Planes on carrier in rough weather

terry, you never cease to amaze me with you storehouse of knowledge.

 

Yea, gasoline is dangerous stuff. I sometimes wonder if it would be allowed if it was invented now. Most have little appreciation for just how dangerous it is.

Was gasoline hard on the motors? Reason I ask is as I recall the old Fairchild Providers had recip and small turbines and ran it all on avgas. I thought turbines didn't care what you fed them as long as it burned.

 

I've heard, several times, from different people that the J-47's on the B-36 were modified to use avgas, not jet fuel (?).

But I have never found anything in print that verifies that.

The jets on the B-36 were not used continuously, usually only for takeoff and fast and/or high bomb runs.

I know that the avgas in the B-36 had high lead content, for those engines.

 

For the recip's or the turbines? My understanding was avgas usually had high lead content anyway. Goes back to the old days when we though the more the merrier. Use good valves and they are better off with none as it turns out.

 

In the early jet age, JP-4 was commonly used in commercial flights leaving Puerto Rico because of its wide use by military units based on the island.

In 1963, a Pan Am 707 crashed in Maryland after being hit by lightning, which exploded JP-4 vapors in the left-hand wing tank. The lack of anti-static wicks on the trailing edges was considered a factor, but so was the volatility of JP-4, which was subsequently banned on commercial jetliners.

 

Jim- Affirmative, the Jet-A the civil industry uses is very similar to JP8, and we used them interchangeably if we had to stop at a civil airfield for fuel. Our F-111s had a credit card that was located in the fuel feed receptacle opening. Not used too often because it was expensive. Tulsa Turnaround had young females in hot pants doing the turns, so that was popular until banned by USAF.

Brian- The -1 for our F-111s mentioned gasoline as an emergency, one time use, and I think they had to pull the engines after that use. The TF-30s were really pushing the state of the art in the 60s when fitted to the F-111 and, later, the F-14, especially in terms of high temperature metallurgy. Not sure what was affected by the use of gasoline, but these were pretty sophisticated turbines for nearly 60 years ago. They were the first production low bypass, afterburning turbofans. By 1970, the TF-30 P9 in the F-111F was putting out 25,000 lbst, the same as early TF-100s fitted to F-15s and early F-16s.

 

I suppose it may be like a recip, the more highly tuned the fussier they are about fuel. I always thought gasoline was an expensive way to run a turbine anyway. I would imagine high output turbines, just like their piston counterparts are squishing the stuff coming in pretty hard and something as volatile as gas might be getting hotter than the hot section can tolerate. But then again maybe I'm full of it.

 

Maybe Solofast will provide some of his considerable insight on this subject.

Along those lines I'm guessing that the volatility can create either unstable/non-uniform burning, which can lead to localized overtemps. Seems the conditions in a turbine combustor are much more ripe for that than in an ICE. Maybe somewhat analogous to detonation in an ICE.

 

That is a pretty accurate description for detonation. Overstressing components is another side effect.

You use the term ICE. Is a gas turbine not a form of same? Sure seems like it to me.

 

The flash point of gasoline is much lower -45F VS Jet-A at 100F also gasoline has a lower BTU rating over Jet-A.

I am not sure but I would bet the reason for having to overhaul a TF-30 on Gasoline is due to the fact that there was more than likely erosion to the burner can from detonation of the fuel. Same thing happens to the pistons of a reciprocating engine if the compression is to high for the gasoline's octane rating. The TF-30 had a compression ratio of nearly 20:1 at this high of a pressure you are at the octane levels of the most exotic race fuels.

 

Should have said 'piston'.

 

J-47's.

I heard that it was so they could use the same fuel tanks as the piston engines.

 

I heard the same reasoning on the Providers.


On those I was told turbines were primarily for take off. Early versions were recip only and I guess with any load to speak of it took too much pavement for them to waddle into the air.

 

Yes, takeoff, full bomb load and full fuel (there was no air to air refueling then).

 

Gas turbines, as a basic engine don't care much what you put in them, they'll burn pretty much anything from natural gas down to bunker C fuel oil and stuff that doesn't burn worth a darn, like JP-7 and even coal dust. While the combustion system has to be set up for the fuel to achieve long life there is a pretty wide tolerance in what you can burn for a while in a gas turbine. Gas turbines meter fuel based on engine speed and turbine temperatures, so if a fuel is lighter in weight (and consequently energy content), then you just need to put more of it in to get the same temperatures, speeds and thrust. If you burn gasoline in a gas turbine you'll just have to pump in more of it than if you burn Jet A. Basically in a gas turbine you inject fuel into the primary zone of the combustor. In the combustor primary zone the air and fuel are burning at stochimetric conditions, so it's very hot, the fuel and air are burning very much like you have in the burner in your furnace and just about anything will burn there,.... once you get it started...

