Tackometer Reading

[badbud]

When trying to maintain an approach/landing speed of 120 KPH and a tachometer reading of 85% before throttling back and flair for touchdown, which pointer do you use........n1 or n2 to indicate 85%? Or at any level attitude, which pointer is the actual RPM percentage reading that I should be using?

BadBud

[doodenkoff]

Are you really landing at 120 kph? You should be touching wheels at something like 180.

[badbud]

Sorry..............meant 220!

BadBud

[Justin Case]

N1 is the rotation of the low pressure fan, which produces most of the thrust so use that one.

 

Edit: completely wrong, disregard!

Edited March 8, 2016 by Justin Case

[frumpy]

N1 is the rotation of the high pressure compressor :)

[Justin Case]

N1 is the rotation of the high pressure compressor :)

 

Well look at that, you're right. These strange eastern aircraft...

[gospadin]

When flying, they usually seem to follow each other within a few percent.

 

Under what conditions do N1 and N2 deviate the most from each other?

[badbud]

And the answer to my post is.........n2.

My worry was: that you can burn up your engine at or over 100% as the n1 pointer indicates when using full throttle. In my short tests in landing pattern doing touch and gos, I found that the n2 pointer stays at about 98% with full throttle.

BadBud

[GeorgeLKMT]

1 (high pressure compressor) is what you need to look at. Everytime you'll read operating limitations, you'll see percentage of HPC (1). During normal operations:

 

Nominal (continuous) is 103 ±1%

Maximum (20 min) is 106.8 ±1%

[WildBillKelsoe]

all thrust settings referred to in the DCS manual are in N1 HPC. This is what you need to follow.

 

The needles deviate the most below 80% N1 HPC IIRC

[RAZBAM_ELMO]

On the L39 C/ ZA the engine used has a bleed air that operates in between 75 and 85 percent HPC RPM so you will see it fluctuate rpm in this range so its normal and correctly modeled. The correct procedure i have found is for a long leg approach to use your airbrakes adn set the rpm just above 70% and use your airbrakes as you decend to lower your airspeed below 300ias. Then while managing rpm below 300ias you lower landing gear T/O flaps and then 100m short of the runway your LDG flaps. The second alternative is to keep engine rpm above 85% and come in for an overhead break to bleed airspeed that way then follow landing checklist again. But yeah dont keep the rpm in between 75 and 85 percent as the bleed air valve continually opens and closes.

[badbud]

The following is a response to my original question from a "real-world" pilot friend of mine that flies the real L-39C:

 

Always use N1.

 

At full power, N1 goes to 106.5% – a reflection of the Russian ability to make do. The tachometer was originally meant for a different airplane/different engine, but rather than relabel thousands of tachometers, they did the sensible thing (much like taking pencils on the space shuttle rather than as we did design a ballpoint pen that would work in zero gravity).

 

So, some numbers: 106.5% for takeoff. Within three minutes, reduce to 103%. Cruise at 95 – 100%. On approach, don't get below 80%. 82% with Gear and flaps down should get you about 4 m/s down, which will keep you on any glide slope. So vary the airspeed with nose position and the descent rate with power, moving back and forth between 78 to 85%. It takes a bit of practice, but becomes absolutely intuitive after a while. Remember, too fast in the landing configuration – lift the nose up a few degrees, then wait for the plane to settle down. Settling too fast? Add a little power.

 

BadBud

[some1]

You know, I don't deny your friend knowledge about L-39, but that sounds more like an urban legend to me. The thing with tachometer showing percents, is that it can be adjusted to show a particular RPM at 100% without the need to be "relabeled". And 100% in aviation can mean various things, in this case I believe it is max continuous RPM (103 and 106% settings have time restrictions).

 

taking pencils on the space shuttle rather than as we did design a ballpoint pen that would work in zero gravity

 

Another urban legend ;)

A pencil is made from graphite, graphite is a great conductor. A piece of graphite from a broken pencil floating in zero g inside a spacecraft can cause a short-circuit in the worst possible moment.

 

Also, graphite and wood are highly flammable in the pure oxygen atmosphere used on the American space craft. Russians started using more earth-like atmosphere much sooner in their spaceships.

[Ramsay]

And 100% in aviation can mean various things, in this case I believe it is max continuous RPM (103 and 106% settings have time restrictions).

103.2% has no time restriction, unless you can offer an alternative source?

From English L-39ZA manual, Engine basic parameters and limitations

 

Maximum - N1=106.8%, 17,600 RPM, 20min max (limitation - engine strength)

Rated - N1=103.2%, 17,000 RPM, unlimited

85% of Rated - N1=99.6%, 16,400 RPM, unlimited

idle-run - N1=56%, ? RPM, 30 minutes max / unlimited when in flight

Edited March 11, 2016 by Ramsay

[some1]

Yes, correct, it doesn't have the time limit (I mixed it with another airplane), but this is near the normally used cruise setting. Which is still quite good considering that in most jest aviation engines 100% N1 is not a meaningful number for the pilot.