Is it possible to loop or roll a 747 jet?

Omniscient: no problem. I was being simple because I don’t really feel like explaining a lot, or having to go to my aerodynamics books to look for the right formulas/equations. :slight_smile:

Drake: this babies are not designed with loops in mind, you know that. The control surfaces will really suffer and most probably won’t be able to provide enough control. And although there is a lot of inertia in a 747 at Mach 0.80, there’s also a lot of DRAG. Now, maybe I can work out all of the aerodynamics, if you want me too. It’ll take me sometime, but I might have an answer sometime soon (day or two).

I still think that the aircraft would stall before reaching 90°. But I’ll check it out.

If you have the tools handy and the free time, I’m sure everyone here would be interested in what you learn.

No offense taken :wink: EE by training, keyboard pounder by trade. More fun and $$ so I can pursue my hobbies: 172 owner/operator & Widgeon (Grumman G44A) Crew Chief, First Officer & First Mate. Sadly not the captain though :frowning:

Oblio:

I really don’t want to get into the mental gymnastics and correlating verbiage it takes to explain the flaws in Drakes reasoning and the general reason why it is not posible. For now I’ll add this: The momentum theory doesn’t work because looking at the motion of the plane and the fact that the momentum acts tangental to the motion of the plane and not in an angular capacity it doesn’t contribute in the necessary direction. Second the aircraft needs a thrust to weight ratio of at least 1 to have a prayer of finishing a loop. 747’s simply aren’t even close.

I realize this isn’t very persuasive to the truely inquizative, and I’ll share the fact that I am a year out of an Aeronautical engineering degree so my knowledge is pretty current to give you some confidence. I can assure you that the aircraft is definately structually capable of the manuvers in question, its a matter of available performance to do it. In the sake of fuel efficiency, they aren’t equiped with the right turbines.

If I have the time and inclination I’ll review Cecils treatment, and get some vitals on a 747 and attempt to explain my reasoning in a laymens terms. In the mean time, keep fighting about it. :slight_smile:


The facts expressed here belong to everybody, the opinions to me. The distinction is
yours to draw…

Omniscient; BAG

Many, in fact most aircraft that perform loops don’t have a thrust to weight ratio greater than one. I think that most aren’t even close to one. Obviously the more the better, but it’s not a prerequisite.

And momentum does play a part. An aircraft in a clean configuration takes a long time to slow down. After all, you don’t start a loop at near stall speed. You start with a lot of excess speed and trade the velocity for altitide and induced drag.

I think the proper thing to look at is how much drag there is at various speeds, altitudes, G loads and AoA. Those and gravity will determine how quickly the aircraft slows down. Can it maintain enough degress per second change in attitude to get well past 90 degrees before the airspeed is low enough that the controls become ineffective?

My initial gut reaction was no way. But the KC-135 is halfway there without hardly trying. It’s mostly about energy management. Though I do concede that a high-bypass turbine is not the engine of choice for an aerobatic aircraft :slight_smile:

This is probably too simplistic but in a pure vertical climb if the thrust to weight is greater that 1:1 then the aircraft will vertically accelerate to a velocity where the thrust (in excess of weight) is equal to drag. IIRC (from ground school) a aircraft in a established climb is doing so due to the excess thrust available and not due to addition lift from the wing. i.e. aircraft slows down, AoA increases to compensate for lower ‘v’ (weight = lift) but the thrust now has a vertical component and the aircraft goes up. Don’t know if this holds for all fixed wing aircraft though.

Oblio

Obilo, shame on you. This is first day of class stuff. You should recall that in straight and level flight Thrust=Drag, and Weight=Lift. If you increase thrust or lift or drag or weight, the aircraft will change its speed, altitude until the T=D, W=L statis is achieved. Now in a perfectly vertical climb, yes the plane will accelerate up until the T=W+D (The wings still generate lift, but in maintaining vertical flight the AOA will be forced to be neutral and the thrust not pointing directly up, but a quick vector diagram will explain) And lift can be ignored.

The point you overlooked is that by saying a plane in an established climb… what you meant is a plane in vertical flight. A standard climb occurs when thrust is increased which in turn increases speed and therefore lift. And lift is exactly what forces the climb. What you are describing is a pure power climb where the thrust is acting parallel to the direction of climb. And the trim is set to negate the effect of lift. The plane doesn’t slow down (ignoring ambient pressure change) it slows accelerating, and reaches a constant speed.

The act of a loop occurs when the thrust is greater than the weight, and then you pitch the plane into vertical flight. As the plane reaches vertical you do not compensate for the lift and the lift is what carries the plane thru the vertical plane and back to the inverted. The lift then transfers from a horizontal direction acting in a positive z-body direction (up relative to the pilot) to a inverted flight where the lift is acting in the negative z-body direction.

