Autorotation RPM change with temperature
 

Hello Ladies and Gentlemen,

What is the effect of temperature on Autorotation rpm?, I have consulted various tombs on FPC and from what i can deduce an increase in DA should cause a increase in ARPM.

The reason I ask is the certain type I have the dubious honor to fly (SH2) does not seem to conform to this rule. Despite the test flying schedule clearing saying this should be the case.

If this happened once I could have put it down to not being rigged correctly in the first place, but this seems to happens all the time.:ugh:

does anyone have any idea why an SH2's auto rpm decreases with a decrease in temperature?

Many thanks

Daver_777

Yes, and conversely, a decrease in DA will cause a decrease in RPM, so
you've answered your own question:ok::ok:

Fail
 

Of course it does, my mistake. I see my error, the last line should read does anyone know why an SH2 auto rpm decreases with an increase in temperature. i.e it does the opposite of what it should.

My Guess...
 

We are used to thinking in terms of an analogy to true airspeed increase with DA increase, but the flaw in that thinking is that in the case of TAS, we are dealing with the vehicle being driven by thrust through the working medium of the air.

In the case of autorotation and rotor RPM, however, we are dealing with the rotor being driven by the working fluid, i.e., the air.

When that working fluid becomes less dense, i.e., the DA increases, there is less energy imparted to the rotor's autorotational driving region in the autorotational descent. Therefore, for a given autorotational condition, i.e. airspeed, the autorotational rotor RPM will be less for high-DA air than it will be for low-DA (that is, denser) air.

What do you think?

Arismount
Good lateral vision there - I'll buy that :ok: - VFR

With a higher DA, you will have a higher ROD in auto. Aside from producing a greater airflow through the rotor disc, a higher ROD increases the inflow angle of this airflow, and produces a greater rotor driving force.

Daver - if your aircraft is failing its flight test schedule in terms of autorevs then there must be something wrong with the rigging. Have you checked with other operators and the manufacturer to confirm the test schedule is correct?

What combinations of PA and temp are you experiencing to calculate your DA?

We never got our SeaSprites up to an altitude where we worried about such things :)

arismount's post sums up the idea quite nicely, but if your FTS has it backward, perhaps there's a misprint?

The world is upside down there isn`t it,or do they hold the books that way...?

For the amount of time it's taken to get the SH2 program spun up down there, it's entirely possible that the basic laws of thermodynamics and physics have changed somewhat. ;)

DA and MR AUTO RPM
 

In auto rotation the angle of attack is (on average, forget for a moment the asymmetries of rotating blades) such that blade drag (plus some remaining gearbox torque=drag) needs to be compensated at a given lift=weight.

For most heli's in auto rotation that maintain correct RPM (see earlier thread on danger of low RPM retreating blade stall with associated blow back in auto rotation) this average angle is well below the optimal angle of attack, optimal defined as giving best lift/drag ratio (geometrically defined by tangent to lift/drag curve) with other words the lift at the normal speed/drag is low.

Making the air thinner (smaller density) will increase this angle because to produce the same lift at lower density the angle must increase. This makes the rotor work more efficiently, so higher RPM is possible because more efficient lift/drag ratio.

Reverse could be true if the heli has highly loaded blades which is the case when heavy and RPM is kept too low, but this is not a design point afaik.

The rigging specification takes TOW, DA, t° and humidity into account (everything influencing air density "ro").
It also needs COG because the body drag also influences the RPM, mostly more nose down will mean less body drag which means higher RPM.

Personally I use the memo technical aid "auto rpm does not like cold and dry" (=heavy air)

Hope that helps

Daver,
The aircraft maintenance manual should have a paragraph giving the parameters required to be observed and a graph to work them through. Different aircraft have different procedures, but the aim is still the same, the flight manual may also provide the procedures. If the auto revs are out of the limits it is normally a fairly simple adjustment by a licensed engineer (duplicate inspections will be required) followed by a re-check. It is better to carry out the check when the aircraft is light rather than heavy. Also make sure that your OAT indicator is correctly calibrated as well as other indicators. Most people will say that too many revs are better than too few, for obvious reasons.

Also, autorotation flat pitch settings checked and set in the high heat of summer, if not subsequently readjusted for denser winter air, may make minimum arpm unattainable - especially if light weight when engine failure occurs. :eek:

Autorotational RPM is based on gross weight and DA, an increase in either one will increase the RRPM during autorotation, and a decrease in either will of course decrease the A-RRPM. the aircraft "should" be set as the MX manual indicates. generally for smaller helicopters it would be the average DA for the area the helicopter operates(seasonal) and the weight would be dictated by the MX manual. the mx manual should also have an equation for how much increase or decrease is expected per 1000 feet of DA or 100 pounds GW for example... 1% RRPM per 1000Ft DA, or 100 GW. so a typical mx pilot would take the aircraft to the test flight area and enter autorotation and record RRPM. after landing he could then decide what the RRPM should be. if the baseline DA is sealevel and the grossweight is 4500 pounds, and the helicopter was actually at 2000 DA and 5000 pounds when the test pilot took his readings. it would be expected to see +5 RRPM for GW and +2 for DA. so the RPM on the mx check should have read 107% assuming 100% is the optimum. this would be the correct RPM and any other RPM would have required adjustment. so now if the average line pilot goes out and does an autorotation at 4000 DA and 4000 pounds GW he would see a +4 and a -4 equaling 100% with collective full down. even though he expected the RPM to be high due to the 4000ft DA, the Gross weight reduction pulled the RPM back down. wow that was a long post. hope it helps clarify. and disclaimer... this info only applies to one design as each will be mx manual dependent!

For the same AUW the autorotational RPM that is set in a high DA enviroment will be lower in a low DA enviroment.
Be safe, have the Autorotational RPM adjusted to ensure the minimum allowable rotor RPM (power OFF) can be attained at the lowest AUW and lowest DA you'd will be operating at.