aircraft icing - metuae716/lecture-9.pdf · •far 25 app. c consists of 6 figures. •has been in...

Aircraft Icing“FAR 25, Appendix C charts”

Prof. Dr. Serkan ÖZGENDept. Aerospace Engineering, METU

Fall 2015

Outline

• FAR 25 and FAR 29– Appendix C charts• Using FAR 25 Appendix C charts• Liquid water content as a function of horizontal extent

and ambient temperature• Liquid water content as a function of horizontal extent

and droplet size• Alternative ways to document test data and compare

with Appendix C• Water catch rate (WCR) and total water catch (TWC)• Icing severity definitions• Variation of icing severity as a function of horizontal

extent and ambient temperature• Comparing test data with natural probabilities

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FAR 25 and FAR 29 Appendix C charts

• FAR 25 App. C consists of 6 figures.• Has been in use since 1964 for selecting values of

icing-related cloud variables for the design of in-flight ice protection systems for aircraft.

• First 3 figures are known as “continuousmaximum” conditions representing stratiform icingconditions or layer-type clouds.

• The last 3 figures are known as “intermittentmaximum” conditions representing convective orcumuliform clouds and icing conditions.

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• Traditionally, continuous maximum conditionshave been applied to airframe icing protection,

• Intermittent maximum conditions have beenapplied to engine ice protection.

• Figures 1 and 4 indicate the probable maximum(99%) value of cloud water concentration (liquidwater content – LWC) expected over a specifiedreference distance for a given temperature andrepresentative droplet size in the cloud.

• Reference distance: 17.4 nm (20 statute miles) forcontinuous maximum clouds,

• Reference distance: 2.6 nm (3 statute miles) forintermittent maximum clouds.

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• The actual drop size distribution (typically 1-30 microns) in clouds is represented by a singlevariable – droplet median volume diameter (MVD).

• Overall MVD≈15 microns in stratiform clouds,• Overall MVD≈19 microns in convective clouds.• The MVD has proven useful as a simple substitute

for the actual droplet size distributions in iceaccretion computations.

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Continuous maximum (stratiform) atmospheric icing conditions, Figure 1

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Intermittent maximum (cumuliform) atmospheric icing conditions, Figure 4

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Using FAR 25 Appendix C charts

• There is no comprehensive guide for using, interpretation and application of Appendix C.

• Design engineers typically select a conventionallyrecommended MVD and a temperatureappropriate to the flight level of concern and usethem to obtain the probable LWC from Figure 1 or4 of Appendix C.

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Using FAR 25 Appendix C chartsSelecting exposure distances (HE)

• LWC values obtained from Figure 1 or 4 are validonly for the reference distances of 17.4 nm or 2.6 nm, respectively.

• If there is a reason to design for a longer or shorterexposure distance, the LWC originally selected maybe reduced or increased by a factor obtained fromFigure 3 or 6 in Appendix C.

• Longer averaging distances will result in lowermaximum LWC.

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Using FAR 25 Appendix C chartsSelecting exposure distances (HE)

• Common applications:– To estimate ice buildup amounts on unprotected

surfaces during a long exposure of 100-200 miles. LWC obtained from Figure 1 is reduced by the factor obtainedfrom Figure 3.

– To estimate ice buildups on unprotected surfaces duringa 45 minute hold. LWC obtained from Figure 1 is used at full value, without reduction. This assumes the worstcase in which the holding pattern happens to be entirelywithin a 17.4 nm region of cloudiness containing themaximum probable LWC.

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Using FAR 25 Appendix C chartsSelecting MVD values

• Common applications:– For computing the impingement limits of droplets

(chordwise extent of ice accretion) on an airfoil an absolute droplet diameter of 40 microns is used.

– In general, MVD=20 microns is used for the computationof ice accretion amounts for standard exposure distance(17.4 nm) or longer.

– Another reference recommends the use of the entirerange of MVDs. The designer is advised to considerexposures to droplets with an MVD up to 40 micronsover distances up to 17.4 nm at least.

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Using FAR 25 Appendix C chartsDifficulties comparing with test data

• Users often wish to plot the points representingcombinations of LWC, MVD and temperature used in– Wet wind tunnel tests,– Computer simulations,– Test flights behind airborne spray tankers, – and test flights in natural icing conditions. on Figures 1 and 4.