Drake, a plane with a less than 1:1 T:W ratio may be able to reach vertical and begin inverting, but the thrust will fail and the plane will basicall fall. The momentum may carry the nose over the back as opposed to the front and it may complete a pseudo-loop, but this is not a true loop. It is just a forced stall where the nose flops over the wrong way. A 747 likely would reach vertical and do a tail slide and fall inverted hopefully to be saved by a skilled pilot. This is not a loop.

Obilo, if in trimmed flight, you increase the thrust the plane will climb and the AOA will stay the same. It Climbs because the lift generated at a higher speed is greater the climb stops where the air pressure drops to a point where the L=W again. I am not quite sure how you are trying to explain the dynamics of a roll, but you are incorrectly using he term lift. You do increase alt. by increasing speed, but when you say increasing lift by the wing, you should say increasing the back pressure on the yoke and in turn increasing AOA. Increasing speed directly increases lift, increasing pitch slows the plane down (except at stall speeds). Becareful to not use lift and pitch interchangably.

This is not any critisism, but it is amazing how little aerodynamics your basic private pilot knows (I have no experience in commercial training). I had to explain most of the actions occuring in flight to my instructor who had over 4000 hours. I was constantly correcting my ground school teacher. He would describe things in oversimplifed ways and negelct any special cases. Depending on the segment discussed he would explain things that worked in practice, but contradicted some other shortcut he explained.

I got off on a tangent here, back to the discussion, i hope this is cohesive.

Omniscient, you’re probably right. It probably wouldn’t be a true loop. Still, if it could get to, say, 150 degrees before it stalled, maybe we could call that close enough? :slight_smile:

While a turbofan isn’t the best engine for the job, I’ve noticed on a few lightly loaded flights how I got pushed back in the seat when the pilot pushed the throttle forward. There’s quite a bit of power there that should be able to get the plane well past 90 degrees.

I dug up some important numbers:

Boeing 747-400 Empty weight: 398,780 lbs
General Electric CF6-80C2 max thrust at Sea level: 61,500 lbs.

4 engines: 61,500x4=246,000 lbs.

Thrust to weight Ratio T:W=246,000:398,780, T:W=.616

That is still alot of thrust, and I suspect that it would be enough to carry an empty 747-400 thru 150 degrees and make a convincing loop happen. The problem is that this Empty weight doesn’t include fuel at over 57,000 gal (6 lbs/gal), 342,000 lbs total, and another 135,000 lbs of passengers and cargo for a total GTOW of up to 875,000 lbs. You’ll see that a loop in any type of standard flight is pretty unlikely, but if Boeing specifically configured a plane to try it, I suspect that it could happen.

Thanks Omniscient!

Omniscient, you’re losing me a bit on this. Could you explain once more why an aircraft needs a 1:1 thrust/weight ratio to perform a ‘true’ loop? Is your definition of a true loop one in which the wing never stalls? Or one which maintains a positive ‘G’ loading all the way around?

I personally believe that thrust has nothing to do with it. Energy is what matters, and it doesn’t matter where it comes from. Aerobatic gliders do ‘true loops’ all day long. Bob Hoover’s Aero Commander has nowhere near a 1:1 thrust/weight ratio, and he does loops with a glass of water sitting on the glareshield.

The whole issue is one of energy management. The issue should boil down to whether a B-747 can accelerate to enough airspeed to give it the energy required to carry it through the manoever without overstressing the airframe or hitting a Mach limit, and whether the airframe is clean enough to hold the energy at that speed without bleeding it off to aerodynamic drag.

Thrust comes into play if the airplane physically can’t fly fast enough in a dive to build up that energy. Then it will need thrust from the engines to add energy to the manoever to carry it through. How much thrust is required is unknown at this point.

Thrust is absolutely critical, if there was a string attached to the top of the plane and a axis in the middle, then yeah momentum would help, but with out that string to act as a force vector, momentum is insufficient, and nonexistant.

I’ll assume your talking about kinetic energy, being that the plane is climbing potential energy is negative. I’ll say this as simply as i can. The plane goes from north to flying south (or any other 180 degree turn). The kinetic energy is in the initial direction, the idea that the plane could change direction 180 degrees and non lose its momentum is foolish.


The facts expressed here belong to everybody, the opinions to me. The distinction is
yours to draw…

Omniscient; BAG

I think dhanson stated the issues very well. He isn’t saying it won’t lose momentum. He’s questioning how much momentum would be lost. An airliner is pretty clean, aerodynamically.

I think the question really is, how much drag is induced by various pitch rates, ie G loads. Of course, that number varies on speed, too. The pitch rate(s) would tell us how long the plane would be climbing in the first half of the loop. And from that we can figure out how much speed is traded for altitude. And also how high it gets, which also affects several of these parameters.

