At the risk of adding to The Hell That Is the SF50 Thread, there was one point raised that I'd like to comment on. The FAR 23 requirement for failure-tolerant pressurization above FL 250 is not an insoluble barrier to a single engine: Rather than using engine bleed air for cabin pressure, use electric pumps. Size the battery to run them for longer than the time it takes to glide down in case of engine failure. We know this can be certified, it's the system on the 787, although they did it for efficiency rather than redundancy.

A knock-on effect is that higher altitude will help the efficiency of a SE jet even more than a twin. Meeting the 61-knot stall requirement makes the wing bigger than an equivalent twin, so the most efficient IAS is lower, so at "only" FL 250 an efficient power setting for the engine pushes it way faster than an efficient speed for the airframe. That's why the best-range speed for the SF50 is so much lower than high-speed cruise. Flying higher would bring those numbers closer together.

You can definitely do this, but it comes at a cost and complexity that greatly exceeds hanging a second engine on the airframe.

When you calculate the air compressor power you need, and the battery size required, and provide for TWO pumps (it has to be redundant, or show failure is improbable, which is unlikely), then it weighs as much as adding the second engine.

Some numbers: FJ33 can supply 50 lbs/min bleed air. That is 667 SCFM. You need that much flow with, say, about 10 PSI pressure increase. That works out to a compressor of about 16 horsepower. Two 16 HP turbine compressors are massive, let's say 50 lbs each, which would be an amazingly light one.

To run them for, say, 10 minutes, will require a battery of about 3 KWH capacity. The standard aircraft turbine starting battery (40AH, 24V) is 960 WH. You need 3 of them at 80 lbs each, 240 lbs of batteries. These have to be in addition to the main ship battery for avionics and other stuff (need 30 minutes of reserve power for that, so it has to be separate).

So we are at 350 lbs roughly. That's more than a PW610F weighs out right.

And, oh, you are going to have to test and replace those batteries every so often at SUBSTANTIAL cost. Go price a turbine starting battery sometime.

The 787 doesn't rely on batteries for pressurization (it has massive multiple generators on each engine, plus two on the APU, 6 total). They spend a LOT of money for their compressors, too.

You want to go to FL410? Get two engines. There is no twin penalty for jets and lots of advantages.

Mike C.

You can see why the 787 went with Li-Ion batteries, saves a bunch of weight -- roughly, at least half. And part of Boeing's calculation was that bleed air is a dead end while the whole world is working on improving electrics, especially batteries.

Anyway, I only point out that there exists a solution to the problem, I made no claims that you would find it preferable.

Not any more after they built the containment boxes to deal with the fires.

Lithium chemistries don't like to be cold, so a cold soaked battery at altitude may end up having far less capacity than at room temperature (one of the reasons EVs are not so good in cold climates). So the energy density advantage can quickly evaporate when used for pressurization backup.

There will be more 787 battery fires, that's just the nature of the Lithium Cobalt cells.

Mike C.

call me crazy but I thoroughly enjoyed the SF50 thread.....

I learned a lot and it was great to see the different perspectives.

I think all pressurisation should be electric. It's just a better way to handle it.

Today pressurisation:

1. Robs engine of power.
2. Toxicity.
3. Not well suited to power variations.

Take my Aerostar as a good example. At high altitude or where max diff is close, any power reduction will pop your ears. Not only that, the aircraft can not sustain high altitude flight at reduced power settings. One can forget 45%, 55% or sometimes even 65% power settings high up in any piston pressurised plane. Won't sustain the cabin.

By having it electric, reduced power cruise would be possible. It would also be much more comfortable. Descents and ascents irresponsible of power settings would now become manageable and pleasant. And in a twin, it would be dead easy to have redundancy in the form of two alternators that can individually feed an electric pressure pump. Don't need a battery that has 10 mins capacity - a bleed air piston or turbine twin doesn't have that redundancy either. Why ask for more of an electrical system when we're not asking it of current systems?

