Thrust Margins !!!???

[Geoffersincornwall]

This one is for the Techies. :8 How do you define 'thrust margin' and how can I calculate it if there is no 'thrust margin' graph in the RFM.

Some offshore decks require a 5% TM and there is a graph in the Ops Manual but where did this come from? Would like to understand the issue a little better if you can oblige.

G

:ok:

[[email protected]]

Geoffers, the thrust margin figures are determined by the maufacturer and essentially entail loading up the aircraft to the point where it will just hover OGE at whatever engine limitation they specify (usually max contingency) and noting the AuM. To get a 5 % thrust margin, reduce that AuM by 5% and at that weight you will have a 5% thrust margin.

This is the procedure I am informed takes place to produce the graphs in our military ODMs and I presume that it is the same for civilian RFMs.

Some military documents specify a rate of climb (often 2-300 fpm) as an indicator of what a 5% TM should give you, others just state that you have the ability to manoeuvre in an OGE hover if you have a 5%TM.

Our graphs on the Sea King allow you to adjust this figure to allow for wind effect or wheel clearance and can also give zero or 10% TM figures.

Many pilots confuse % torque in hand as indicated on the torquemeter with a Thrust margin but the two are quite different.

[soggyboxers]

It's normally a figure based on the twin engine HOGE weight for take off from an offshore deck. HOGE is based on twin engine max continuous power and if you were e.g. pulling 100% OGE you would have no more left for vertical performance. If the hand of God sent a little puff of downward moving air or you attempted to move forward you would descend. So take the weight for your OGE figure at the appropriate OAT and height (=100% thrust) from the RFM HOGE graph. If this were e.g. 10,000 units, then 10,000 x 100/105 would give you a new weight of 9524 units. If you were hovering at this weight you would have a 5% thrust margin in reserve for your vertical performance.

[Geoffersincornwall]

So the issue seems to be "what limitations (engine/torque?) properly describe the appropriate OGE conditions?"

Is it Max Contingency or Max Continuous? Can be quite a difference.

Geoffers

:ok:

[[email protected]]

Geoffers, I would say Max contingency in a single (or a twin OEI) and max contiuous for a twin

[NickLappos]

The term "thrust margin" is a poor one, because thrust is wrongly calculated if one uses the WAT curve to calculate it. However, the concept of applying a 5% performance margin based on hover weight is certainly good practice, even if mis-labeled as "thrust". Simply look up the HOGE max weight, and subtract 5%, which then produces your "thrust margin" weight.

Something to note is that 5% of all-up HOGE weight is probably more like 12% of total useful load, and 20% of payload, based on a helo where useful load is 40% of all up mass, and half the useful load is fuel. This is a steep price to pay for a poorly designed helideck, where such power margins are needed to overcome the crappy design.

BTW, Max Thrust is the maximum force generated by the rotor under the atmospheric conditions, and is calculated based on rotor capabilities, not engine power limits, since kinetic energy is a higher source of power than the engines, and is easily put into play, as anyone who has done an autorotation can attest. Aerodynamic blade loading (Coefficient of thrust on solidity) is the best estimator.

[tyto]

what about if you have a SE failure while coming in for a decklanding??
no matter how much the thrust margin you cant Hover! it has to be a running landing....so is it that the term 'TM' is only for normal flying conditions????

[Shawn Coyle]

Thrust margin is a very poor term, as thrust equates to weight in a 1G situation. In strict terms, a thrust margin implies a weight margin.
The term only appears to take into consideration the ability of the rotor to produce thrust and does not take into account the ability of the engines and transmission to produce that thrust.
Unfortunately, the civil charts do not provide a satisfactory answer as they (nearly always) only calculate ability to hover, and do not take into account above spec engines.
Far better to have the military-style (at least US military as I know the UK military used to use thrust margin, which I don't think many understood) charts that have power required to hover and power available charts.
With those charts, you could get an idea of the power margin or weight margin for nearly any situation. Can't say the same for the other charts.

[[email protected]]

tyto - yes it refers to 'normal' operations - our graphs show the MAUM for OGE hover at various density altitudes and then give (weight) adjustments to apply to get different thrust margins. For the Sea King, the normal OGE graph gives the weight for a 5% TM and then you adjust it up or down as required.

