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Low G Pushovers?


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So here's a question. My understanding of the low-g push-over issue was that it was a byproduct of gyroscopic progression: Rotating the disc around the lateral axis (by way of torqs) would cause the disc to roll to the right (90 degrees later) around the longitudinal axis and because you had also unloaded the disc the blades would tend to stay low and possibly strike the boom. I recently flew with an instructor and he mentioned the rolling (Along the longitudinal) part but made it sound like the only real danger was the aircrafts (possibly) unexpected roll. I'm sure he was trying to dumb it down or maybe he didn't want to scare me but the question is - What's the real danger of the Low-G pushover? Missing tail rotor or just unexpected flight characteristics?

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ok this is going to be longwinded without diagrams but i'll try:

 

First off you need to understand what is happening to main rotor lift in a low g pushover. If the cyclic is pushed forward rapidly whilst you are in forward flight the momentum of the helicopter will mean that it will try to continue in its original direction before the cyclic was moved, however the helicopter is now in a nose down attitude and more of the disc is presented to the air flowing in, which increases the induced velocity massively over the entire disc, the angle of attack of the blades is reduced as a consequence and so the total lift from the disc dramatically reduces.

 

so the helicopter is now in a nose low attitude with little lift from the main rotor, however the tail rotor thrust hasn't been reduced and the tail rotor is now above the center of gravity of the helicopter, this produces the rolling motion, to the right in american helicopters.

 

The helicopter is now in a nose low right roll attitude (the roll being much greater than the nose down) the instinctive action would be to put in left cyclic to stop the roll. IF this is done the disc tilts rapidly to the left but becasue there is greatly reduced lift on the main rotor the helicopter fselage does not respond. in this situation the helicopter rolling to the right with the disc rolled to the left it is possible (probable as well) for the main rotor head to contact the mast. The mast cannot be designed to take the loads imposed by mast bumping and so its a catastrophic failure and the main rotor seperates (or in some cases it strikes the tail boom as a result of the mast bump either way it's game over).

 

S'o thats what not to do.

 

If you get in a low g situation and feel teh right roll the disc has to be producing sufficient lift for the helicopter as a whole to respond to the cyclic inputs, this means the induced flow has to be reduced (as thats what is causing the low gf in teh first place) to do this it is aft cyclic to regain the lift and then only after the main rotor lift has been regained put in left cyclic to stop the roll.

 

diagram s will help definately ask your instructor to show you on a board what is happening.

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Query:

How about those birds one sees in the movies, or some military birds, that do perform pushovers-or radical manuvers? Do they have a different kind of connection between the rotor and mast? Can you elaborate on helos that perform loops and rolls, and the difference between them and civil helos?

 

Just wondering.

 

Later

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Witch,

 

The short answer is 'yes'.

 

Generally, that nap of the earth stuff is only possible with a rigid rotor head design. That's because the helicopter will always follow the disc.

 

Most civil helos have teetering (semi-rigid) or articulated rotor heads. These are both susceptible to mast damage.

 

AdamSTL,

 

That's an interesting concept, but the effect of gyroscopic precession would be so small, that it wouldn't be noticeable.

 

No, right roll is pretty much as PhilJ explains, as a result of the tail rotor thrust being above the longitudanal axis of rotation.

 

It can happen in an instant causing loss of control, and the natural reaction is incorrect, which could result in mast failure. That's the danger.

 

The correct reaction must be drilled into you. As it is not easy to recreate this sort of emergency, the best way you can learn it is to use the very effective tool of 'VISUALISATION'. Go through this often, imagine getting it and imagine getting out.

 

Joker

 

Question for Instructors: How many demonstrate 'right roll tendency to students'?

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As a side note, mast bumping is usually associated with semi-rigid rotor systems. It occurs in articulated systems, but has a different name: droop stop pounding.

 

Question for Instructors: How many demonstrate 'right roll tendency to students'?

 

Also, what method do you use to demonstrate right roll tendency?

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I saw a vid of a 407 first do a loop, then a roll. I assume it was a semi-rigid design, so mast bumping would occur in these instances?

 

I also saw a Jet Ranger hit the ground pretty hard (at 80 mph) to cause it to jump back into the air. Might this also cause a kind of mast bumping? I'm suprised the skids didn't collapse

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As I said, 'Generally' rigid heads are required to go inverted...

 

However, with some clever flying, it is possible to invert in other helicopters. It requires careful timing in order to keep load on the disc and / or to only use cyclic inputs at the right times.

 

I of course haven't a clue how exactly it's done, but I suspect the 'roll' after the 'loop' that you saw on the video was for exactly that reason - to keep load on the disc. (Was it a full loop or more like an immlemann turn?)

 

immelman-anim.gif

 

Anyway, you'll have to find another pilot for explanations!

