Cowl induction for jet aircraft engines , is it possible?

The bold word is the key word here. Whatever happened with that 'speculation'?
Going by what I know about the laws of physics, oh and all the aviation classes (including aerospace propulsion) that I took while I was at Middle TN St. Univ studying for my degree in Aerospace admin, I am going to have to conclude that the air passing over and past the hood, and over the windshield, is going to, by Bernouilli's principle, suck the air that is gathered under that reverse cowl.
Air is coming in through the grille, at force when the car is going fast. That build up of air has two means of escape; underneath the car, and out through the back of the cowl. We can also conclude that the hot air is going to rise, and thus be forced out by means of incoming air, and by means of the aforementioned Bernoulli's Principle through the back of the reverse cowl.

It's not taboo, but anyone who is even slightly knowledgeable in physical science knows that when you change the direction of air/liquid flow, it slows down the flow and drains a bit more energy from the power source.
You are right, there are examples of curved piping designs; The Boeing 727 and the McDonnel Douglas L-1011. Both of those have S curved exhaust ductwork. They are both very successful airliners. But to say that they wouldn't be more fuel efficient had they had straight exhaust piping rather than the S shaped, would not be telling the truth.

Care to cite some examples? Better yet, cite some modern examples of jet-turbine engines(not turboprop or turboshafts) that are like that.

Uhhhh, you might want to do the same there bud, as you have no links, graphs or charts to back up what you stated either.

 

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cos; Thank you for thanking me!

 

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My dear BillyT;

Perhaps you are perplexingly unaware that the early 1970s Trans Am had one of the lowest drag coefficients of any car ever manufactured in America. The car was intentionally designed to be a winner of the sanctioned Trans Am races. If you somehow are confusing a 1973 Trans Am with a model T, then you are overripe for the funny farm. Correction: drastically overripe. Further correction: unbelievably past due.

Not that I would ever crave to embarrass you by pointing out to you your obvious ( possible ) ignorance of a subject about which you vociferate; but, there seems to be some confusion in your discussion re drag coefficients. To offer you correct instruction, purely because of helpful intentions for your benefit,: turning air 180 degrees is a contender for the BEST way to INCREASE the car's DRAG COEFFICIENT. You may be confused about the use of the DRAG COEFFICIENT. Correct use is this: when the number is large then there is a lot of drag. When the number is small then there is a little bit of drag.

The 1970s Trans am was first designed to have a low drag coefficient. Then, the wind tunnel tests showed a pressure rise in the vicinity of the windshield. Then, the pressure rise rise was exploited via the backward facing shaker hood scoop so as to provide a small but definite free supercharge effect to the horsepower output of the motor. These engineering measures were carefully tested and verified by wind tunnel tests in conjunction with motor dynamometer tests. Pontiac was determined to gain reputation by winning in the Trans Am races and so was not farting around. The rearward facing hood scoop really gave the cars an additional horsepower or else they would not have ever been made.

 

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Well, bud, uhhhh, I did not make posts with the intention of getting into a cat fight with a nit picker, so, I'm not going to do so now with you.

You seem to be oblivious to the obvious fact that I said nothing about airplane or jet engine efficiency in my post, so I am not going to respond to non sequitur comments.

As for jet engine designs, you are either trolling ( Is there a site rule against that? Is that rule ever enforced? ) or you are so ignorant that you do not know anything about the layouts beginning with the earliest, the Frank Whittle designs. Begin your education re jet engine air flow systems by doing your own homework and researching Whittle Power Jets.

Now, buzz off. I have important things to give attention to.

 

Translation:
Mike knows way more than I do about physics and internal combustion engines in general so I better make up some crap so I don't wind up looking foolish.

The laws of physics apply to both turbine and reciprocating engines. The air flow into both types of engines follow the same rules.

