Slowing down without gaining altitude

I've been practicing with the mi8 and I have a really tough time slowing down from 200+ kph.

Here is what happens:

I drop collective to keep my altitude around 40m off the deck, and slowly pitching up to slow down.

Eventually I get around 50 kph and I get some shaking - I check my vertical airspeed gauge and it shows i'm descending 2 m/s, I check the Doppler shift and it shows I'm climbing 1.5 m/s. The shaking gets worse but there's little indication that im entering a VRS, so I start to raise collective, next thing I know I've climbed to 100 meters and I need to do the approach again.

I must be misunderstanding something here, because when the instruments are accurate I'm able to control myself and not enter VRS, but when the mi8 is shaking, and both vertical speeds show I'm well above 3m/s descent, I have no idea what is going on - am I entering a VRS?

I have the same problem when flaring left or right - get shaking like I'm about to enter VRS, try to raise collective a tiny bit to compensate, then I rocket up 40 meters into the air.

Any tips?

That sounds as if you were just coming out of translational lift and not that you are about to enter VRS. VRS is only a threat if your airspeed is less than about 20 kph (iirc) - i.e. when you slowing down and thus leaving the translational lift region.

What I would try if I were you: keep the collective where it is a bit longer and the shaking should stop eventually. And as you are entering in-ground-effect, drop the collective a bit more. NOW be very carefull to not enter VRS ...

The mi8 shakes something horrible under 50kph. Nothing to worry about.

TJ

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It's either entering ox exiting translational lift and yes, it shakes and buffets quite a bit.

Also, keep note of the when exiting translational lift, e.g. on landing, add copious amounts of collective to not enter VRS and keep vertical speed stable. When you drop below 50km/h keep a close eye on your vertical speed and keep decelerating unti either a hover or touch-down.

As the other chaps have mentioned, the shaking always occurs as you exit transitional lift and is a superb indicator that; now is the time to take care and pay attention to the VVI as you further slow, VRS generally starts below 50 and I would guess around 40 - 30 depending weight. If you decelerate slowly you will see the VVI slowly drop, try to ride the drop down by allowing the chopper to fall but slowly increasing collective at the same time to maintain a manageable descent rate (it will be quite a bit of collective).

Sometimes she can catch you out with 2 VRS dips.

I saw a video on YouTube of a guy who bragged that he could (in DCS) stop the Mi-8 without any shaking. After watching the video, I was not impressed. He was basically throwing the airframe around in high-G maneuvers that would risk damaging the airframe and make for one painful ride for the passengers.

Many people seem to be of the impression that "vibration = VRS" when vibration is your friend. Vibration tells you when you enter translation, which is a que to add collective.

Also, don't watch the Doppler hover indicator when slowing down, just the VSI. Keep it above -3m/s and you'll be fine. The Doppler hover indicator is for when you're established in a stable hover.

And the perfect approach/landing is like the perfect hover. It "doesn't exist". You will always slip and rotate a bit when hovering, same thing with an approach. You can minimize the fluctuations, but chances are you'll never do away with them completely. Don't worry about it.

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Any vertical speen >4 m/s and airspeed < 50km/h is going to give you a VRS, reduce either one and the other will follow, it's called the dead-man's zone and you want to cross that zone as quickly as possible.

If you want to experience the VRS, set up a high hover for your self and slowly decrease collective until you reach > 4m/s descent speed, watch and learn how to recognize a VRS develop. After that, you'll be able to recognize it much easier and even differentiate a bit between entering/exiting translational lift and VRS.

IIRC, the shaking caused by loss of transitional lift is minimised if you decelerate through it gradually. I'm not at home to confirm at the moment though!

Any vertical speen >4 m/s and airspeed < 50km/h is going to give you a VRS, reduce either one and the other will follow, it's called the dead-man's zone and you want to cross that zone as quickly as possible.

The dead man zone refers to combinations of airspeed and altitude from which a safe autorotation cannot be made. It's nothing to do with VRS.

One thing I found, that helped me alot, and which I never heard mentioned is the blade pitch angle indicator.

I had LOTS of trouble stopping and landing the Mi-8. I watched lots and lots of videos and read through stuff. All good tips indeed.

But no one ever mentioned the blade pitch indicator.

Now, the gauge sits down left, on the Pilot/commanders console, and is not totally unlike a throttle for fixed wing aircraft.

When pulling the collective, it changes the blade pitch - not the actual throttle - as it is already set to full during the startup.

However - this is exacly why you do a hover check, before you take off.