 

Joker

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We used to fly NOE all the time in OH-58s with Cobras flying behind us (both semi-rigid systems). Apache's do it also and I believe they have fully articulated systems.

 

That being said, NOE flight is not fast or aggressive. It's more "sneekin and peekin" than "yankin' and bankin". We were typically at 30 kts or less and were often below ETL. It's pretty hard to get into low-G conditions if you don't make any abrupt control inputs so that's what they teach.

 

The answer about how you demonstrate low-g conditions is that you don't. The right-rolling moment is faster than you can imagine and accelerates as you roll. Several highly experienced test pilots have been killed exploring these conditions.

 

What they told us at Robinson is that you apply aft cyclic and once the disk is re-loaded, the rolling moment will result in a turn to the right. Just let the aircraft turn and then straighten out once you're able to swallow again. They don't want you to even think about left cyclic.

 

As for the video showing the 407 doing aerobatics, it's my understanding that the FAA was not impressed and the pilot is in hot water. The AC has been grounded since Bell said that it exceeded the design limitations.

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I saw a vid of a 407 first do a loop, then a roll.

 

I've got that one too although mine came with a supposed copy of the letter from Bell that effectively grounded the helicopter until it was more or less overhauled. (Can a manufacturer do that?)

 

 

I've heard the Bo-Super Five (BO-105CBS5) is the only civilian helicopter certified for Aerobatics?

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If gyroscopric precession was playing any part the roll would be to the left.

 

I can't rememer what the number is but its surprising how fast you have to be going to get into a low g situation (in a robbie) where the right roll is a problem, if you get the right instructor on the robbie safety course they will demonstrate the effect of a rapid forward cyclic movement at various speeds. I don't suggest anyone trys to figure out the airspeed at which right roll becomes a problem themselves!

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The answer about how you demonstrate low-g conditions is that you don't. The right-rolling moment is faster than you can imagine and accelerates as you roll. Several highly experienced test pilots have been killed exploring these conditions.

 

Do any of the flight simulators (FAA-approved simulators like FLYIT, not MS Flight Simulator) demonstrate this tendency in a realistic manner. Enough so that a pilot could experience the effects of the proper correction (as well as the improper one) in a safe environment?

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When I was saying if gyroscopic precesion had any part in the roll it would be to the left I was trying to show it wasnt occuring and probably in a not too clear way.

 

gyroscopic precession in a nutshell is precession of the body 90 degrees after the moment (force) causing it, (or the opposite case of bearing force as a result of precession). It is not precession as a result of precession.

 

Breaking down what happens in the case we are talking about the low g is initiated with abrupt forward cyclic, this in turn moves the control rods to cause a moment around the longtitudinal axis of the helicopter, the resulting precession causes the disc to tilt around the lateral axis (pitch forward). So the discs movement is already the result of precession.

 

What you seem to be saying is that the pitch forward of the disc causes precession which makes the disc tilt to the left, we can see this doesn't happen simply by following through what would happen if it were true. the pitch forward would cause the disc to then roll left, the roll left would then cause the disc to pitch aft, etc etc.

 

As for flyit, isnt it essentially a fancy set of controls and display for MS flight sim, with a little tweaking?

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When I was saying if gyroscopic precesion had any part in the roll it would be to the left I was trying to show it wasnt occuring and probably in a not too clear way.

 

gyroscopic precession in a nutshell is precession of the body 90 degrees after the moment (force) causing it, (or the opposite case of bearing force as a result of precession). It is not precession as a result of precession.

 

Breaking down what happens in the case we are talking about the low g is initiated with abrupt forward cyclic, this in turn moves the control rods to cause a moment around the longtitudinal axis of the helicopter, the resulting precession causes the disc to tilt around the lateral axis (pitch forward). So the discs movement is already the result of precession.

 

What you seem to be saying is that the pitch forward of the disc causes precession which makes the disc tilt to the left, we can see this doesn't happen simply by following through what would happen if it were true. the pitch forward would cause the disc to then roll left, the roll left would then cause the disc to pitch aft, etc etc.

 

As for flyit, isnt it essentially a fancy set of controls and display for MS flight sim, with a little tweaking?

 

I have also been told that FlyIt simulators run MSFS. I believe X-Plane would be your best bet for helicopter physics simulation. The author(s) claim it is the most advanced flight-model available to the average consumer. The trouble I have with x-plane is that the ground texture resolution (Even with the large texture set) is so low it's worthless near the ground. I haven't spent much time with it.

 

As gyroscopic precession goes - I have to admit that I sort of zoned out when torqs came up in physics but - It seems to me that a force must be responsible for the disc rolling forward about the lateral axis. I assume the disc itself is providing the force. If that's the case you are going to have a precession 90 degrees later in the same direction. They make a big deal about it in fixed-wing training (P-Factor.) Obviously its academic at this point and I agree that it is not responsible for the right-roll tendency. It just seems to me it must be there.