Ok. You'll be a bit more believable if/when you cite some examples.
I'm well aware of the engines you speak of. They are called centrifugal flow turbine engines.

http://www.solarnavigator.net/aviation_and_space_travel/jet_engine.htm
One problem with both of these early designs, which are called centrifugal-flow engines, was that the compressor worked by "throwing" (accelerating) air outward from the central intake to the outer periphery of the engine, where the air was then compressed by a divergent duct setup, converting its velocity into pressure. An advantage of this design was that it was already well understood, having been implemented in centrifugal superchargers. However, given the early technological limitations on the shaft speed of the engine, the compressor needed to have a very large diameter to produce the power required. A further disadvantage was that the air flow had to be "bent" to flow rearwards through the combustion section and to the turbine and tailpipe.
And like I said, when you change the direction of airflow, you lose energy and velocity.

What exactly are you trying to argue anyway? My whole argument is that when air flow changes direction, it loses energy and velocity. That is a physical law. You can't argue it.
Does that keep people from engineering mechanisms where airflow changes direction? Of course not. Just because the airflow does lose a bit of energy if it changes direction, does not prevent us from making successful and reliable mechanisms that to utilize airflow direction changes.

 

I hope for your sake that they are not ideas as silly as this one of thread you started. Several had several times explained why adding a reverse cowl intake is very bad idea. My first post admitted it was "possible" but went on to explain the physic as to why it is a stupid idea.

You have not only never thanked any of the posters trying to help you understand but now attack one - too much for me to remain silent about so I post again.

 

[Watching from behind the safety railings]

"My God, it's cruel to put so many trolls in one cage"

 

The most basic way of explaining it that i can think of is the fact that the Jet engine is meant to suck in air and push it out at high pressure out the back, the only reason your system works on cars is because the air is only being used for combustion.

 

I believe the pressure rise you are refering to is the one that occurs when air becomes turbulent at the bottom of the windshield. On the sharp upward corner formed between the windshield and hood of many cars (including the '73 Trans Am), a pocket of turbulent air pools there since the air cannot otherwise flow smoothly over that inside corner. I think this turbulent area does have a higher pressure, and is in fact a source of drag on the car, since pressure on a forward surface of the car pushes against forward motion.

It sounds like the Pontiac guys did the right thing in tapping what they could from this for extra airflow, but really all they did was exploit a flaw in the design: the sharp corner between windshield and hood. Notice that many high-end sports cars today and aerodynamic "green" vehicles like the Prius try instead to have the windshield flush with the hood of the car, eliminating that corner entierly.

So can a reverse scoop give a horsepower gain? Apparently, if engineered right, sure, but only if the car has this kind of aerodynamic flaw.

I wish I could find that image from my textbook that showed the turbulent region I'm describing... sorry bout that.

Also, Uno Hoo, try to treat us with some respect, even if others don't neccecarily return the favor.

 

Dear weed eater guy;

Last things first: From my very first post on this site, long ago and far away, I was very respectful to everyone. I did not ever get back such respect, but, rather, from ordinary members, staff, and favored special members, got back disrespect, insults, and grossly unfair treatment of even the worst kind possible on this site. With only a few welcome exceptions. I have become Pavlov trained to expect the worst from any encounter on this site until somehow proved different. If I have jumped the gun and unjustly offended you in particular , I humbly beg your forgiveness.

Anyone on this site or anywhere in the visible universe will be treated with respect by me if they give me mutual respect. Anyone who starts play when they wake up by assuming that they are superior to me or anyone else and who first hassles me can only expect to be hassled in return.

Now to interesting science stuff as you have raised in your post. I presented the 1973 Trans Am in particular because i bought one new and learned to genuinely appreciate it as a well designed and engineered car. It was presented here as the earliest example that my humble limited knowledge is aware of, of a rear facing air scoop as spoken of in the opening post. Several important points:

The rear of the hood was sealed against the firewall by a rubber gasket across the width of the car. This prevented appreciable flow of air out of the motor compartment upward onto the windshield.

The shaker hood scoop was open to air at the windshield region and composed of a sealed ductwork apparatus permitting air to flow pristinely from the windshield region to the carburetor.

