in order to do aerobatics safely in any aircraft the thing must be designed for it. i don’t actually own a 747 but i suspect that really bad things will happen if you turn the engines upside down. they might just quit. or they might catch fire a la the concorde.
as far as pilot skill goes, aileron rolls and loops are beginner stuff. snap rolls, slow rolls, and barrel rolls are significantly more difficult to perform. the aforementioned restriction on maintaining positive g’s pretty much eliminates snap roll and slow roll from the 747’s list of approved maneuvers.
wings generate lift as long as air is moving across them and the angle attack is less than that required to “stall” the wing. it doesn’t matter that the 747 has its nose pointed at the moon. or one wingtip is pointed straight down.
tex johnston chose to barrel roll the 707 because it’s a spectacular looking maneuver that’s relatively easy on the aircraft. it maintains positive g’s all the way around so the engines won’t konk out. it doesn’t require a high entry speed like the loop so it won’t run out of airspeed going over the top and fall into an inverted spin. and it won’t place wing breaking loads at entry and pullout.
ÀQue? Why should the engines conk just because they’re upside down? I don’t imagine they have gravity fed carbs.
Judging by the punishment the airframes take during design & testing I suspect they would survive rolls or loops, provided they had enough airspace to regain control if things did go mucky.
It would be interesting to have a crack at a loop in a simulator.
February 19, 1985, a China (Taiwan) Airlines 747SP, registration number N4522V, did a high-speed dive and right aileron roll.
It was cruising at 41,000 feet when #4 engine started acting up. The pilot’s attention shifted to the problem, and he forgot to fly the plane. The pilot regained control at 9,500 feet. During the dive vertical acceleration was over 5Gs, and presumably the 747 went supersonic.
There was considerable damage to the plane - horizontal stabilizers (little horizontal wings on tail) were ripped apart, landing gear popped out, and the wings were bent upwards a few inches. None of the engines were seriously damaged, and in fact none of them flamed out. The plane managed to land safely, was repaired, and put back in service. A few years ago it was donated to a non-profit peace group.
“but i suspect that really bad things will happen if you turn the engines upside down. they might just quit. or they might catch fire a la the concorde.”
As far as I know, no Concorde’s engines have ever caught fire. if you are thinking of the French crash, I thought this was caused by debris on the runway damaging the wheels and breaking the wing fuel tanks, which were then lit by the sparks from the wheel hub.
A jet motor should run perfectly any way up, in fact it’s hard to see what would constitute upside down, as they are symmetrical about their longitudinal axis!
“A jet motor should run perfectly any way up, in fact it’s hard to see what would constitute upside down, as they are symmetrical about their longitudinal axis!”
fuel tanks and fuel delivery systems aren’t symmetrical. engines won’t run long if you’re trying to suck fuel out of the bottom of a tank when all the fuel’s at the top of the tank cause you’re inverted.
If I remember my aeronautics correctly lift is NOT created by an increase in pressure on the under wing, but due to decreased pressure on the top surface of the wing caused by the venturi effect
there are two types of lift. impact lift is generated by air molecules being deflected downward. stick your hand out of your car window while speeding down the freeway and angle it up and down to feel the force. the other kind of lift is based on the bernoulli principle: faster moving fluids exert less pressure than slower. wings are shaped such that air travelling across the top travels a greater distance than air that travels along the bottom. both paths take nearly the same time. hence the pressure above the wing is decreased, producing lift. airplane wings use both kinds. so the pressure below is increased and that above is decreased.
timmerov, this has been discussed to death on this board. Both paths (over and under the wing) don’t take the same time. This is often given without question in the Bernoulli’s explanation of a wing’s lift, but it’s wrong.
absolutely. it’s a much smaller error than stating airplanes don’t generate lift during a loop when the tail is pointed at the ground as was asserted without question in the original article and prompted this thread.
air masses above and below the wing reunite significantly offset horizontally in simulations of a cessna’s wing. they reunite much closer together in simulations of a faster aircraft’s wing. like a 747’s. which is the topic of discussion.
jumping tracks, the extreme maneuvers performed by the china airlines flight 006 certainly justify the suicidal descriptor. photographs of the aircraft safely on the ground show significant portions of the tail destroyed.
