non turbo charged and turbo charged aircraft

[aslan1982]

Am I right in think that a turbo charged aircraft would reach 2000 feet faster than a non turbocharged aircraft because

the turbocharged engine is simulating sea level pressure all the way up to 2000 feet so no power is lost

whereas the non turbocharged aircraft would be losing power while climbing to 2000 feet

thanks for opinions

[JUST-local]

A turbocharged aircraft will get to 2000 ft quicker than the same non-turbocharged aircraft because the turbo will be giving more than sea level pressure well above 2000 ft!

In practise is makes little difference from a sea level airport.

Having flown the non-turbo Arrow and the turbo Arrow, and the Seneca 1 non-turbo and II turbo from the same sea level airport.

The turbo aircraft will start to show the real benafits above 5000-6000 ft

JL.

[fly_antonov]

Your turbo is a compressor that allows your engine to process more air.
By processing more air and by injecting more fuel you can increase power on each engine cycle.

So yes, you will have more power for the same engine at any levels.
As Just-local said, the power benefit will make the most difference at higher density altitudes.

You must obviously compare the same aircraft with the same engines otherwise there is little point to it.

[Pilot DAR]

Your turbo is a compressor that allows your engine to process more air.
By processing more air and by injecting more fuel you can increase power on each engine cycle.

So yes, you will have more power for the same engine at any levels.

Well... That's the physics of it, but the reality can be somewhat different.

In many light aircraft engines, the role of the turbo is much more to maintain power up to the maximum permitted for that engine, as altitude increases, through a much higher altitude. Generally little use is made of the available increase in power at sea level. This is because most turbo installations were accomplished as changes to existing engine designs, which were already operating at the limits they were designed for. Increasing the maximum power output of existing horizoontally opposed aircraft engines has not been universally successful.

Most turbo systems employ mechanical means to assure that the pilot does not extract more than the maximum permitted power (which would usually be very close to the non-turbo version of the engine at sea level). A few engines do not have such a system, and there is high pilot work load during takeoff to set the power carefully. An "overboost" mistake can be damaging and expensive.

Turbo aircraft are great, if the normal operation for the aircraft is above 5000'. Other than that, turbo systems are heavy and expensive, and of little benefit near sea level.

[hvogt]

whereas the non turbocharged aircraft would be losing power while climbing to 2000 feet

Actually, the power output of a normally aspirated engine will initially increase during climb from sea level. This is because the back pressure on the exhaust side decreases, and that effect overcompensates the power loss due to decreasing air density.

[pistongone]

Pilot Dar is about right, the only thing i would add is that the term i have heard used is "turbo normalised" Which means maintaining sea level pressure at the manifold at higher altitudes. So basically what Pilot Dar said!

[IO540]

Am I right in think that a turbo charged aircraft would reach 2000 feet faster than a non turbocharged aircraft because

the turbocharged engine is simulating sea level pressure all the way up to 2000 feet so no power is lost

whereas the non turbocharged aircraft would be losing power while climbing to 2000 feet

Your theory is right.

However, ASSUMING you bolt a turbo normaliser (i.e. no increase in the max engine power) onto a plane, then climbing from 0 to 2000ft I doubt you would see any difference. Any half decent plane (of the kind which might have a turbo option) will climb at +1000fpm or so, minimum, so 2000ft comes up pretty fast.

If you said 15000ft then there is a significant difference - probably 1/2 the time.

Also the turbo normalised plane will have a ceiling of ~ 25000ft whereas the same but non-turbo one will have a ceiling of about 19-20000ft.

Actually, the power output of a normally aspirated engine will initially increase during climb from sea level. This is because the back pressure on the exhaust side decreases, and that effect overcompensates the power loss due to decreasing air density.

Do you have a reference, with some data, showing a power increase between a given IAS at sea level and the same IAS at some higher level?

The back pressure drop does improve matters but nowhere near enough to compensate for the MP drop. If it made that much difference, nobody would need a turbo

[tarmac-]

Quote:
Actually, the power output of a normally aspirated engine will initially increase during climb from sea level. This is because the back pressure on the exhaust side decreases, and that effect overcompensates the power loss due to decreasing air density.



I very much doubt this. Exhaust back pressure on aircraft is quite minimal compared to car engines etc, and even modifying the exhaust on a n/a engine for more power still yields very small gains. Higher alitude, less air, = leaner mixtures = less power. Exhaust plays a minimal role.

[Islander2]

Actually, the power output of a normally aspirated engine will initially increase during climb from sea level. This is because the back pressure on the exhaust side decreases, and that effect overcompensates the power loss due to decreasing air density.

As queried by IO540, I've also never seen any data to support that contention and, assuming the climb is undertaken at Wide Open Throttle, it seems highly improbable.

