Is it possible to loop or roll a 747 jet?

A minor quibble. What you described in your answer was an aileron roll. I once saw an interview with the pilot who performed the barrel roll in the 707. He points out that a barrel roll is a 1G maneuver. IOW, the wings generate lift towards the top of the aircraft the entire time. Almost any plane should be able to perform one.

Here's how it works. As you start the maneuver you pull the nose above the horizon. Then you apply back pressure at the same time as you apply roll input. If done correctly, the angle of attack (the direction from which the air hits the wings) should remain constant throughout the roll.

Since you have to start with a climb, some aircraft may not be able to perform several barrel rolls in quick succession. An aircraft without a lot of excess power might lose airspeed and/or altitude with each roll. Eventually, the ground will get in the way.

Another pilot on that same show (sorry, it was years ago and I don't remember where I saw it) pointed out that an expertly executed barrel roll is imperceptible to passengers. He gave an anecdote where he proved this to his co-pilot by performing a barrel roll without provoking comment from the businessmen who were deep in conversation in the back of the plane. He also demonstrated on-camera that he could pour a cup of coffee while performing a barrel roll.

Whoo boy. This comment has so many holes it looks like a "no hunting" sign on a Texas highway. But I ain't gonna dump on a junior member, no way.

Welcome to the SDMB, Drake!

And yup, for those of you interested - the moment I replied to Drake's topic, he became a regular plain old E-flat member.

That means "fair game."

Go for it. I think I did a reasonable job of describing a barrel roll. The quote by the pilot is accurate. What holes do you see?

One addition I should make might help people visualize the difference.

With an aileron roll, the fuselage of the aircraft goes pretty much straight and the plane just spins about that axis.

A barrel roll can best be described as a corkscrew threw the sky. It's as though the plane is flying around a huge barrel, thus the name. If you think about it, the only way to get the plane to rotate and move around the barrel and keep the top of the plane pointing at the barrel all at the same time is if there's some back pressure on the stick. That back pressure produces the percieved G force, making this a 1G maneuver.

The pilot to which Drake refers to as the other pilot on the show was the legendary Bob Hoover. I, too, have seen the same show and have seen several videos of Hoover pouring water or coffee from a pitcher into a cup sitting on the top of the instrument panel while the plane was in a roll. Hoover was the only one at the controls at the time and was flying a Rockwell Commander, a twin-engine turbo-prop business aircraft. It was without any modification for aerobatics. He also used this same aircraft to perform a standard airshow routine, including loops, rolls, Cuban 8s, and Lomcervaks (sp?) with one engine feathered.

Drake is also right about the 1G stress of a properly performed roll, as stated by the pilot who rolled the Dash 80 (prototype 707) at its 1st public showing. Now, can you loop a 747? An entirely different question.

Actually, he didn't do Lomcervaks (who knows? :-) but he did do all the rest. And it wasn't one prop feathered. It was both props feathered. He was demonstrating energy management and did something like a four-minute routine without engines, all momentum. I think he finished it off with a dead-stick landing.

Not just any old dead-stick landing, but a dead stick landing after coming out of the bottom of a loop, followed by rolling the aircraft without power off of the runway, up to the airshow flight line, and stopping it within a couple of feet of the fence.

I've seen Hoover do this routine a couple of times at the Reno Air Races.

He also does aileron rolls in the Commander with one engine feathered and the other producing power. If you ask me, that would be more difficult than doing it with both engines shut down.

Well, ok. What's the quibble again? Cecil was describing an aileron roll when what he meant to describe was a barrel roll? I don't think so. Show me how you made that determination.

Or perhaps you meant he was wrong about it being "an extremely foolish action" to attempt a barrel roll in a 747, or do you mean it would be foolish only if what he was describing is an aileron roll? Why? Because you have determined that "almost any airplane should be able to perform one"? (Which? Both?)

Maybe it was Cecil's "loss of lift" and "when the wings cease to function" statements that threw you. Did you think he meant the aircraft stalled? Perhaps quibble with Cecil's use of the word "lift" to describe "(cease to) hold the aircraft up,"
but it was quite obvious to me what he meant.

