Nacelle and Engine Bay Cooling and Ventilation

Part 2/3

ES3AERO's overview of the test content and parameters to be considered

Test content

In general, testing will involve operating at the predicted worst cases conditions for nacelle or engine bay ventilation and cooling. Worst cases can be identified using a combination of experience, judgement and the results of modelling and simulation.

Tests would normally take place on the hottest practical test day to allow extrapolation to the specified MHD (Max Hot Day). Minimum acceptable ambient temperature for testing should be agreed in advance. If extrapolation is required, the engine configuration may need to be adjusted. For example, it may be necessary to force open any coolers or ejectors that typically open/close with oil temperature to recreate the configuration that would exist if temperatures were that of a MHD. If ground tests are being used to de-risk flight tests then any weight-on-wheels logics used in scheduling of ejectors, etc., may need to be overridden. Similarly, a pre-test on a cooler day will help to de-risk instrumentation and understand general system performance before exposing the system to extreme environmental conditions or deploying the aircraft to a hot location.

The following parameters must be considered during the preparation and execution of a cooling and ventilation test campaign.

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Solar radiation over aircraft (Image: NASA Images)

– Stabilization criteria: The criteria at which a parameter is judged to be stable should be agreed with specialists in advance. A rate of change of less than 1°C/minute (or 2°F/minute) is usually considered stable

– Solar radiation readings are recommended to be taken during the test, while also taking into account the aircraft heading relative to the Sun position. Black (or any other color) plastic film can be applied over the aircraft skin to increase absorptivity properties as required. Special attention must be taken to avoid blocking any sensor during plastic film installation

– EGT/ITT correction may be required to account for engine deterioration that may result in increased exhaust gas temperature (EGT) and inter turbine temperature (ITT), causing higher engine bay temperatures. This effect needs to be accounted for when extrapolating temperature data to MHD to cover for the worst engine scenario. Since the test engine might not properly represent the AC 25-7D “Minimum Engine”, engine bleed can be used to achieve an EGT/ITT level equivalent to that found in a deteriorated engine. This needs to be defined prior to the cooling test between the airframer and the engine manufacturer

– Cooling climb profile: Worst case thermal conditions for civil transport aircraft are usually achieved by flying a cooling climb profile at ambient conditions as close as practical to MHD. The existing literature presents the following sequence as an example of a cooling climb:

1. Stabilization at ground idle maximizing bleed and power extraction on the test engine (1 Bleed/1 Pack, maximum IDG Load) followed by a single engine taxi out (usually a 1 mile taxi)
2. A max rated take-off followed by a simulated One Engine Inoperative (OEI) climb up to the maximum time allowed with OEI thrust rating (usually 10 minutes). Airspeeds and Rate of Climb (ROC) should be representative of those found during an OEI climb
3. A climb at Max Continuous Thrust rating with aircraft speed and rate of climb managed by adjusting the non-test engine thrust to simulate an OEI climb
4. In some cases, once temperatures have peaked or the ROC has reached a low value (around 150-200 feet per minute) or after a stabilization at a set altitude an All Engines Operating (AEO) climb profile is then flown up to a typical cruise altitude/Mach
5. A normal descent, approach and landing is then conducted, using max reverse thrust
6. Test engine shutdown after the minimum on-ground engine cool down is completed followed by static soak-back

– A static ground test is required for civil transport aircraft certification tests. This involves a stabilization at ground idle, 5 minutes at rated take-off thrust, the required cool down period at idle, followed by a soak-back test with the engine shutdown. Many engine bay components present its peak temperature during soak-back; this last can easily take 90 minutes or longer. Also, during the soak-back portion of the test, any nacelle or engine bay heat sources that can remain powered (e.g. the EEC) should remain powered on

– Wind should be avoided as much as practical due to their influence on cooling and ventilation during ground operations. During engine soak-back after shutdown, the test engine will be positioned on the downwind side of the aircraft to minimize any cooling effect

– Bleed, hydraulic and electrical off-takes: All off-takes should be configured to represent a worst case for heat generation into the nacelle or engine bay; this is usually maximum bleed off-take and power extraction loading

Additionally, a typical climb profile may not be the worst case for some systems (e.g. air cooled oil coolers). A high altitude low speed case may result in lower cooling than at typical climb / cruise speeds and may need to be assessed. Similarly an atypically light aircraft may achieve a higher altitude or be capable of flying at a slower speed at altitude and therefore these cases may also need to be considered.

Did you enjoy this overview of the Nacelle and Cooling and Ventilation test content? Visit Part 3/3 of this guide to discover ES3AERO's recommended practice and a useful checklist for this tests

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