Starting is another story entirely and starting jet engines was a real issue in the old days before multistage fuel nozzles were developed. When you start a jet engine you spray fuel past a spark igniter a if the fuel isn't highly atomized it doesn't light off. Or when you finally get it lit you can have so much fuel puddled in the bottom of the combustor that you can get a hot start and burn things up. That is why JP-4 was initially developed and was a lot more volatile than later fuels. As igniter and fuel nozzle technology advanced less volatile fuels could be used. Some engines used to use a single starting nozzle and injected the fuel at high pressure past the igniter in a highly atomized mixture to get it to light off.. then they switched on the main fuel nozzles after they got the engine lit. The fuel used in the SR-71 is JP-7, which has very low volatility and is notoriously hard to get lit. To light off the J58 they injected tetraethel bromate that spontaneously ignites in the combustor to get the engine started.

In some jet engines fuel is used to actuate exhaust nozzles or cool other things so that you do not want to use volatile fuels as they could boil if they picked up too much heat, and in other cases the fuel could boil on its way to the fuel nozzle which would mess up fuel delivery and could make deposits in the fuel nozzle (which can happen anyway with the fuels that are supposed to be used)... And the fuel pumps need a certain amount of lubricity so fuels like gasoline that have very little lubricity aren't a good thing for most turbines. Finally, avgas has lead in it and lead can lead to stress corrosion problems in hot sections parts made of certain materials (which is why you don't even use a pencil to point at parts in hot sections, lest you get lead on it and cause a failure) so you don't want to run fuel with lead in it.

Fuels with lower volatility can be worse in that they tend to make "coke" or hard carbon combustion deposits. We had a lot of problems running JP-10 in missiles. JP-10 was developed to be heavier (have more mass per unit volume) because volume was limited in the missile and they wanted as much range as you could get. JP-10 was also notoriously hard to start because it didn't have the volatility of lighter fuels. Once you got it lit it was fine, but it was hell to get it started with an igniter. In the missile we used a flare that shot a 10 second a jet of burning magnesium and Teflon into the combustor and that lit it off without fail, as you might expect.. Of course in the test cell that was a pain, so we used a torch... that used hydrogen and compressed air!!! Hey, you gotta do what you gotta do...

Some engines do allow limited operation on gasoline, the Allison Model 250 had a allowance to run for short periods on gasoline but inspections and replacement of the fuel control could be required due to lubricity issues.

So while it isn't a good idea to run turbines on fuels they aren't supposed to run on, it isn't that they won't run just fine on those fuels for a while, they could have problems due to deposits, lubricity, vapor lock, or stress corrosion even if they ran fine for a while.

 

Aha, thanks.

How do you measure or calculate compression ratio in a gas turbine?

 

As I said, you can drown a turbine if you put in too much water, but that's pretty hard to do.

Turbines have excess air in the process and to drown them out you have to have so much water that you run out of air to burn. There could be other issues with compressor performance, or fuel control limits that could give you problems at high rates of water ingestion. The problem is that the manufacturer generally tests to the FAA limits and if you do that successfully the you are done. In the real world it is always possible to get into a storm that could significantly exceed that amount of water, and if you do that you're on your own.

Newer engines run hotter and have less excess air so there isn't as much margin (excess air) in the cycle, so it is easier to lose power if you put in very high quantities of water in a newer technology engine.

Part of the problem too is that in very high precipitation conditions you generally aren't running at high power, you are typically down closer to maneuvering speed. With newer engines that actually reduces the margin available for water ingestion as described below.

The older T-56 in the P-3 and C-130 were single spool machines. That is once you set the prop speed to 100% the engine is then running at 100% speed. Which means that at part power you have the same amount of air going into the engine as at full power, but you aren't heating it much, so you have huge amounts of excess air in the engine, and that leaves you with much more margin for water in that engine. The old single spool engines have a fundamental advantage in that regard.