In any event, why would an aircraft stall during a properly executed aileron roll? The angle of attack does not change. Unlike your barrel roll, an aileron roll will result in momentary negative g's - but the barrel roll is technically not a 1 G maneuver, either, unless you don't count the "pull the nose above the horizon" part of it, during which you have not only increased the angle of attack, but the G load as well.

Tell me again what that minor quibble was?

Here's the original article, for those of you interested.

http://www.straightdope.com/classics/a1_262.html

Here's the line that caught my attention:

> Enough forward speed must be maintained during the roll to compensate for the loss of lift that occurs when, in effect, the wings cease to function.

Basically, he's mixing in his description of an aileron roll with the discussion of the barrel roll done by the 707. In a barrel roll, one is actually gaining altitude in the first quarter of the roll even though the wings are perpendicular to the ground. That's because one has been raising the nose and climbing while rotating up until that point.

The reason I mentioned raising the nose at the beginning of the maneuver in order to gain altitude to compensate for how much altitude would be lost while the plane was upside down while still maintaining something near 1G. If one doesn't care about the loss in altitude then one could start from straight and level.

I guess the main point of confusion is that the original question was worded imprecisely. Todd J. simply asked if a large plane could be rolled. If one allows barrel rolls then most any aircraft can safely perform one. I've even seen video of a helicopter do one.

> Enough forward speed must be maintained during the roll to compensate for the loss of lift that occurs when, in effect, the wings cease to function.

I agree with Drake, I also cringed at the wording used. Technically, the wings are functioning throughout the roll (aileron or barrel). One wing is providing positive lift and the other negative, hence the roll. Explaining that lift and therefore roll rate is a function of indicated airspeed and relating that to the aileron roll dynamics would have been more appropriate than the 'forward speed to compensate ...' explanation. Of course Bernoulli, adverse yaw, etc. have been left out but I'll let some experts jump in now.

Oblio,

Assuming the wings are at a positive angle of attack then the downward-moving wing is still providing a net positive lift even during the roll. It's simply providing less lift than the upward-moving wing.

After reading Cecil's explanation one more time, what bothers me is he goes through three ideas in that one paragraph. He starts off with the discussion of the Dash-80 barrel roll. He changes to a description of an aileron roll. Then he states that the plane must have enough velocity for knife-edge flight (using the fuselage for lift) in that one sentence. In a barrel roll, that's just not the case.

Changing to the other subject of the question, I'm wondering myself if one of those aircraft could perform a successful (read: survivable) loop. What came to mind just this minute is the NASA KC-135 "Vomit Comet." http://zeta.grc.nasa.gov/kjenks/kc-135.htm

Hmm, according to the accompanying text, the plane only pitches up to 50 degrees and down to -45 degrees on the other side. That's a long way from going past 90 degrees. It's hard to say whether it would be controllable at the top of the loop. And then of course there's the question of whether they can get the nose of the plane back up to level on the bottom side of the loop before the speed got so high that it ripped something important off.

Waffling back the other way, they've had aircraft plummet out of control for tens of thousands of feet a couple of times and survive. I think it just might be possible with an exceptional pilot and a little luck.

Forget the LOOP! In an aircraft as heavy as a B747 (even if it's almost empty), by the time you are a quarter of loop-up, your airspeed will be so low, that the wing will be stalling. Notwithstanding the powerful engines (Rolls-Royce RB211 for a typical B747-200), it just won't climb to complete the first half of the loop before you reach stall speed. I don't think there's a pilot crazy enough to try to do a loop with an airliner.

Now, regarding the roll, maybe a barrel roll is feasible for a B747, although it is definitely NOT recommendable! There have been other large aircraft known to have done similar stuff but under very different circumstances. For example, a B727 "performed" a spin after stalling at high altitude, and the pilots recovered it. But an aileron roll would be a little trickier, although not impossible (maybe).