The rear of the hood had a unique upturned quirk across its width which probably acted as a spoiler to initiate turbulent air at the base of the windshield. It is very important to recognize that subsonic aerodynamics in the range of speed of 150 MPH, more or less, has an important characteristic in that disturbance of airflow not only has effect behind a disturbing device, but, also, the disturbance is distributed in front of the device. Therefore, the shaker scoop, being a number of inches in advance of the base of the windshield, was nonetheless in a location able to tap any potential pressure build up.

The sanctioned Trans Am races were track races. Maneuvering for favorable position on the roadway is deemed a highly desirable ability of a racing car in a track race. For best maneuvering ability, a professional racing car must have the greatest amount of power and traction upon the roadway. Theoretical considerations of top speed, limited in one sense by ultimate possible drag coefficient, is important but is absolutely positively not greatest concern. So, while possible turbulent air at windshield might very well have increased car drag and be considered a flaw in terms of not getting best drag coefficient, it might also be considered an performance plus because it provided something like 20 horsepower extra at high speed, which could be of great importance to allow a driver to execute a favorable maneuver demanding acceleration ( which, of course was demanding of horsepower ).

By the late 1960s, when the Trans Am F body was conceptualized, enough was known about practical aerodynamics that the interplay between best drag coefficient versus getting 20 free horsepower was certainly a priori known, and the Pontiac engineers decided that it was better to give the racing driver extra 20 horsepower to maneuver with, than to get best drag coefficient for ultimate top speed of 191 MPH instead of 190 MPH with shaker hood. Watch NASCAR races on tv sometime, or, GO TO A TRACK and see a race. Maneuvering is most important. Top speed is over rated.

Now, as for application of car shaker hood concept to jet engines in their usual regimes of barely subsonic to supersonic speeds. The aerodynamic regime of 150 MPH, more or less, the home of the shaker hood, is totally alien to the aerodynamic regime of 650 MPH to 2450 MPH ( published top speed of SR71 Blackbird ), the home of the jet engine. At an aircraft speed of 650 MPH ( more or less ), there is no ( no, as in zero, zip, zilch, nada ) counterpart to the pressure rise happening in front of a protuberance acting as a spoiler. At high subsonic speeds there are already shock waves in all, except the most carefully engineered special cases, which could not possibly enable a rearward facing intake to enjoy any benefit of a helpful pressure rise.

If a very carefully engineered case were enacted, in the high subsonic speed regime, to provide a ( very small ) pressure rise to the jet engine inlet, it is certain that the aerodynamic drag of the spoiler would be of greater importance than the slightly increased power of the engine.

There are those who may not understand or appreciate the difference between what happens at the inlet of a jet engine versus what happens inside a jet engine. The airflow into the burner cans must be at subsonic speed. Supersonic shock waves into burner cans = flame out. The airflow from the compressor turbines into the burner cans must be subsonic and designed so that there are no local occurrences of supersonic shock wave. So, whatever it takes to slow the airflow to comfortably subsonic into the burner cans is what must be. An astute observer will notice that a typical burner can has a glow plug at one end. In current layouts the glow plug is at the "front" of the burner can. An astute observer will notice that airflow apertures are typically "behind" the glow plug. An astute observer will then discern that the airflow must come from the compressor turbines, from front to back, get to an aperture in the wall of the burner can, traverse an aperture in the wall of the burner can, then reverse course 180 degrees to approach the glow plug, get ignited by the glow plug, explode and travel back 180 degrees again and head for the exhaust pipe.

Yes, Virginia, this means that aerodynamic drag must happen to slow air to definitely locally subsonic speed, whether such slowing takes place around inlet, or, whether slowing happens inside engine around burner cans. During more than a half a century of developing jet engine technology, clever engineers, some of whom may have even been armchair physicists sitting and playing with their theories, and some of whom may even have been PhDs, have determined, by actual experiments, that is more efficient over all to slow air to subsonic locally inside the engine than to slow air to subsonic locally at inlet.

Bottom line:

Shaker hood cool trick on car.

Shaker hood no good on jet.

 

No Uno, you did get initial respect, as everyone else here does. The reason you eventually got no respect is that you apparently think you know everything about the physics of air flow. God for fucking bid that others here know a bit about it as well...so when myself and a couple of others corrected you on your design, YOU were the one that became disrespectful including comments that we did not know what we were talking about.
After that, how else did you think we were going to react??