It may not be the perfect solution but knowing you are not right at the edge of the flight envelope means you always have a little bit of performance in hand for the 'wife and kids' or just in case.

Various military ODMs (RFMs) have stated a 5% TM is equivalent to a 250 fpm rate of climb, the ability to deal with light turbulence or sufficient to allow manoeuvre in the OGE hover - I don't think any are an industry standard but it does give you some idea of what 5%TM equates to in the real world.

Interestingly we also have charts that predict Tq in the hover but they are very pessimistic and not representative of real world performance.

[Geoffersincornwall]

IMHO the best safety margin you can have when landing at a strange helideck (i.e., stepping into the unknown) or landing on one that is known to bite (i.e.. one where turbulence and/or turbine efflux causes loss of performance) is vertical performance in hand. The only graph that counts in the commercial world is therefore the HOGE data and this should be factored to take account of the dynamics of landing. HOGE is essentially a static condition. Factoring can be 2.5%, 5% or 10% but that's what is going to save your backside if you run into trouble.

Of course there are some who would put Cat A on the table for use in this context but they would probably be folk who are lucky enough to operate to the perfect offshore helidecks and have no dive-boats, barges, double (lit) flare booms, decks with unlit whip aerials on the edge of the OFS and missing deck or obstruction lights or other peculiar structures all of which have crews expecting their crew-change flight. There are times, as with HEMS and similar ops , where performance is not the highest risk factor for that landing. Hitting something you don't see during the latter stages of the landing because you were preoccupied with converting the 40knot approach speed to zero on a dark windless night peeing with rain and looking through that bit of the windshield that the wipers don't cover means that sometimes a little less speed and a little more 'slowly-does-it' is the right answer. That's when your HOGE = Thrust Margin is your saviour.

But in how many operations around the world do I see them just not bothering to worry about performance calculations? Too many!

G.

[Shawn Coyle]

crab:
"we also have charts that predict Tq in the hover but they are very pessimistic and not representative of real world performance."

How can the torque charts be pessimistic and the thrust margin charts be OK? What specifically is wrong with the torque charts?
A thrust margin chart based on density altitude is not going to take the effect of air temperature on engine performance into account.
At the risk of beating an ailing (but not dead) horse- power available from a turbine engine is dependent on pressure altitude and air temperature, (and not density altitude) and power required is based on density altitude.
-40°C and 9,000' PA is the same density altitude as +40°C and 2,000 PA, but the power available from a turbine engine in those two conditions is going to be wildly different (unless torque limited by something else).

[212man]

Shawn - what is density altitude if not pressure altitude corrected for temperature? PA is based on 1013.2 mb (or hectopascals as we are supposed to call them now) in a standard ICAO atmosphere with a lapse rate of 1.98 degrees C per 1000 ft - to get your DA you add or subtract 120 ft for every degree you are away from the ICAO figure for that altitude (add if you are warmer than ICAO subtract if colder).

Oh dear Crab!

a) I think Shawn has more than a passing familiarity with how to calculate DA from PA, and

b) I suggest you get into your Gas Turbine theory books again (perhaps after deleting your last post before too many people read it ;);))

[[email protected]]

You are right, I have re-read Shawn's post properly now and am no longer on the outside of several beers and nice red wine!

[HeliTester]

If the OGE hover weight is determined from the Flight Manual OGE WAT curve and then reduced by 5%, operating at the resulting weight will be the equivalent of operating with a 5% thrust (weight) margin for HOGE. This is likely the most accurate way to make the determination because the Flight Manual OGE WAT curve takes into account the effect of air temperature on both turbine engine power available and rotor power required.

It is true that -40°C and 9,000 ft PA produces the same density altitude (4000 ft) as +40°C and 1,000 ft, and that the engine power available is substantially different when operating at those two conditions. It is also true that the rotor power required will be different when operating at those two conditions due to the different blade tip Mach numbers (compressibility effect).

I note that the Flight Manuals of several modern helicopters (I think the Eurocopter Dauphin Series and the Sikorsky S-76 Series) provide power assurance procedures that quantify above specification engine performance and WAT curves formatted to take performance credit for those engines that produce above specification power.