Mast Bumping in Helicopters
Welcome back to Helicopter Lessons in 10 Minutes or Less!
This video’s topic covers Mast Bumping. This subject can be a sensitive subject for some because it usually deals with expensive repairs of rotor components and, in some cases, catastrophic damage resulting in death. That said, it’s a subject that requires understanding if it can affect your helicopter. I say “if” because it only applies to semi-rigid or “teetering” rotor systems. If you’re not familiar with what exactly a semi-rigid system is, I’d recommend watching my Types of Rotor Systems video (https://youtu.be/7gM3rMDpJt4). Mast bumping, just like the name implies, is where the main rotor hub contacts or bumps the rotor mast. This causes damage to the mast and can cause complete rotor separation in extreme cases. Mast bumping can occur for a few reasons. The first being takeoffs and landings from sloped terrain.
If a helicopter attempts a takeoff or landing to or from a slope there can be points where cyclic limits may be reached. By applying cyclic in the direction of slope with little or no collective applied it is easy to see of this can happen. To help prevent this, be sure to have some collective applied prior to displacing the cyclic for a slope takeoff and try to avoid landings which require maximum cyclic displacement in any direction. This type of mast bumping is far less severe than the other type : low-G flight.
Low-G flight is usually accomplished with a “pushover” or the leveling off / dive entry following a cyclic climb. You’ll notice those maneuver being demonstrated in the introduction to this video prior to the Apaches conducting diving rocket engagements. Keep in mind that fully articulated and rigid rotor systems can do this type of maneuver. Semi-rigid rotor systems cannot without risking mast bumping. If a semi-rigid system attempted this it could have catastrophic damage resulting in main rotor separation.
Let’s take a look at why:
1. As the helicopter reaches the crest of the climb, the helicopter reaches weightlessness also known as “zero-G’s.” Main rotor thrust is drastically reduces as it unloads.
2. At this point, the tail rotor still provides thrust to offset the tail rotor. But now there is no main rotor thrust to counteract. So the aircraft side-slips and rolls towards the right.
3. The pilot recognizes this right roll and tries to correct with left cyclic. But this has little effect because the main rotor is unloaded.
4. The pilot applies even more left cyclic and the rotor reaches its flapping limit. The rotor hub and mast make contact. This force can be sufficient enough to bend or break the mast. The result: the main rotor flies away and the fuselage falls to the ground.
The best way to prevent mast bumping is to avoid pushover-type maneuvers when flying semi-rigid rotor systems. If you ever find yourself in this situation by recognizing a feeling of weightlessness/right roll you should immediately and smoothly apply aft cyclic to load the rotor and establish level flight.
The biggest takeaway from this video is that mast bumping is a very dangerous condition in helicopters with semi-rigid rotor systems. Well that wraps up the subject. Thanks for watching! Don’t forget to hit like and subscribe below. As always, safe flying!
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If you’re just getting started and want more information, pictures, and more explanations, I’d recommend reading the Rotorcraft Flying Handbook – http://amzn.to/2ifPlnZ. If you’ve already got a basic understanding, and want to further your professional helicopter education with advanced helicopter concepts, I’d recommend reading Cyclic and Collective, by Shawn Coyle – http://amzn.to/2ifQGLx