Now, to call a maneuver a "1G" maneuver is just pilots lingo. There's no such thing as that. Even if momentarily, but you will always exert a force of a little more and a little less than 1G whenever you change your flying conditions (remember Newton laws?).
And please, the wings never cease to function! At least not as long as they are attached to the fuselage, and you have enough speed. What happens is that when the roll angle equals 90°, the whole lift is perpendicular to the direction of the weight of the aircraft, that's all. To say that the wings "cease to function" is misleading.

And, Cecil: increasing the speed increases the lift OVER the wing, and THAT is what holds the airplane in the air. The pressure under the wing helps a bit, but it is a very small help compared to the lift created in the upper camber. It is a common misconception to think that it is the other way around.

¡Salud!

Assuming the wings are at a positive angle of attack then the downward-moving wing is still providing a net positive lift even
during the roll. It's simply providing less lift than the upward-moving wing.

Drake, I thought that the downward traveling wing actually had a negative AOA (angle of attack) due to the upward deflected aileron. Or is this in reference to the dynamics for a barrel roll only. IOW are you saying the 'pitch up' AOA is greater than the aileron down -AOA. Is this due to the fact that most ailerons are not full span, angle of incidence & asymetrical camber ? Do most aircraft wings (w/asymetrical camber) in general have a net positive AOA even with full up aileron ? Of course, I realize this is all in reference to roll/looping big iron, but I was hoping for some additional insight as to the dynamics of a barrel roll.

Thanks, Oblio

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A point in every direction is like no point at all

Ooops, up, down, left, right ...

... greater than the aileron down -AOA ...

Should be

... greater than the aileron UP -AOA ...

And, Cecil: increasing the speed increases the lift OVER the wing, and THAT is what holds the airplane in the air. The pressure under the wing helps a bit, but it is a very small help compared to the lift created in the upper camber. It is a common misconception to think that it is the other way around.

I'd be careful here especially when you really don't know any more than what you see on the discovery channel. It all depends on the camber, AOA, and type of airfoil. A flat plate for example could be more accurately descibed to fly by increasing the pressure below. Finally seperating the pressure below and above the airfoil is foolish in that they are directly related, in relative terms the pressure above cannot decrease with out the pressure increase below. Relative terms are the only ones that apply in discussing lift/drag.

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

Omniscient; BAG

E1skeptic, I checked your profile, so I'll take back that Discovery Channel smack. But, I think you'll agree that your response was extremely narrow and over simplified.

Hows things over at Raytheon?

Damn, not my day!

Obilo is the one at Raytheon. E1skeptic, he has this thread pretty well covered, and assuming he's not some schmuck Comp. E at Raytheon he'll be a bit more experienced than I.

E1skeptic, even though I got the wrong profile last time, the pilot title should still warrant my retraction.

E1skeptic, yes, the 747 is heavy. But that also means massive. That's a heck of a lot of inertia at .8 mach. Considering that the KC-135 can get to 50 degrees with enough inertia to "coast" for about 15-20 secs before hitting the top of the parabola, I think it just might be possible for one of those to get past 90 degrees under power.

Now, will it still be controllable at or near 180 degrees? Or will it pretty much just stall and flop over the top? The latter seems likely, though not certain. A good pilot who is careful to conserve his energy could probably keep it controllable much longer that one would initally guess. Keep in mind Bob Hoover in his business twin. He might not produce a nice, pretty near-circular loop, but he should be able to get over the top.

Oh, and the "1G maneuver" phrase is intended to mean, "no undue stress beyond what would likely be experienced in straight-and-level flight." IOW, no high positive G's and nothing close to zero or negative G's. For all practical purposes, 1G.

Oblio, yes, the downward moving wing is producing lift. Think of it this way. The plane is being held in the air by the wings. A 747 weighs over 500,000 pounds. A fully-loaded 747-400 approaches one million pounds. If the downward-moving wing was not producing lift then all 500,000+ pounds of lift would be coming from the upward-moving wing. That much differential would cause an enourmous roll rate :-) With the mechanical advantage of the ailerons out on the tip of the wing I suspect it doesn't take more than a few hundred to a few thousand pounds of differential lift to induce a roll.

OTOH, an aerobatic plane might experience negative lift on the downward-moving wing. I'm not sure about that.

