Helicopter Questions

[Frank N. O.]

Thank you for the explanation, I'm sure now that I did understand what was going on, I just wasn't good at understanding it, and my example with the Thrust SSC was misunderstood I think, but it all worked out in the end so nothing for me to be sad about

I just looked in my Combat Aircraft book about the Apache that it does not have a enclosed t/r, I think I was thinking too much of the Commanche when writting the former post. I googled fenestron and it does indeed look a lot different than a normal rotor and from you it seems like the benefits outweigh the problems (weight and power-use) and that's great since I heard that a lot of accidents with helicopters were due to t/r problems, either failure or ground-strikes.

Btw, where did Mike write about the Gazelle? The only mention of it I can find is in my post, although I didn't know it was the very first with a enclosed t/r just that it was early, and one of a very few types with it which is why I mentioned it.

Frank

[Roland]

@ Frank

Ahh, gosh, you’re right. It was YOU mentioning the Gazelle. Sorry, mate! I’m an elderly, you know …

The Gazelle was, as to my knowledge, the first helicopter build in series with this fenestron (“fene-“ comes from the French “fenetre” = window). Another reason why other, especially bigger helicopter do not use the fenestron concept has to do with stability, mainly in hover. If you look at bigger helicopters like the Huey, Apache, any Sikorsky and so on you will notice the tail rotor is mounted on top of the vertical stabilizer. This brings the point of all forces for the thrust to the side (created by the tail rotor, that is what it is for) into the same level as the point of all forces for the thrust down (created by the main rotor, that is to lift the helo from the ground*). This guarantees me an equilibrium of this two main forces an the helicopter does not tilt to one side. It makes it easier to hover (somehow).

You can understand that it is not so easy to place a fenestron-concept on top of a vertical fin. But you are right. A fenestron is well protected not only against ground strike. On one of the Dolphins we had a huge bird strike on one of our fenesrton’s which took out one of 13 blades. The pilot only realised an increase of vibrations in flight. On a  conventional tail rotor system this bird would have taken out the whole tail rotor with all its consequences in this case. All we did was a structural fibre-glass repair on the shroud and put in the new blade, some balance, that was it.


 *) As everyone can see now, it is not the ugliness which is repelled from earth. Helicopters are NOT ugly.

Writing this I see the saluting smily! Thanks, Stef! 

[SkyKing]

Best training helo??

Robinson, Entrom, or Schweiser?? I've looked at getting my swing wing rating, but I'm not to keen on the Robinsons.  Especially since you need 100 hours PIC to carry a passanger (I think  )

You will never see me in a Robinson.  I like Enstroms, Bell and Hughes.  For that matter, I like the Rotorway Exec and the Scorpion II, and the Scorpion single-seater was a fun ride.

You can learn in anything. 

[SkyKing]

Is the learning curve steeper than an airplane? Or is it easier?

As much harder as it was to learn to fly after learning to drive, it's that hard to learn to fly rotors after learning fixed-wing.

That is, yes, it's harder but you pick it up fast and get comfortable soon.  However, if you are learning rotors, DO NOT FLY FIXED-WING during the process.  You want to develop your motor skills for the new way of doing things without confusing them with the other way.  Once you are comfortable with rotors, then you can go back and forth easily.

[SkyKing]

Can the rotor slice a tomato ? or something rougher like a stone ? and what happens when the main rotor shuts down ? and what happens when the tail-rotor shuts down ? 

No.  The leading edge of the rotor is just a smaller version of an airplane wing, rounded and blunt.  The trailing edge is sharper.  And hitting anything hard with the rotor, the rotor generally loses.  The real strength in a rotor blade is the long way, from the hub to tip.

[SkyKing]

Hi Frank, et al,

The wirecutters are precisely that: devices to cut wires (Electrical, telephonic, etc.). Those come handy when you are flying low and accidentally find them on your path. The devices cut the wires before they hit any vital part of the chopper like the rotor mast or get entangled on the skids.

