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?
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5$\begingroup$ Why should there be vertical speed limit? The turbine only cares whether it is facing the relative wind, not how it is oriented. $\endgroup$– Jan HudecAug 23, 2015 at 11:13
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4$\begingroup$ I refuse to vote for close as I am hoping for it to be a typo as that question would be interesting if the horizontal speed is considered... $\endgroup$– Maverick283Aug 23, 2015 at 22:21
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1$\begingroup$ Does he mean the resistance of high wind to avoid the mechanics to drop it out? $\endgroup$– TesterMen TesterAug 24, 2015 at 6:37
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3$\begingroup$ Yes, this could be an interesting question if the OP clarifies a little about what he's looking for. I would assume that any answer (to the first part, at least) will be type-specific. $\endgroup$– reirabSep 1, 2015 at 3:29
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1$\begingroup$ Looks like a good question now. Hope someone comes by with a response or two. $\endgroup$– FreeManSep 2, 2015 at 15:46
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2 Answers
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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.
Uncommanded Ram Air Turbine (RAT) extensions - Airbus
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.
Embraer 190 - Airplane Operations Manual
- 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.
More:
- RAT article on Wikipedia.
- A320 RAT extension and retraction on Youtube.
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2$\begingroup$ @FreeMan, the first thing you want to do when your engines fail is to push down to accelerate to the best glide speed which is something like 205 knots at flaps 1+F and at least 220 knots when clean. The batteries have enough energy to keep ELAC1 and ECAM alive and the hydraulic accumulator has enough pressure to move the elevators to get you there. $\endgroup$ Sep 4, 2015 at 13:30
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2$\begingroup$ Batteries & hydraulic accumulator - makes much more sense now! Thanks! $\endgroup$– FreeManSep 4, 2015 at 13:32
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3$\begingroup$ @FreeMan, indeed. The RAT takes some time to deploy and spin up, so you need batteries to bridge the gap anyway. The reason for RAT is that batteries wouldn't have enough power for the around half an hour you can be gliding down from cruise altitude (Air Transat 236 glided for 29 minutes). $\endgroup$ Sep 4, 2015 at 13:45
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2$\begingroup$ @mins, APU has maximum altitude that is in most cases lower than typical cruise altitude and many of the all-engine-out cases were because of lack of fuel anyway. Windmilling engines can indeed produce some hydraulic power (the A320 training manual mentions it), but for jet engines it requires speed that is too high for efficient glide. $\endgroup$ Sep 4, 2015 at 15:16
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1$\begingroup$ There is also no maximum RAT deployment speed on the Embraer 170/190-series commercial jets. $\endgroup$ Sep 4, 2015 at 21:02
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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.
answered Sep 28, 2019 at 17:51
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1$\begingroup$ I'm not the downvoter, but rotational speed is commonly kept constant by a speed control governor, loaded or not. There is also a mechanism to prevent overspeed and blades damage/release in case of governor failure. If there are exceptions, which is always possible, please provide some reference. $\endgroup$– minsSep 30, 2019 at 19:01