 

the pitch forward would cause the disc to then roll left, the roll left would then cause the disc to pitch aft, etc etc.

 

Call me crazy but when you try and rotate a spinning disc you actually do encounter a force in the opposite direction! =) If I'm not mistaken that is why we can ride a bike.

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I have also been told that FlyIt simulators run MSFS.
Two installations of FS2002 to be exact, with customized instrument panels, control mapping, aircraft profiles and two stations - instructor and pilot. Instructor controls weather and failures. Its a pretty good setup, although a bit pricey for a hobby setup!
As gyroscopic precession goes - I have to admit that I sort of zoned out when torqs came up in physics but - It seems to me that a force must be responsible for the disc rolling forward about the lateral axis. I assume the disc itself is providing the force. If that's the case you are going to have a precession 90 degrees later in the same direction.
You have to remember that it's precession that got the disk to pitch forward to begin with - forward cyclic causes the advancing (right side) blade to flap down - the blade reaches maximum downflap over the nose. The rolling encountered in a low-"g" situation is the airframe rolling due to tail-rotor thrust - the M/R disk is still flying level to the world in the bank axis. If the pilot pushes left cyclic, that causes the blade to try to flap down over the nose - precession causes the disk to tip left. Airframe right, disk left, enjoy the ride...
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1. Forget about the gyroscopic effects, when you get into low-g the rotor system is "floating" and not dragging the fusaluge with it so the effects won't be felt as a roll. In this case it will add to the tilt of the disk if you add the input, just as it normally will.

2. The phrase to discribe the reason for the roll and eventually mast bumping is "intertial load". The faster you go the worse it gets- the more energy you have in a particular direction.

3. Below 35 kts it's very hard to get mast bumping from a sudden and quick displacment of the cyclic unless you stick it and hold it in the extreme position. Below 15kts won't happen unless you are on the ground. 45kts or better, happens every time.

Advise: learn about know how to prevent it and get out of it then move on. Don't focus on low-g with so much more to learn about.

I know this posting is a little broken but i'm all typed out tonight, you fill in the blanks by reading the privious posts.

I was one of those guys that used to test limits and lived to tell you why its a bad idea to test things in aircraft.

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The Blackhawk has an articulated rotor system and we regularly experience low-g situations. In fact, I instruct students in creating low-g situations, the effects of trying to fly the aircraft in these situations, and finally how to apply the corrective action.

 

By applying forward cyclic (after an abrupt cyclic climb) everything in the helo gets light and checklists/equipment/anything not tied to the floor floats. There's enough weightlessness for passengers in the cabin to do back flips. If you try to roll the helicopter in this low-g state, you will notice a delayed control response and inability of left/right cyclic to make the nose track in a turn. Basically, the aircraft maintains a straight ground track regardless of applied bank. No, the aircraft does not start any sort of roll on its own and the HH-60 has a huge tailrotor far above the CG and provides 2.5% of the total lift. I have never noticed any rolling tendancy.

 

We demonstrate the HH-60s reluctance to turn when unloaded by entering the low-g and applying 30 degrees of bank, first to the left and then to the right. The aircraft maintains a straight groundtrack throughout the entire maneuver. Only when you load the rotor with aft cyclic will the helo snap into a sharp turn and begin to track in the direction of the bank.

 

All of this being said, the semi-rigid design behaves differently in a low g state because of its unique design characteristics. Since the rotor teeters on the mast, it is virtually independent of the fuselage in a low g state. Control inputs have no affect on the total vector of the fuselage or rotor system but they do affect the angle between the rotor and mast. (As a rule of thumb, 12 degrees in the maximum angle allowed between rotor plane of rotation and mast.) This lag in control response (due to an unloaded rotor) sets you up for mast bumping. It's the over-controlling, mast bumping, and subsequent destruction of the rotor mast that will kill you not the nominal roll produced by gryroscopic effect.

 

The FAA recommends never exceeding .9 VNE (R-22/R-44 ASW-95-01 January 10, 1995) due to increased possibility of mast bumping. However, this advice could be applied to all helos with teetering rotor systems. Considering the kinetic energy of the rotor system at high airspeeds, you can see that low g can be encountered with deadly results even with minor pitch deviations.

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Nobody has mentioned it, but when I started lessons in a R22 back in 1986, they used to teach us Low G maneuvers..take the bird up into a Neg G situation and when it started to roll right we would use aft cyclic to correct. I guess it got removed from the curriculum at some point..Now when I fly a 22, I am very aware of, and I avoid like the plague, any movements that may cause a Neg G to occur....funny how experience changes things !

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