If the induced drag is low enough at reasonable pitch rates then the plane will be able to perform the loop. If one must haul back on the stick hard in order to get the nose over before getting too high then the engines might not be able to provide the extra uumph needed.

Once again, remember Bob Hoover’s Aero Commander. If he yanked the plane around he would burn all of his energy in a few seconds. Instead he keeps his pitch/turn rates low enough so that he doesn’t cause a lot of induced drag from his wings. He can perform aerobatics without engines for minutes. That’s energy management. That’s the key.

Can a lightly loaded 747 pitch over quickly enough without burning its energy reserve? There’s probably a pitch rate where the drag goes up dramatically. If that rate is only a couple of degrees per second then the plane would use up all of its energy climbing. If the rate is more like 20 degrees per second then that gets the plane to 180 degrees in only 9 seconds. The “spike” is probably somewhere in between.

Omniscient, please answer these two simple questions:

  1. Can a Glider perform a ‘True loop’?
  2. If so, why?

I’m a pilot who has performed loops in airplanes, and I can state categorically that thrust is not required. In fact, even in powered airplanes some looping manoevers are performed with the power pulled back to idle.

I’m having a hard time understanding how a graduate in Aeronautical engineering wouldn’t know this, so I’m assuming that you are using some special definition of ‘true loop’ that escapes me

And just in case you didn’t know, a loop in a glider is performed without the wing ever stalling, and with a positive G loading on the aircraft all the way through the manoever. The loop is also completely circular if performed right.

In an exceptionally clean glider, the airplane may lose so little energy during the manoever that at the completion of the loop it will only lose 50-100 feet of altitude.

Basically my definition of a True loop was ones where the aircraft never eft powered/controlled flight. So this says things about the glider, being that it is merely falling gracefully and not flying by defintion. Gliders are uniquely balanced and could not be flown with an engine as is, nor could a plane glide for very long with engines off. These differences are why it is so easy to “loop” a glider and a plane take much more thrust. A plane is balanced to have very positive stability and therefore requires a large amount of input energy to break that stable flight (which an unpowered loop is). A glider on the other hand is designed on the threshhold of dynamic instabiliy and therefore tend to be easy to force into unstable flight. Comparing a 747 to a glider is not useful, they are too different, you’d be much more accurate to compare a 747 and a Cessna 182. They are balanced much more simalarly, and that is what limits the manuverablity in flight.


The facts expressed here belong to everybody, the opinions to me. The distinction is
yours to draw…

Omniscient; BAG

Another thing, you keep talking baout a clean aircraft, and that indicates a low parasite drag. In the loop the parasite drag causes a negligable speed loss compared to the induced drag.

Name one powered aircraft that can loop with the power out? Also, I’d love to know what aircraft you are current on. You seem so certain of the dynamics that i spend 4 years of my life learning, yet you seem so vague on the proper terminology of the activities occuring. It confuses me.


The facts expressed here belong to everybody, the opinions to me. The distinction is
yours to draw…

Omniscient; BAG

Lastly, could someone give a link to anything concerning Bob Hoover. I don’t know what he did, and never saw any programing about him. I want to see what type of plane he’s flying.

And now you are making me wonder about your credibility. You are making a claim that a glider is unstable, and therefore a loop in a glider is not performed in ‘controlled flight’? There are so many things wrong with this statement that I don’t know where to begin.

First of all, there is nothing about a glider that says it has to have less stability than a powered airplane. Some gliders are VERY stable. Many powered aircraft are designed with very low dynamic stability (an Extra 300 comes to mind). Some gliders are also powered aircraft, and stow the propeller when the engine is shut down to improve glide capability. Basically, the only universal difference you will find between a glider and a powered aircraft is that a glider is, well, unpowered.

You want me to name a powered aircraft that can loop with its engine shut down? Okay, a Rockwell Aero Commander Shrike. This is a business twin, with I believe two Continental IO-540 engines. Bob Hoover shuts both engines down, feathers the props, and loops the airplane. I’ve seen him do it on several occasions.

What airplanes am I current on? Well, since there are no specific currency requirements for light aircraft I could simply list the airplanes I have been checked out on and flown as pilot in command in the last few years. They include a Grumman AA1 which I owned for 6 years, A Glasair III, Mooney M20, all sorts of Cessnas and Pipers, A Stinson 108, Maule M5-210C, and probably some others I am forgetting about.

I also studied physics in university and used to be a ground school instructor.

I don’t know where you can find information about Bob Hoover, but I suggest you try doing a web seach. You might also look up Manfred Von Radius, who is an airshow performer who does an entire aerobatic routine in a glider including loops, rolls, Split-S’s, wingovers, etc.

Dan

You might consider this when thinking about a loop in unpowered flight:

  • a wing is a reaction engine

  • A typical loop is ented with a high-G pullup (3g’s or more).