Most helicopter types have them, although I'm not sure if they are mandatory. Mike? Roland? Can you guys help us out here?

They are not mandatory, and most helos don't have them, but they have become popular, with good reason.  Helos spend a lot of time near the ground, where wires may be found.  Cutters are even more popular on crop dusters.

[SkyKing]

Aha, but a cutter between the roof and the rotor? That would be extreme luck/skill if a wire came right between that and didn't hit the rotor uintil it came to the cutter right infront of the rotor-head wouldn't it? The nose-one is more understandable. Thanks for the info, one more thing though, what's a "Ag" aircraft?

Frank

Look at the shape of the helo.  A wire that hits the nose can only go two ways, down into the landing gear or skids, or up into the rotor mast. 

[SkyKing]

I can't see how a helicopter can "crash" because of ground resonance.

In order for a helicopter to get into ground resonance you need to be in contact with the ground to transfer the shock into the rotor system and then back into the skids and then back up and so on...
So there is an easy fix if you feel ground resonance coming on while touching down. Lift the ship back up!
The centrifucal forces will straighten the blades out again.

Think about this. Remember the Four Basics of Powered Flight: Stall, Spin, Crash and Burn -- I mean, Lift vs Weight, Thrust vs Drag.  In the case of a rotor, Thrust = Energy = Power.  Increasing lift increases drag, which requires more power.

Let's play the numbers in ground resonance.  Imagine that your helo weighs, say, 2000 lbs.  In order to lift off the ground, your rotors have to provide MORE than 2000 lbs of lift, to land you need less than that.  To fly you pump a lot of power into the rotors so that they turn fast, then increase lift by increasing pitch, getting light on the skids but not getting off the ground (say, 2100 lbs of lift).  This increases drag.  If you don't provide enough EXCESS power to the rotors, they will slow down because of the drag.  Slower rotors provide less lift, so your helo get heavy again (1800 lbs of lift).  When the weight transfers back onto the skids, drag on the rotors decreases, so they can speed up again.  They speed up, develop lift (2100 lbs), and your helo gets light again, just as the rotors slow down again from the increased drag (1800 lbs), and the cycle repeats.  After a few of these cycles (which can happen several times per second), the high (skids-light) curve can have enough power to lift the helo off the ground, and the low (skids-heavy) curve may come while the helo is still a couple of inches up, slamming the aircraft back down hard -- and slingshotting into the next cycle!  Another factor is the elasticity in the rotor system, both practical (flex of the blades) and virtual (mechanical tolerances of the control system and rotor-tilt).

If, as you suggest, you roll on more power to overcome the drag and take off, it takes time for the rotors to speed up (there is a MASSIVE amount of centrifugal force to oversome).  During that period, the rotors will pick up enough power to lift you several inches or even a couple of feet during the high curve (2200 lbs), so when the low curve arrives (1900 lbs) you have lot farther to fall (and hit the ground harder, even if you have more lift). 

Worse, the rotor disc will tilt one way or another, and the pilot has NO CONTROL over where it will go (remember, the cyclic only changes the angle of attack of the rotors, and is only effective when you have sufficient, even lift).  Thus, the impact point may only be one end of one skid, so the effect is like bouncing a football, or exceeding the weight-carrying capability of that part of the structure.

Instead of trying to take off, what you do is ROLL OFF power -- it's a lot less likely to slam into the ground when you're firmly sitting on it -- then zero your pitch and wait for the rotor disc to flatten out.  When you have full control again, keep your pitch zero, THEN add power, pumping enough energy into them to overcome the drag before you bring in any pitch. 

Whether taking off or landing, you want to cross through this portion of the lift range steadily and rapidly.

[SkyKing]

The NOTAR system hasn't really added safety in my mind. They are actually getting away from that again because it's sluggish and slow and doesn't have as much power as they thought it would have.
The only cool thing about it that you can't get your T/R hooked in anything (see Ted's post).

