# What are the pros/cons of ram air turbines?

Commercial aircraft are equipped with ram air turbines that can be deploy to provide power in case all other sources of power are lost. Ram air turbines have the benefit of providing power as long as the aircraft is moving through the air. However, aircraft are almost always designed with internal generators, driven off of the engines.

There is a limited case of adding generators to older aircraft. But they are also used to power large accessories, like the jamming pods on the EA-18 Growler. The EA-6B, which the Growler replaced, was also a modified attack aircraft using jamming pods with ram air turbines. Larger aircraft that are modified for the electronic warfare role don't seem to go this route though.

Obviously the turbine will add drag, and it seems like internal generators are more efficient, since almost all aircraft are designed to generate power that way.

So my question is: what are the benefits or drawbacks of using ram air turbines to generate power? While they are generally saved for emergencies, what makes them more useful in cases like jamming pods, compared to other options?

• One obvious drawback as the sole source of power is that they don't generate power when the aircraft isn't moving through the air. And presumably would generate very little at taxi speeds. – jamesqf Dec 7 '16 at 20:39

The EA-6B, which the Growler replaced, was also a modified attack aircraft using jamming pods with ram air turbines. Emphasis and links mine

If they were designing an electronic warfare aircraft from scratch, I'm sure that all the electronics would be powered by generators driven by the main engine(s). These engines would be sized to provide appropriate thrust for the expected mission profile and have excess capacity to provide power for the electronics.

However, since they are modifying existing aircraft for a new role they've found there is not enough excess capacity in the existing engines for the expected flight regimes and electricity generation, so dealing with the extra drag from the RAT is a cheaper and more expedient alternative than re-engining the the FA-18 (or the A-6 before it). Also, the cables needed to bring the power from the generator to the wing station must be added, and in tightly packed combat aircraft the space might not be there to add the wiring later. The RAT-driven pod only needs to be hooked up to a communication bus and brings its own power, so it can be designed once and used in a variety of aircraft. Multi-function displays in the cockpit just need a software upgrade, and the aircraft can perform reasonably well in its new role.

The E-3 Sentry (for example) is powered by the CFM56-2 producing 18,000-34,000 pounds of thrust while the 707 it was based on was powered by the P&W JT3D-3B produced only 18,000 pounds of thrust. Per Wikipedia, the E-3 used CFM56-2A-2 version producing 24,000 pounds of thrust, thus it had the power for its normal flight regime plus power to spare for all the extra electrical demands of its electronic warfare role.

• This is a good point and that's why I brought it up. I'd like to know a bit more detail... what might the limiting performance requirements be? How does it work out that added drag of a RAT is better than reduced power for more generator capacity? Larger aircraft tend to use generators so it seems to be something specific to the size and role. – fooot Mar 17 '16 at 15:38
• @fooot - Paging Dr. Kämpf. Dr. Kämpf to the red courtesy phone. The specifics on that seem to be much more up his alley than mine. You may have noticed that my answer was full of vague generalities - there was a reason for that... :) Though I appreciate that someone gave me an upvote! – FreeMan Mar 17 '16 at 15:47
• @fooot Just making presumptions here but to generate more power you'd first need a bigger generator. Then to drive the bigger generator will require more bleed air. As peter commented on my answer you'd have to accommodate the higher bleed air requirement with a more powerful compressor. To do that you'd probably have to either make the spools larger or add stages. I'd bet space in an F/A-18 is at a premium so you'd not only have to redesign the generator and the engine but possibly the airframe to fit it in. At some point you'd have a new plane. – TomMcW Mar 17 '16 at 19:03
• @TomMcW - that's exactly my point about the EA-18 & EA-6B - they were modified from existing platforms, and the engine upgrades necessary to provide the additional electrical power were deemed more expensive/time consuming/etc than the additional drag of "slapping on" a RAT for the power. – FreeMan Mar 17 '16 at 19:06
• Excellent addition, thank you Peter – FreeMan Mar 18 '16 at 2:40

You have kind of answered your own question. The down side to ram air turbines is they cause drag.

The up side is that they provide power outside the internal generating capacity of the aircraft (e.g., in emergencies when there is no power).

In the case a of a jamming pod, having its own power source eliminates/reduces the need to rely on internal power. That makes the pod modular. If the pod did not generate its own power, there would need to be a generator inside that sits there regardless of whether the pod were attached.

• I don't quite understand your last paragraph. Presumably the plane the pod is attached to already has a generator so having an "unused" generator sitting in the plane doesn't seem to be much of an issue since it's not unused. Conversely, having a generator in the pod that's unused when the pod isn't fitted hardly seems to be a problem, since the whole pod is unused when it's no fitted! So, presumbaly, the problem is that the plane's generator can't supply enough power for the pod and that a fuel-powered generator within the pod isn't feasible for other reasons. – David Richerby Mar 16 '16 at 17:29
• @DavidRicherby: Jamming needs lots of power. Your assumptions are correct, adding the capacity outright is not sensible and a RAT is a simple way to create the needed over. – Peter Kämpf Mar 16 '16 at 19:57
• How big is the benefit of not relying on an aircraft power source? It probably improves the aircraft when not carrying the pods, but pictures seem to show that the aircraft isn't often without them. – fooot Mar 17 '16 at 15:41
• @fooot, the advantage lies mainly in simplifying the conversion as they don't have to replace the engine generator and don't have to run additional wires to the wing hardpoints. – Jan Hudec Mar 18 '16 at 8:59

Another advantage to a RAT as opposed to using the engine powered generator is that it reduces the requirement for bleed air so the engines can be more efficient and responsive.

• Hmm - what is the more responsive engine good for when the RAT adds so much drag? It is really about the efficiency, because pulling all that power from the engine will shift its compressor far away from the design operating point. – Peter Kämpf Mar 16 '16 at 20:00
• @PeterKämpf I think that's what I'm trying to say. If you pull too much bleed air out the engine will run less efficiently. With a less efficient engine you have less reserve power. As far as overall efficiency of adding a RAT vs designing the engine around a bigger bleed requirement the later would seem to be more efficient. If the RAT was more efficient overall I think we'd see use of them on commercial aircraft. – TomMcW Mar 16 '16 at 20:54
• Some of the engine efficiency v RAT efficiency trade off is likely due to the fact that an existing combat aircraft (A6, FA18) is being converted into an electronic warfare aircraft. In its new role, it's probably much more economical and easier to add a RAT than re-engine the aircraft to provide the extra power for the new electronics. – FreeMan Mar 17 '16 at 14:37