Lastly, bot not least. Air quality and toxicity. In all turbines they take compressed air before the combustion chamber and use that. They always say "it's impossible to have it contaminated, because it's upstream". Bulls*it. Anyone who's ridden in a turbine or jet will tell you that you can immediately tell when they power up and the smelly, oil misted, contaminated air comes into the cabin. Aerotoxic diseases are a reality and it has killed at least two (BA captain and FO):

http://www.dailymail.co.uk/news/article-2708365/BA-pilot-killed-toxic-fumes-flightdeck-constantly-exposed-fuel-leaks-board-passenger-jets.html

If it was so clean, then people like Peter Schiff Aero would not be in business. They are offering STC'd air systems that have the contaminated bleed air drive a turbo that then pressurises with fresh air. Not only that, it saves fuel, increases horsepower and lowers temp of the engine:

http://www.peterschiffaero.com

Electric pressurisation is the future and there could be a big dormant retrofit market for all piston singles and twins for the right person and the right business.

I'd add "4. Inefficient" The system has to be designed for the worst case, all the rest of the time it's too big. The bleed has to be enough for highest altitude, minimum power so at all other times it's compressed (and heated) more than needed and you're throwing away the power that went into that extra compression, and more power to cool it for use. An electric system just compresses only as much as needed.

This is the same logic that in cars has "electric, on demand" systems replace lots of "engine driven all the time" things like A/C compressors, power steering pumps, radiator fans, etc.

Use a ram air turbine to generate cabin pressure. Light weight, deploy as needed.
You can either have the ram turbine compress the air or generate power. Plus has the additional advantage of providing drag allowing for a steeper descent.

Tim

Adam,

thanks for trying to introduce reason to the certification complexity argument, but that's not how it works with the FAA.

Just look at the hoops our electronic panels are required to perform when the air-driven instruments aren't:

Old A/I driven from pressure/vacuum from both engines, or a single engine.

New Electronic instrument powered by alternators on 2 engines, or a dual alternator on single engine? built-in backup battery? dual AHRS? secondary display with additional backup battery? it goes..

Funny thing is that they'll let you take out the Altimeter and ASI, which are virtually fail-proof.

Adam,
Long response to your comment:

The only time my cabin pops my ears is close to the ground, where the differential is low (but not zero) and engine power is reduced (reducing MAP below ambient). With both engines running I have no problem maintaining max differential (for me 5.5psi at FL250. I have never had a pressure bump reducing power from Climb Power to cruise power.

You should find a chart in your POH that shows what cabin altitude can be maintained at what MAP. It shows how it requires less MAP to maintain a given cabin pressurization up high than down low, this is because MAP is absolute and ambient pressure is variable (depending on what your altitude is), cabin pressurization is a differential from ambient, so higher altitudes mean a higher cabin altitude/ lower cabin pressure (once max differential is reached).

Bottom line:
If your pressurization system can't easily maintain pressurization (differential) up high, you have a leak (or leaks) somewhere.

The nice folks at AAC pressure tested my plane up when they were installing the 5.5 pressurization upgrade (good to 30K (before the days of RVSM) IF I had the 700HP engines), they found (and fixed) lots of leaks. But even before, all the leaks were fixed, I had no problem maintaining 4.1-3 differential at all altitudes (with both motors running).

Hint on looking for leaks:

If the eyeballs stop blowing air up high, the problem is in the supply ducting, if they blow a lot of air at high differential, suspect a cabin leak.



For all those folks advocating (and flying) S/E planes that cost more than $250K and fly higher than 14,500', a challenge:

You pick the the start and finish airport (at least 100 miles apart).

We both take off and form up a mile apart (close formation flying isn't for me), on your call we each shut one engine down.

Last one to the destination buys lunch.



Forrest

Adam, none of what you wrote describes any turbine I have flown in. My experience is just the opposite. Perfect pressurization without any smells or ear-popping events ever.

I would kill for a set of 5.5 valves for the Duke..

Rocket Engineering was trying to get it certified, specially for the RTD which spends more time at FL270-280 than the rest of us at FL230-250. But it didn't happen, at least not yet.

But you have had it do that when you go to idle or even below cruise settings. It doens't have to be that way. With electric that would be a thing of the past.

Another thing is that we now rob a lot of pressure from the engines and then let an outflow valve regulate the cabin in a fast manner. In a well designed electric system, you could get rid of the outflow valve (or at least reduce it in size considerably) and have fast acting electric compressors regulate the inflow instead. Much more efficient.

I flew my 421C at FL270, at under 60% power, and maintained max diff without an issue.

The only time there was any ear popping is when the dummy pilot forgot to reset the cabin ALT.

Adam, your A* either had a lesser system or had issues...

Best,

Marcus you can get the valves and controller, you just have to relabel them for your mechanic