Engine tachometer readings.

[Fozzer]

Hello fellow aviators...

In my pretty little Cessna 152, fitted with an Avco Lycoming 0-235 air-cooled flat-four engine, the engine tachometer has a thin red line corresponding to 2,400 RPM.

The green area, (normal cruise), starts at 2,100 RPM and ends at 2,400 RPM.
The maximium revs at full power are 2,700 RPM.

Does this thin red line warn the pilot that the engine speed must not be continuously maintained in this small area, due to damaging, critical engine vibrations...?
Or are there other reasons...?

Many thanks...!

Cheers all...!

Paul.

[SilverFox441]

You got it right...it's a warning not to exceed.

There can be all the reasons you list, plus there can be things like prop limits, torque limits, cooling limits that all factor in. The engine has it's own physical limits, but depending on installation that limit might be slightly lower in any given A/C type.

[OTTOL]

I would have to see the Cessna manual but most manufacturers have what's known as a transient limit. The way that I see the markings as you state it; 2400 is your normal redline and 2700 would be your transient limit. Usually in real fixed pitch prop aircraft you wouldn't even reach redline at takeoff (the type of prop(cruise vs. climb)would be a big factor in this). In a an aircraft with a constant speed (variable pitch) prop and especially in a Supercharged aircraft, over-boosting and over-revving is a potential occurence on every takeoff. Very often the normal limits ARE exceeded while the pilot fine tunes the power and the props and engines settle down during the initial application of power.
The representation of power dynamics in FS is one of my biggest gripes.
As a CFI, a term that we often use when  discussing performance planning is "those were the numbers for the new aircraft".
Meaning when you plan a takeoff and the graph in the manual shows a takeoff distance of 1000feet required under certain conditions, you'd better throw in a couple hundred more for good measure. The reason being, these performance charts were developed using a NEW airplane. The airplane that you fly in the sim is that same NEW airplane. MS or the developer throw in the dynamics of new airplane that isn't down a few PSI on compression and doesn't have carbon in the cylinders.

Basically all of this verbal coleslaw that I'm throwing at you is a longwinded way of saying "the real airplane will probably never exceed 2400rpm".

[Fozzer]

Hi OTTOL...!
Thanks for the reply...
Maybe Microsoft/aircraft designers didn't get the tacho indications quite right as you suggest.
With a fixed pitch prop on my, (sim), Cessna 152, the revs reach 2,700 RPM on take-off and climb.
When reaching cruise altitude I back-off the throttle and re-trim, to cruise in the green, also weaken the mixture slightly when above 2,000 feet ASL.
I can fly at full throttle, against a head wind, or air racing, (2,700 RPM), at the expense of high fuel consumption.
This may stress the engine somewhat, but probably doesn't cause the engine to fail in the short term.
It's that little thin red line at 2,400 RPM in the Cessna tacho' which confuses me...?
On a real '152, maybe the load exerted by the fixed pitch prop on the engine in climb and cruise, prevents the revs from exceding 2,400 RPM...and only reaching 2.700 RPM in a dive, (disaster?)...!
On my motor bike everything from 10,500 to 12,500 RPM is is in RED and must not be entered at any time...!

On VP props I use the usual throttle/prop/mixture levers, manifold pressure, and EGT, and fuel flow, to keep everything in the green during the cruise... as you do...!

Cheers OTTOL...!

Paul

[XP1900]

with lycomings 2700 is the usually redline regardless of plane type.  that is not saying that you shouldn't go above 2700.  i am building a homebuilt and been reading about how to decide on diameter and pitch for the correct prop.   in my reading 2700rpm should be reached at cruising altitude at full throttle.  if you plan on cruising at 3000' that is where you want to have a prop with the correct pitch so that it will reach 2700.  this doesn't mean that it won't reach 2700 on the ground. there is a static rpm that you must have.  the static is on the ground the engine must reach a min rpm for takeoff at full throttle.  with the lycoming O-235 it is 2200rpm.  

[jknight8907]

In some planes, there is a small section of "no-fly-zone" marked on the tach with a red line or band, this indicates that the engine should not be run in this RPM range due to harmonic vibrations (among other things) between the engine/prop, and the airframe. Harmonic vibrations can cause the plane to literally come apart in flight.