When you take off - you are *supposed* to do a hover check. This is just to make sure that your helo can manage a hover, with the fuel quantity and loadout.

At this point, pay close attention to your blade picth.

If you do - you already know more or less exacly at what blade pitch your machine wll hover (in ground effect or not, if you raise out of it during the check) *without* transitional lift.

Now, of course, this will change as you burn fuel and expend munitions/unload cargo - but you will have a close idea.

For me, this works all the time.

After performing the hover check, I memorise my blade pitch angle at hover.

Then after flight, when approaching my landing/hover zone, I start to decelerate, and when the shaking starts, I know I am about to lose transitional lift - so just gently try to match the blade pitch of your hover check.

Easy as pie.

Sorry for a long *** reply, but yeah.

Hope it helps

IIRC, the shaking caused by loss of transitional lift is minimised if you decelerate through it gradually. I'm not at home to confirm at the moment though!

 

 

 

The dead man zone refers to combinations of airspeed and altitude from which a safe autorotation cannot be made. It's nothing to do with VRS.

 

I stand corrected! Thank you.

Regarding the angle indicator, good tip that I'll start using as well.

When it comes to the original question - what is the best way to slow down without gaining the altitude?

Unless dropping the collective to the floor, slowing down the Mi-8 in a straight flight takes a lot of space. Something around 2-3 km depending on the speed and altitude. And speaking about the altitude if I understand correctly before starting the approach and slowing down the altitude should be lowered to around 100m. What also helps (at least for me) is to follow the speed altitude rule from the rolling landing approach - maintaining speed 20 km/h more than the altitude.

However the fastest way to slow down seems to be doing the 90 or 180 degree turn. Like on the video, starting at 1:50:

A question regarding usage of collective during approach and landing. For some reason I remember that the collective shouldn't be dropped below 3 deg. I can't find however nothing in the manual. Is it a real rule or something I did invent?

I remember that the collective shouldn't be dropped below 3 deg. I can't find however nothing in the manual. Is it a real rule or something I did invent?

Sounds like you confused the 3m/s rule? You should not decend faster than 3m/s not to end up in VRS.

A question regarding usage of collective during approach and landing. For some reason I remember that the collective shouldn't be dropped below 3 deg. I can't find however nothing in the manual. Is it a real rule or something I did invent?

It's rather a technique than a procedure but it's quite right.

Dropping the collective below 3° will also drop engine RPM quite a bit. The big turbine engines of the Mi-8 take quite a while to spool up again, once there is a higher power demand.

So this is what can happen: you're approaching hot and high with 0° rotor pitch, then bleeding off energy. Coming into flight regime below 70-80km/h your power demand will increase drastically. You pull collective, but the engines can't supply the power demand, as they are in their low RPM range. Min rotor RPM drops and you're dead.

Engine spool up time is one reason why commercial jets require about 45% N1 minimum during final approach (below 500ft). This way, they can immediately perform a go around, if required.

I've once flown a low altitude go around, where the engines were already at idle. It took about 7 seconds, until I had go around power!

Thanks Viktor. What you write perfecltly makes sense. I know exactly what you're talking about. Especially with a heavielly loaded helicopter, dropping the collective down and trying to regain the power afterwards can make for some "exciting" wild bronco ride like moments :)

Such a low collective setting could put you into an auto rotative state also, which should be avoided.

I never have problems with slowing down with choppers anymore, the exit from transitional lift is visually seen but the drop into VRS is not.

If you are well into choppers in DCS and you see a solid future that will keep you playing, then I cannot recommend the vibrating jet seat enough.

As you exit transitional lift, then the jet seat instantly vibrates your butt and spine just like the visual cue you see, a slow bumping frequency.

This is not VRS.

Once you clear transition and continue to slow, a fast frequency vibration slowly grows through the jet seat and no visual cue is seen. The fast frequency vibration grows in amplitude as you drop....... this is VRS.

The jet seat simply moves you into a different league of feedback and reduces the need for your brain's requirement to concentrate on something that is now fundamental.

I think when I pulled the harrier through a tight right turn and the jetseat starting vibrating on the right side as right wing stalled.... then I started to think..... yeah this is great.

As I pulled the corner harder the jetseat vibration spread from right to left and she was moving into the stall and loosing velocity quickly.

I eased the joystick forward and the left vibrating wing slowly disappeared and the right stalling wing remained....the curve was cleaner and the speed maintained.

Once you use the awesome design of your body correctly, you will start to understand how easy it is to get completely absorbed into this game!