The 2100 is a newer technology free turbine engine. That means at the prop speed is decoupled from the gas generator speed and you're running at lower gas generator speeds and it is easier to drown the engine since you've got less heat in the compression process that would vaporize the water, and less air in the engine and less air in the cycle. At high rates of water ingestion, your compressor discharge temperature drops down to the boiling temperature of water at that pressure. At that point additional water doesn't boil, but passes through the compressor as water and then into the combustor. At that point if the amount of water is so great it could cause power loss as the engine just doesn't have enough air to make the power desired. Another issue could be how the fuel control, which is a lot more sophisticated than the T56 control, could be is limited. There well could be limits in the fuel control that are related to speed and would not let the engine run because of the high amount of water in the engine.

Every engine is different in terms of how much water it can accept before it starts to lose power or get drowned out. The old single spool T-56 was extremely reliable and had no issues with ingesting horrific amounts of water without missing a beat. That the 2100 isn't as good as the T-56 isn't a surprise, in fact based on the configuration of the engine it's unlikely that it would be as good as the older single spool engine.

 

The pressure ratio of the engine is simply the pressure at the compressor discharge divided by atmospheric pressure. Pressure ratios vary all over the map from model airplane engines that run pressure ratios of 2 or 3 to one, up to large turbofan engines that run pressure ratios of over 30:1.

The hotter you run the engine the higher pressure ratios you want to optimize efficiency and the power available. If you run a high pressure ratio without high temperatures, you don't make as much power per unit airflow (and per unit engine weight), but generally you want as much pressure ratio as you can get.

Small turbojets run pressure ratios of around 5:1, helicopter engines run from about 8 to one up to about 15:1, and big turbofans run from 20 up to over 30 to one. which is why they they can be very efficient in big power generation systems.

 

Forgot to mention that the pressure ratio is based on ABSOLUTE pressure.. GT guys think in absolute pressure... as opposed to boost or gauge pressure.. except when you're calculating case pressure stress, which is based on gage pressure......

Anyway, a compressor pressure ratio of 4 would equate to gauge pressure at the compressor exit of 3 bars... hope that makes sense...

 

sf- Thanks for all the good info on turbine engines.

 

All this fancy talk scrambles my head. It's a very simple process. SUCK, SQUEEZE, BANG, BLOW.

 

So it has no equivalency to a CR in a piston or positive displacement engine. That's what I suspected. In a piston engine at cranking speeds the pressures are much higher than measured CR suggests and at running speeds much higher still.

 

It is similar in the sense that the more you compress, the greater the expansion you have and the more energy you extract from the gases

What you're seeing in compression test (if it were perfect and there was no leakage and the valves close perfectly at bottom dead center) is close to an adiabatic compression process. That is when you compress the gas in the cylinder the pressure is raised, and that process also creates heat (the work you put into compressing the gas). Since there isn't much time for the heat to get transferred to the cylinder walls the resulting measured pressure is significantly higher than just the dimple ratio of the volumes. The amount of heat created is pretty amazing, with a compression ratio of 18 the temperature is close to 1000F.

With both kinds of engines, you add heat after the compression process. With a constant volume process (like in a piston engine) the pressure goes up even further, because of the heating of the gasses in that closed volume. In a gas turbine you add heat, but the pressure is (essentially) the same as the compressor discharge. This is a key difference between the two engines.

There has been a lot of work lately to try to get that "extra pressure" from the combustion process into gas turbine, since would improve the efficiency of a turbine by a good bit. There have been some "pulse detonation" concepts that would add a "topping" cycle onto a turbine, but there hasn't been much real success at getting them to work. Rolls-Royce was working on some systems that were similar to the "comprex" supercharger, but these haven't worked all that well yet.

Think about it this way. For any internal combustion engine, the amount of energy you extract is directly related to the expansion ratio. If you expand the volume 20 times (like you do in a diesel) then you've extracted more energy than if you expand it 11 time (like in spark ignition engine). So the compressor pressure ratio in a gas turbine is directly related to the efficiency, just like it is in a piston engine. But because a piston engine is constant volume combustion the pressures are higher and the work extracted is higher for a similar pressure ratio. In order to get similar thermal efficiency you'd need a good bit higher pressure ratio in a gas turbine to get similar thermal efficiency. That's possible because the combustion process is continuous and pressure ratios in gas turbines of over 30 are possible.

 

Well we're usualy near the ocean so we wash planes every 14 days at home and every 7 days while on ship.

As for the video they would probably wash everything that was on deck after they got to better weather.

Heavy weather chains and wait it out.

The thing is even.when the water isnt credting the bow, airplanes on ship are always rusty.

Its why every squodron has a corrosion control shop with in the airframe shop.

 

Johnny- Very rough on weapons, too, especially air-to-air missiles. One reason why the Navy insisted on guns on their fighters during the early days of the AAMs. Failure rate was just too high.