this post

Link to post
Share on other sites

A possible factor:


While many ‘traditional’ helicopters have aerofoil-section fins to provide sideways anti-torque thrust as airspeed increases, these are virtually always fixed(and therefore pretty simple).  


As a NOTAR helicopter increases speed, the engine power required to allow that acceleration increases. Increasing the power output through the gearbox to the rotor naturally increases the torque (twisting) reaction, which obviously necessitates an increasing anti-torque sideways thrust as airspeed increases. The ducted NOTAR fan that provides the air to the tail boom takes its drive from the main gearbox, just like a tail rotor, but on its own would take an increased power demand from the engines as airspeed increases - just when the main rotor requires an increased power demand. 


To get around this ‘running-out-of-power’ issue, McDonnell Douglas designed the MD900/902 to have a pair of large vertical stabilisers with a marked aerofoil section, but these constantly alter pitch in forward flight to provide the optimum sideways lift/thrust to offload the ducted fan/blown air system.  One benefit of this is that it is possible to continue forward flight if the ducted fan fails (landing is more awkward!), but it definitely adds complexity.


Another possible factor:


In the hover, assuming wind speed is below about 30kts, the stabilisers are ineffective as they provide no sideways lift/thrust. The ducted fan has to provide all the anti-torque effect and yaw control.  Simplistically, this means that applying left pedal (increased anti-torque) results in a fairly significant power increase (I have often seen a 10% torque increase). If you are already at high power settings this can be awkward.


If you combine this with the Coanda effect from the tailboom slots breaking down due to airflow over the fuselage (dependent largely on wind direction and strength), then it is possible for the power demand to increase to the point that applying more left pedal would result in an overtorque. When this happens, you basically run out of left pedal. I was careless enough to allow this to happen twice: on the first occasion I was at 1000ft and was able to fly away comfortably. On the second I was hovering over a runway with a strong wind from the right; I stopped pushing the left pedal just below the torque limit and had to let the aircraft pirouette to the right until the power demand dropped so that I could regain full yaw control. Mildly embarrassing!  A good example of this is the MD902 that features in many YT helicopter crash compilations, which was carrying out a rescue pickup on a mountain and started to spin uncontrollably before crashing. It seems pretty clear that the wind effects, weight and power demand contrived to make the pilot run out of anti-torque left pedal with insufficient time and space to fly away to offload the ducted fan system.


In short, I think that NOTAR is an interesting concept with some benefits mentioned by previous posters, but the add-ons to make up for the deficiencies make a traditional tail rotor more attractive.


Here endeth the lesson!


Jon (1200 hrs MD902, in a previous life)

Edited by Jonners

Share this post

Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this  

  • Create New...