• The problem is that these figures are valid only for thefixed averaging distances.

• A better way is to convert Figures 1 and 4 toequivalent, distance based envelopes where the LWC curves have already been adjusted for the distanceeffect.

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Continuous maximum LWCs converted todistance adjusted values

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Intermittent maximum LWCs converted todistance adjusted values

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Appendix C curves converted todistance based format (MVD=15m)

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LWC as a function of HE and Ta

(Continuous maximum, MVD=15μm)

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(Intermittent maximum, MVD=20μm)

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LWC as a function of HE and MVD (Continuous maximum, Ta=0oC)

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LWC as a function of HE and MVD (Intermittent maximum, Ta=0oC)

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The entire supercooled cloud database(660 icing events, 28 000 nm in icing conditions)

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Graphing flight data (texp=10 min, V=150knot)

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Graphing flight data (texp=10 min, V=150knot)

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Sample flight data compared with Appendix C Continuous maximum, Appendix C, MVD=15μm

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Sample flight data compared with Appendix C Continuous maximum, Appendix C, Ta=0oC

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Icing tunnel test points on Appendix C envelopesContinuous maximum

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Icing tunnel test points on Appendix C envelopesContinuous maximum, MVD=20μm, V=174 kt

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Water catch rate

• In some applications, such as in testing thermal anti-icing systems, the rate of water catch is important.

• For a given amount of LWC, the speed at which the aircraft flies through it and the droplet collection efficiency of the wing is important in determining how much heat is required to keep the leading edges at a required elevated temperature. Water catch rate is calculated from:

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LWCVWCR tot

Water catch rate

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Total water catch

• Another item of interest for an icing encounter may be the total amount of ice accreted on certain components, such as unprotected surfaces.

• Here, the rate of water (ice) accumulation may not beimportant, but rather the total water catch during the encounter(s).

• The TWC may be useful for estimating the weight of ice accreted on aircraft components, except for any losses due to shedding or melting. Total water catch is calculated from:

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averagetot LWCHETWC

Total water catch

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Acceptable Exposures

• What is an adequate exposure, or how much exposure is enough?

• This can be set in terms of TWC.

• Maximum TWC from the envelopes for a 17.4 nm exposureat the same temperature as the available icing conditionsduring the test flight can provide a reference.

• This can be used as the target TWC to be achieved duringthe test flight.

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Icing severity definitions

Icing severity Time expired for0.25” ice formation

Trace t > 1 hour

Light 15 min < t < 60 min

Moderate 5 min < t < 15 min

Severe t < 5 min

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• Test exposures can be reported based on whether theencounters correspond to trace, light, moderate or severe icing conditions.

• Icing severity can be calculated from:

r

a V

dt

dB

Continuous maximum, Appendix C converted toicing severity envelopes

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Sample icing intensity compared with cont. Max., Appendix C, converted to icing severity envelopes

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Comparing test data with natural probabilitiesThe differences between FAR 25 App. C and nature

• The envelopes in Appendix C do not show all the valuesthat can exist in nature.

• They also do not give information about the probability of encountering various LWCs, MVDs, temperature durationsin icing conditions.

• Only the probable maximum (99% percentile) values of LWC are shown. Designers of ice protection systems formilitary aircraft would like to consider lesser percentilevalues of LWC to accept more risk as a tradeoff againstextra weight, space and electrical power reqirements.

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Comparing test data with natural probabilitiesFlight tests and icing wind tunnel tests

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Natural 99% limits vs altitude for highesttemperatures available at the altitude

(MVD=15-20m)

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Natural probabilities for LWC averagesat altitudes < 2500 ft AGL

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Natural probabilities for LWC averagesat altitudes 5000 ft ± 2500 ft AGL

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Sample flight data compared with naturalprobabilities for LWC averages at altitudes< 2500 ft

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Natural HE limits and 99% LWC limits for differentMVDs in stratiform clouds at 0oC to -10oC

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Sample flight data compared with natural 99% LWC limits for different MVDs

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aircraft icing - metuae716/lecture-9.pdf · •far 25 app. c consists of 6 figures. •has been in...

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