Is there any airspeed limit for deploying a ram air turbine? Can airspeed be determined from the rate of its propeller rotation? Could that data be used to determine altitude?
Is there a maximum airspeed to deploy a ram air turbine?
The ram air turbine (RAT) can be extended and used at any speed above a minimum speed, because under this minimum speed, power is difficult to generate.
Generated power may be decreased below 130 knots, and extension is prevented below 100 knots because of additional drag produced for nearly no power obtained.
The RAT can be extended automatically (loss of engines or pressure) and manually, e.g. logic for RAT extension on A330 (note the inhibition by airspeed below 100 kt and by on-the-ground sensor:
Source: Airbus A330 FCOM - Hydraulics
Can airspeed be determined from the rate of its propeller rotation?
No, the RAT turns at constant speed to produce a constant AC frequency or hydraulic pressure or both. The rotational speed is stabilized by a governor which adjusts the angle of attack of the blades according to airspeed.
If the governor fails to maintain the speed, there is a security to prevent overspeed and blade damage/release.
Emergency Airplane RATs, Michael J. Zolidis, Hamilton Sundstrand, 2006.
A ram air turbine from Hamilton Sundstrand has a mechanical speed governor to vary the RAT blades angle according to the airspeed, and maintain a constant rotational speed.
At low speed, the blade angle is maintained near-perpendicular to the airflow to extract the most energy possible. When the ram speed increases, the angle is progressively reduced.
HS seems to measure the RAT power at 130 KEAS (Knots Equivalent Air Speed), e.g. the A380 RAT is 62” (1.57 m) diameter and delivers 85 HP (63 kW) at 130 KEAS. This mean at 130 KEAS or above, the full power is delivered.
The RAT auto-deploys when both engines fail, or manually from a panel overhead. It's prevented to deploy on the ground, and below 100 KCAS
The speed value is provided by the ADIRU.
The RAT is able to be extended in all flight envelope conditions.
- There is no maximum altitude or speed for RAT deployment
- Deployment occurs whenever AC buses are not powered or manually.
- RAT deployment takes 8 seconds.
- RAT has its full capacity at 130 KEAS or above.
- Below 130 KEAS, only the AC bus is powered. DC buses are powered by the batteries, as well as the standby AC bus through a DC/AC converter.
For ETOPS, three electrical power sources are required. If the RAT is one of them, it must be demonstrated as reliable (extension and generation).
The extension mustn't rely on engine power :-)
The most popularized RAT use may be for AC143 Gimli Glider, with a dual engine loss.
The rate of spin of RAT in the no-load condition is closely related to TAS, so by comparing this to IAS, you can calculate altitude, or at least density altitude.