Oh, and the wing itself still has a positive AOA. The aileron is in effect "dumping" some of the lift. Similar to what the spoilers do on landing.

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.

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.

assuming he's not some schmuck Comp. E at Raytheon

No offense taken 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

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.

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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 :-)

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? :-)

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.

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.

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.

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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.

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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.

I'm a pilot who has performed loops in airplanes, and I can state categorically that
In fact, even in powered airplanes some looping manoevers are performed with the power pulled back to idle.

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.

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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).

Dhanson:

I have to say that Omniscient is right. You two guys are talking different languages. It seems to me that you are trained as a Private Pilot, and your terminology (even if you studied physics) is a little too coloquial at times.

Omniscient said that a glider is designed at the threshold of dynamic instabillity, NOT that the glider is unstable.

You say that the basic difference between a glider and a powered aircraft is the engine. That's what everybody thinks. It looks obvious. But the most important difference, is the Aspect Ratio.

I can see that you know a lot about flying, I'm not saying you don't. But some of your analogies don't make sense, at least to me. A wing being a "reaction engine"? Could you explain?

And I am going to look for info on Bob Hoover, 'cause what I have seen is him doing a loop with ONE engine shut down and the propeller feathered, not with TWO engines down. But I could be wrong.

Ok, I see I could be wrong. I just found some very high level info on Bob Hoover, and it says that he performs some maneuvers with two engines down (it didn't say which ones).

I'll keep looking for more.

Here's an interesting article I found while looking for Bob Hoover links. Titled: Energy Awareness and Energy Management. http://www.monmouth.com/~jsd/fly/how/htm/energy.html In section 1.2.6:

The airplane's ability to convert airspeed to altitude and back again is the key to many aerobatic maneuvers. There is no way you could perform a loop using engine power alone; you have to zoom. Bob Hoover's airshow routine typically closes with a spectacular energy management demonstration. After shutting down the engine, he performs a series of complex aerobatic maneuvers, including an eight-point roll and a hammerhead. 7 He then returns for landing and coasts to the reviewing stand, all without restarting the engine. It is quite a fascinating lesson in pilot technique.

Here's a link that includes a photo out the window of the Dash-80 when it was inverted. It also has a picture of Hoover's plane with the props feathered: http://www.n-w.de/top/airshow/airshow.htm

Here's a series of photos of Hoover doing his deadstick loop: http://www.ebom.com.au/thom/avalon99/ Choose "Bob Hoover's Shrike Commander aerobatic routine" from the drop-down list.

The MOST important difference between a glider and powered aircraft is that when you are over the numbers in a glider a go-around is NOT an option

High aspect ratio powered AC: U2, Daedalus
Low aspect ratio gliders: Lifting bodies, STS, F-16 (when the engine flames out)

Any others ??

I can't recall why high aspect wings are not used more in powered AC. High L/D, but what is the negative aspect of using them ?

Saw Hoover (at Lakeland Fla) finish with a loop (OK, he was falling gracefully with a terrified crowd onlooking), followed by a aileron roll (four point IIRC), and finished with a teardrop reversal to his signature landing on left, right and then both mains. All this with the fans off. And he does the same with one turning, including eight point rolls.

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)

Why is a loop unstable, due to having wing anhedral (sp?) when inverted ? I don't see how pitch or yaw stability suffers during a loop. Has the aircraft moved to the right hand side of the plane ? Gotta stop these puns, it's Fri. so I'm celebrating Or by unstable, do you mean stalling the wing e.g. snap roll entry? If that's the case then you botched the loop.

I love this thread, mixing raw theory with real word aviation.

Oblio

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A point in every direction is like no point at all

Actually, I know quite a bit about aerodynamics. I've been speaking colloquially because this is not a scientific panel meeting.

And gliders are not necessarily designed at the threshold of dynamic stability. The only thing a glider would gain by being at that threshold would be the reduction in induced drag that comes from requiring less downforce on the tail. But that's not necessarily a good thing if it requires more pitch inputs from the pilot, which can increase drag and increase the workload for the pilot. In high-endurance gliding, this is a major factor.

