Jump to content
Autojohn

Bell 206 N2 governor - static droop?

Recommended Posts

I’m a relatively new pilot transitioning to turbines and training in the 206. While studying the systems,  I had a look at the Bendix manual:

http://www.justhelicopters.com/Portals/5/SkolaC28 Bendix Book.pdf

Page 17 decribes the collective linkage to the governor as providing “compensation for droop characteristics”. I take this to mean static droop which I understand to be an intentional speed reduction with load to prevent overshoot. If I have this right so far, how is static droop created in this system? The internals of the governor look extremely simple - just a set of flyweights with a thrust bearing that opens the Pg vent. (Page 22)  With my very limited understanding of static droop, I expect to see something modulating the speeder spring tension so that after a load is applied it governs back to a slightly lower RPM.

Where have I gone wrong?

Share this post


Link to post
Share on other sites

The linkage is set up so when a load is applied to the engine, such as when raising the collective, the governor arm is automatically increased to provide more fuel so the engine will not droop.

Share this post


Link to post
Share on other sites

agreed,  but that’s not quite my question. Suppose the linkage were gone. If Im at 100% N2 and raise collective there will be transient droop as the rotors create more drag, N2 governor will kick in and add fuel. Once the N2 governor is back on speed the RPM will be less than 100. This difference in RPM is the static droop. So my question: does the Bendix governor exhibit static droop, if so, how is it achieved? I believe the collective droop compensation will address both transient and static droop. In the first case, the lag is reduced by immediately allowing greater fuel flow when the collective is first moved, in the case of static droop, the higher speeder tension lets the system return to equilibrium close to the original RPM. (Elsewhere someone said the 206 is overcompensated)

Share this post


Link to post
Share on other sites

  From a pilot standpoint 97%-100% N2 (power on range). Small droop (1-2%) is normal pulling power. A crappy governor will over speed up to 5% percent when reducing power especially when heavy.

 The droop compensator is a mechanical linkage that auto inputs commands to the governor so the system doesn't lag to much when you pull or reduce power. Remember this is old analog tech. I'll see if I have a picture from the 212 it's very clearly visible.

Share this post


Link to post
Share on other sites

Here is a 212 droop cam compensator. As you can see it is a mechanical linkage. It is connected to the collective and as the pilot increases power the linkage leads the engine N2 to prevent droop caused by the requirement of the main rotor loads. It will make much more sense once you have a few hours on a turbine machine utilizing this type of system. But for now I hope this helps answer your question.

images.jpg

  • Thanks 1

Share this post


Link to post
Share on other sites

Theoretically, the governor will keep the engine at 100% rotor speed all the time.  The droop compensator will help keep it at 100% when making larger power changes in either direction.  The 206 had 2 types of systems to mechanically rig the power lever(collective) to the governor.  One section controls the range of speed and the other controls how quickly the change is applied.  As a pilot you need to know what it does, trying to figure out how it does it and why is a whole other course.

  • Thanks 1

Share this post


Link to post
Share on other sites
2 hours ago, Heliian said:

 As a pilot you need to know what it does, trying to figure out how it does it and why is a whole other course.

Roger that, I'm just curious about the control theory aspects and how it was all achieved (with steam essentially)

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

×