Actually, the really cool thing about NOTAR systems is that you eliminate the major failure points in the antitorque system.  No belts, shafts, clutches, swash controls or 4-foot rotating curb feelers.  The antitorque drive is part of the main transmission and all that goes back into the tail boom is air and the director control cables.

The sluggish feel is eliminated with the addition of a "booster fan" in the tail.  If the booster fails, you still have control, but with the booster turning you have a very high pressure into the director cone, right where you want it.

[SkyKing]

Ag planes have them as well.  I've heard of them being used more than once.

I'm here to tell ya!

[SkyKing]

The guy was a well liked news chopper guy, and he was a terrific military pilot from his Vietnam days, but I suspect he was rather ... hmm... arrogant... in terms of safety anyway.

Unfortunately, military training induces this.  In the same way that cops are generally not as accurate shooting as private citizens, military aviators are not as motivated as private pilots.  They are training in a compartmentalized environment, while the guy (or gal) who paid for flying lessons by fishing coins out of the piggy bank with a knife is far more likely to be familiar in detail with the aircraft.  Yeager mentioned in the first book that he was interested in how things work, while most of the other Army pilots he knew during WWII had the mindset that it was the mechanic's job to know that stuff.

[Ted_Stryker]

The guy was a well liked news chopper guy, and he was a terrific military pilot from his Vietnam days, but I suspect he was rather ... hmm... arrogant... in terms of safety anyway.

Unfortunately, military training induces this.  In the same way that cops are generally not as accurate shooting as private citizens, military aviators are not as motivated as private pilots.  They are training in a compartmentalized environment, while the guy (or gal) who paid for flying lessons by fishing coins out of the piggy bank with a knife is far more likely to be familiar in detail with the aircraft.  Yeager mentioned in the first book that he was interested in how things work, while most of the other Army pilots he knew during WWII had the mindset that it was the mechanic's job to know that stuff.

I would concur.  But then again, it's true of any such discipline.  If you have an active interest in it, one tends to excel at things.  Some things can be institutionally ingrained, as with some of the bad habits one can carry in with them from a prior intensive training experience, such as military aviation.

By the way, I like your posting name!  SkyKing was a great program! 

[Mike]

Say SkyKing, are you a rotor pilot?

I have to say, I have never heard anybody explain ground resonance the way you do in all my years of flying. I have to admit though that I never went deep into the physics and aerodynamics about the whole thing. However I gotta tell you, I have gotten into resonance a couple of times for all kinds of different reasons and it happened each time on the landing and everytime I pulled the ship back into the air and was fine. I have to mention there though that it was only the onset of resonance (the-skid-to-skid bounce) every time and I never let it go far. A couple of times it happened in a Schweizer after touching down when the student doesn't fall through with putting the collective down all the way and as soon as you do it goes away, so I agree with you there.
Your explanation also doesn't explain how the blades would get knocked together and shift the center of rotation away from the mast...

So can you elaborate on your theory a little?
I am eager to learn!

Also, how come you're preferring to be a guest instead of signing up. You're definetly invited !!

[Mike]

Actually, the really cool thing about NOTAR systems is that you eliminate the major failure points in the antitorque system.  No belts, shafts, clutches, swash controls or 4-foot rotating curb feelers.  The antitorque drive is part of the main transmission and all that goes back into the tail boom is air and the director control cables.

The sluggish feel is eliminated with the addition of a "booster fan" in the tail.  If the booster fails, you still have control, but with the booster turning you have a very high pressure into the director cone, right where you want it.

Have you ever flown a NOTAR?
If yes, please elaborate! I have only a few hours in them and I never see them anymore, especially not longlining and on fires....

[Frank N. O.]

I was wondering, how does the classic Bell 206 and 222/230 compare to helicopters like the Eurocopter 120 Colibri, Sikorsky S76 and Augusta 109? Are Bells like old Cessna and Pipers vs Lancairs and Cirrus' ?

Frank