High-performance gliders are simply designed to have as low drag as possible. This usually means high aspect ratio wings to reduce induced drag, narrow fuselages to reduce parasitic drag, and light composite materials to reduce weight. They may or may not be designed to have a low dynamic stability, depending on the mission. Some gliders have small engines with stowable propellers. Some normal powered aircraft actually make pretty decent gliders. In other words, there is no intrinsic difference between a glider and a powered aircraft other than that one is designed for gliding flight to be its most efficient regime.


Oblio: High aspect ratio wings involve design tradeoffs like any other. They are generally more efficient, but they also can add structural weight because of their length, and the longer wings make the airplane difficult to manoever on the ground. They also reduce the roll rate of the aircraft. Winglets on airplanes are one way of solving the same problem (reduction in drag from wingtop vortices), and are often used instead of increasing the aspect ratio simply to reduce the span of the aircraft.

A loop is not an unstable manoever. The airplane is in controlled flight at all times, with the wing producing lift and the lift vector being perpendicular to the direction of flight just as it is in straight-and-level flying. Basically, the way a loop works in an airplane without the 1:1 thrust/weight ratio mentioned is that the aircraft is put in a dive to build up excess airspeed. At the right entry speed, the pilot pulls back on the yoke, cause the airplane to pitch up and over the top. With enough airspeed, thrust, or a combination of the two, the angle of attack on the wing never exceeds the stalling angle of attack, lift is maintained throughout the manoever, and the pilot feels positive G forces all the way through. On the backside of the loop, the power is reduced to idle to prevent excessive airspeed buildup, and the aircraft converts the potential energy it has gained back into kinetic energy by accelerating. Performed perfectly, the aircraft will return to straight and level flight at exactly the same airspeed and altitude at which it entered the loop.

My point about the wing being a reaction engine is that it is essentially producing the thrust required to change the direction of the airplane. If the airplane were in a vacuum, thrust from the engine would have to do all the work (and of course, the engine would have to be a rocket engine of some sort).

Dhanson: That was a very good explanation of how to perform a loop. I'm impressed, really.
And I see your point now, about the wing providing the "thrust" during a circular maneuver in which the centripetal force is being provided by the lift. Your use of the word "reaction" is what has me confused, because the reaction engine takes advantage of Newton's third law, whereas the lift produced by an airfoil is the product of a differential pressure.

Maybe I'll let you fly my plane, after all. You seem like a good guy.
May I fly your Grumman some day?

Actually, you can describe the lift generated by the wing both in terms of differential pressure and in terms of Newton's 3rd law. An aircraft stays aloft by moving a volume of air in a downwards direction. If you look at the downwash behind the wing of an airplane, it has a net downwards vector. In this sense, a wing is a reaction engine, pushing a mass of air down in order make the airplane stay up.

Unfortunately I sold my Grumman last year. I really miss it. The AA1 is a beautiful airplane to fly. Fingertip pressure on the controls, bubble canopy, and a roll rate just this side of a Pitts Special. But I wasn't flying it enough, and I felt bad about it. Airplanes were meant to fly.

I am not an airline pilot, nor a rocket scientist, nor an engineer. I did not even graduate from college. But I did see, in Joplin MO, and in Kalamazoo MI, Bob Hoover do loops in a normally powered (Shrike Commander) aircraft, with the engines shut down.

Now, if those loops were not "true loops", they apparently slipped past my untrained eye. I did not have the opportunity to measure the roundness on his path, but they looked good to me. It is my opinion that these were loops.

I do hold a commercial pilot rating, and a ground instructor's license, although I have used neither in more than 20 years. But I still saw it.

[Cardinal]

I just came across actual video of Tex Johnson rolling the prototype 707 through a Schondell (I have no idea how to spell that, and Google isn't helping).

http://www.aviationexplorer.com/707_roll_video.htm

[Jonathan Chance]

I am astonished.

Not over the roll. I don't know enough about airplanes to have an opinion.

I'm astonished at someone tracking down evidence for a five year old thread.

Cardinal, I salute you! Truly, there's no statute of limitation on fighting ignorance.