Like I said, I was an Aerospace Admin major at MTSU. I know a thing or two about airflow and different types of internal combustion engines.
I used aviation examples of air flow beause, guess what? The same exact laws of physics apply to aviation engine air flow as it does with autmobile engine air flow.

Any modern functioning examples of the reverse cowl hood?

Uno, I do believe that we explained this to you, and why, on the first page of this thread.
It was then that you blatantly and openly refused our explanations. Then upon our responses to that is when you apparently got the indication somehow that we were not respectful.
You asked us if something would work on a jet engine, we told you why it wouldn't. You wouldn't accept that so that's how it turned out.
I don't understand how you thought any of us were initially disrespectful to you, but whatever.

 

Please, please. You're winding each other up! Where's this going to go?

"Oh, ok, you're right and I'm a douche" - I think not!

 

I was commenting on the '73 Trans Am that was mentioned, saying that, well, taking Uno's research as credible, it's apparently possible to get it to work, but I'm speculating that this involves exploiting an otherwise unwanted turbulence buildup.

Seriously, guns down everyone!

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Thread Summary (I hope):

First sentence of the OP is:

"Why can't jet aircraft engines have a reverse cowl induction as a few cars have used on their air intake systems? "

Several have explained why "Not a good idea." is the correct answer and others that when done on cars, it decreases the efficiency, but if collecting denser, partially stagnated, air for combustion can boost HP, that may be more important in races, especially if quick burst of power for maneuvering / passing, etc. is important. Then it may be an advantage to be less efficient.

In further answer to thread’s question:
I seriously doubt that a jet airplane could fly at even half its normal speed if forced to use only air that had be turned 180 degrees prior to entry into the engines. Turning the air INSIDE the engines need not cause any extra drag, just as lady seated inside the plane turning air 180 degrees with her fan does not increase the drag.

 

My dearly respected colleague mike;

Your post is a hopelessly garbled mixture of quotes of several member's posts, and misquoted references, even somehow including one of my own. I am always willing to take full responsibility for anything and everything I say, but you are unfairly attacking me and trying to start a fight regarding things which were said by persons other than myself.

I am totally disinterested in being sucked into the cat fight that you are trying to provoke.

Take enough time for the chemical imbalance in your metabolism to get back to normal equilibrium.

Take enough time to then figure out accurately who said what that you consider unjustly offensive.

Then, if you, in a lucid state of mind, will coherently and calmly explain how I, individually, have grieved you individually, state it accurately and plainly.

I will assess your assertion and either explain to you how you misunderstood some innocent comment of mine, or, if somehow you make a valid assertion, I will humbly apologize for any unjust insult i have provided to you.

Mike, just between two tight buddies like you and me; I have a secret to tell you: airflow at 150 MPH, involving NO supersonic airflow, either local or outright, is TOTALLY different than airflow at ( round about ) 650 MPH to 2450 MPH involving ubiquitous possibilities of local supersonic airflow at the least, and past the Mach, only outright supersonic airflow. Perhaps you flunked out before you learned that subsonic airflow works real different than supersonic airflow.

 

To orderly re summarize the summary:

In terms of the opening post, let us first look at the car.

The first example of a backward hood scoop that i am aware of is the 1970/1970 1/2 / 1971 ( depending on who's controversial opinion about model years one gives credibility to ) Pontiac Firebird Trans Am. The car was designed with strong intention to be winningly competitive in the sanctioned series of Trans Am track races. The famous shaker hood scoop was concocted to provide cool air at higher than normal atmospheric pressure to the motor, for a small but measurable boost in horsepower, compared to the boost provided by the already developed Cold Air Packages which employed "normal" forward facing scoops located near the front of the hood. It was considered, and proven in wind tunnel tests, that the shaker hood provided as much or more horsepower gain with less air drag, than the "normal" Formula scoops indigenous to the Cold Air Package options. It is very important to note and to remember that there were certain immutable parameters. One was the requirement that the car have a windshield. Any drag caused by the windshield would be there whether or not there was also a shaker scoop nearby. So, the wise decision was to have the lower hood drag of the shaker scoop scheme ( nixing the Formula scoops ) plus the slightly greater free supercharging benefit made by air piling up on the base of the windshield. Remember that in professional track racing, like NASCAR, to this very day, if the crew chief can find even one or two more horsepower, he will be sorely tempted to torture his sainted grandmother if necessary to get them.