I very much doubt this. Exhaust back pressure on aircraft is quite minimal compared to car engines etc, and even modifying the exhaust on a n/a engine for more power still yields very small gains. Higher alitude, less air, = leaner mixtures = less power. Exhaust plays a minimal role.

But I don't believe that to be correct either! For a constant MP (together with constant RPM & peak-power mixture), altitude makes a significant difference to power output. Whilst this is actually the impact of lower temperature as well as reduced exhaust back pressure, it's quite wrong to suggest that the exhaust element contribution is minimal.

Some hard data for the power generated by a Continental IO-550 at 21"MP and 2100 RPM on an ISA day: sea level = 146 BHP, 8500 ft = 169 BHP. That's approx 2% increase per 1000 ft.

[tarmac-]

Yes your quite right, didnt take into account temperature.

[englishal]

The power output of a TC'd aircraft will increase with altitude -for example the seneca II is rated at 200hP at SL but at altitude 215HP which is why one can officially do an FAA High performance endorsement in it.

As mentioned there ae two sorts of turbo aircraft - Turbo normalized aeroplanes, like the T182T which maintain sea level MP up to altitude by varying the amount of pressure the waste-gate dumps. These can be flown like any other normally aspirated aeroplane and the throttle can be firewalled on take off and at low altitude would perform the same.

Then you have turbocharged aeroplanes, like the Rockwell Commander and Seneca II. These actually generate more than atmospheric manifold pressure at sea level, which means if you firewall the throttle on the runway you run a serious risk of over-boosting the engine. Normal take off in a Seneca II uses 39" of MP, and the commander can use up to 42" MP. I assume this increases performance even at low altitude. They do have wastegate valves to dump excess pressure if accidentally over-boosted but it depends on the speed of operation as to whether the engine will suffer any damage or not.

The DA42 actually uses over 60" of manifold pressure - not that you care because the FADEC takes care of it....

[Pilot DAR]

which means if you firewall the throttle on the runway you run a serious risk of over-boosting the engine.

Yes....

The turbo Cessna 207 I used to fly was such an arrangement. Though it had a wastegate, the flight manual conatined a warning that for the first takeoff of the day, great caution had to be used, as overboost was still possible.

While setting up the turbo following maintenance, the shop asked me to begin the takeoff roll normally, and at about 50 knots, for only an instant, firewall it, and note the manifold pressure, but certainly not let it dwell there. On the split manifold pressure/fuel flow gauge, I did see the manifold pressure well into the fuel flow side! It took several of those takeoffs to get the whole system adjusted, so it would no longer do that.

That plane was a homesick angel. After an (approved) overgross takeoff, with quite a bit of somewhat draggy external equipment mounted, and while generating 3 kilowatts of AC electricity all the way up, it still rocketed through 17,500 feet, the intended top of my climb. But, I'm glad I was not paying the bills for the turbo!

[IO540]

The thing is that if you have a wastegate set to increase the MP, all bets are off as regards high altitude performance.

I also don't think many turbocharged engines make TBO. Even comparing with a turbonormalised engine, the effect of the immediate power drop on a normally aspirated engine as one climbs prob99 has a huge effect on engine longevity, so even if one holds the CHTs etc steady, the maintenance of sea level MP for the next 10 or whatever minutes will still knacker the engine a lot faster than normal aspiration.

My memory may well be imperfect but I don't recall ever meeting a turbo pilot whose engine made the 2000hr TBO without cracked cylinders or some major work.

[eocvictim]

Very common over here in aus IO540. Sounds like the guys you speak of thrash the engines. Compensation for higher MP usually means a lower RPM which is far more beneficial for any reciprocating engine. Also it's not as though the turbo is just bolted on, the engines are designed to handle the increase in MP.

The problems are usually poor fuel management resulting in hot spots (always going to happen with any old clunker) and shock cooling. They need very careful leaning and gentle power adjustment. Even in the ausy climate most turbo'd engines are reduced by 1 in/2min. Once set the only time you touch the throttles is to land or go around.

[IO540]

Yes, could well be.

However,

Compensation for higher MP usually means a lower RPM which is far more beneficial for any reciprocating engine.

I have not seen evidence for this. Can you find any?

Lower RPM does result in very definitely better MPG - some 10% better between 2200 and my max of 2575. But that's not the same thing as a proof of longer engine life. Also, some engines have a limit on the MP at lower RPM values.

[englishal]

Ours has a TBO of 1800 hrs and it was over 1600 when we bought the aeroplane. As it needed a shock load inspection anyway we decided to zero time the thing too. We're also going to fit an EDM930 or similar to keep track of temps and shock cooling and set warning limits (i.e. MP) so I hope we get close to TBO next time as I'd rather not spend another £28,000 too soon!

Ironically you can buy a Turbo Normalizing kit for the Commander so that you don't overboost, BUT....it is not certified under EASA as clearly it is unsafe...right? (Come to think of it I don't think the EDM930 is certified under EASA either...all good reasons to change state of registry....)....