Now for jet engines in jet planes. The need for a windshield has absolutely positively nothing to do directly with engine air intake. Period.

Air must be subsonic at entry into burner cans. Whatever intake geometry conniptions are necessary, they must be done to slow air down to subsonic, with no local supersonic excursions. Slowing air down to subsonic when the aircraft is going past the Mach means wasting efficiency to air drag within the intake tract and inside the engine.. But is is essential to keep the compressor turbines compressing and keep the burner cans burning. This is a kind of indirect counterpart to the car situation of having to have a windshield and its drag.

The simplest and aerodynamically neatest configuration is seen on every commercial jetliner. The engine intake is simply a round hole in the front of the engine housing. The engine housing has a minimal cross section width so as to have a minimal frontal area and therefore the lowest possible drag when calculated in conjunction with its drag coefficient. Past the lip of the housing, whatever geometric conniptions as are necessary to insure local subsonic airflow to the engine are done. Including the inevitable drag caused by reversing the airflow 180 degrees approaching, and /or inside, the burner cans.

If we design the engine housing to have an exterior protrusion in the form of a collar going round the original housing, in conjunction with a bullet nose at the very front, ( with a rearward facing opening ) which would catch oncoming air and stagnate it, we will suffer much greater drag due to increased cross section size of the housing. There will be a pressure build up, but this pressure will be no greater than the ram air pressure build up available with the normal open fronted housing. So, with such a scheme, we have greatly increased ( unnecessarily ) aircraft drag, but no increase in ram air to the engine. I respectfully disagree with properly esteemed colleague billyT re compressor turbine efficiency. However, I am not interested in further debating turbine efficiency in a scheme which is a doomed scheme because of the vastly greater aircraft drag caused by the hypothetical greater cross section width of the engine housing.

 

It is hard to disagree with me as I have not said anything about "turbine efficiency." In fact, do not think I have even used the word "turbine" in any earlier post in this thread!

I have said "engine" and for me, the turbines of a jet engine are only that part of the jet engine that compresses the air prior to combustion. (Someone else did mention that radial flow turbines were less efficient but cheaper. Perhaps Uno Hoo has confused me with them?)

I do not know much about the design of jet engine or their turbines but assume the turbines always work in a subsonic flow. I also think that some supersonic jets have a "spike like" extension* on the nose so that engines intakes, at least, if not all of the control surface flaps etc., are always in the post-shock-wave subsonic flow. (Control is more complex with surfaces in supersonic flow and design is further constrained because at times these same control surfaces must work in subsonic flow. I.e. it is best, if possible*, to have only subsonic flow over the control surfaces even is the plane is flying supersonically.)

Most posting here, and certainly me, think it very difficult and extremely inefficient to have 180 degree reversed air flow for the combustion air entering the jet engines AT EVEN HALF THE SPEED OF SOUND, so no rational person would even consider using that 180 reversed air flow pattern for capture of the combustion air used by a supersonic jet airplanes. (I have always assumed that the OP was asking ONLY about subsonic airplanes.) I think, but have not gone back over thread to check, that only Uno Hoo has even mentioned supersonic jets.

------------
*Often, I think, this is not possible in many supersonic fighter jets as the nose has a forward looking radar in it.

 

its not about rational people capturing air at 180 degrees, but rather that the concept wouldnt work to begin with, because air does not work that way.

 

My first post in this thread said something like: "It is possible , but not desirable." and then went on to explain why, I think.

 

your words quoting: "all of the natural described above compression be lost"

reiteration of your words: the plane will not fly, since there is no pressure difference, as a result of no compression