Being a former and retired member of the U.S. Air Force, I heard lots of rumors concerning aircraft. If my memory serves me, one specific rumor was that a B-52 made a barrel roll in the course of evading a surface-to-air-missile while on a mission over North Viet Nam. The teller of this tale recounted that the B-52 started the maneuver somewhere around 35,000 feet altitude and ended the maneuver somewhere around 15,000 feet altitude. Loosing 20,000 feet altitude in the process. If you have ever seen the wing structure of a B-52, the rumor sounds plausible. (A weird airframe design, the thing even takes off nose down unless the pilot is in a real big hurry to get off the ground.)
Since I didn’t have to go to Nam, and I didn’t talk a crewman of the acrobatic '52 or a witness of this maneuver, I can’t vouch for the veracity of this rumor.
But rumor also has it that the bumblebee has been aerodynamically proven to not be able to fly. (But don’t tell the bee. :o)
Shortly after 9/11 I was talking with a commercial pilot. I asked him what he would do if he found himself in similar circumstances. Without a moment’s hesitation he replied, “flip it upside down”. Now, I’m not claiming that he had fully thought through the possible scenario befor 9/11, but at the time I was talking with him he was pretty confident both that the plane would withstand some aerobatic antics and that it would sufficiently disorient an aggressor with a pocket knife.
“But rumor also has it that the bumblebee has been aerodynamically proven to not be able to fly.”
yeah, that proof was literally on a napkin at lunch in the early 1900’s. the calculation went like this: a fixed wing aircraft the size of a bumblebee flying at the speed of a bumblebee would not produce enough lift to support the weight of a bumblebee. we’re all familiar with where the religious cranks took this one. ;->
“fuel tanks and fuel delivery systems aren’t symmetrical. engines won’t run long if you’re trying to suck fuel out of the bottom of a tank when all the fuel’s at the top of the tank cause you’re inverted.”
I thought we were talking about an engine ‘catching fire’ because it’s upside down. Of course a plane won’t fly inverted if its systems aren’t designed for this - though a barrel roll should maintain positive g, and is a possible manoeuvre for many large planes. The Vulcan is a big plane (though smaller than a 747), and they were regularly rolled at displays.
I’m sure the issue with the bee started in a lecture on fluid flow. Airflow is often calculated using the Navier-Stokes equations, which are approximations that work so long as the Reynolds number stays within a critical range. Airflow over insect wings is so far out of this range that this technique will fail to produce accurate answers - this is a known and obvious limitation of this mathematical modelling technique.
I imagine that a lecturer fed in some figures for a bee one day just to illustrate to the students how the calculation breaks down at these scales, and the story that “Professor ??? has just proved that a bee can’t fly” was born.
Its not a matter of weather the plane could do it but its a matter of whether or not the pilots muscles would be strong enough to keep the tension on the control surfaces with the amount of pressure on them, and sense a majority of them are hydraulic I don’t think they would respond fast enough to to anything tight. it would have to be a long thought out process with everything done at precise moments to get the desired effect.
Is this some type of record? 5 first time posters in 1 thread!
Aside from that, a very interesting discussion. As to fuel and delivery systems being designed for inverted flight, i’m sure that Boeing and M/D didn’t intend for these planes to be used aerobatically, but as a safety issue, they may be (FAA) required to be able to sustain inverted flight for a short duration; does anyone know one way or the other?
My money says ANY large commercial aircraft could be easily (and quite safely) barrrel-rolled…not a straight line roll as fighter/acro aircraft love to do. My pilot of choice for any trials would have to be Mr. Bob Hoover; if you never saw his show using a Rockwell Commander business aircraft (I forget the model, but it’s a straight wing twin, piston pounder) that does a heck of an airshow with a glass of iced tea on the cockpit glareshield. Barrel rolls are perfectly coordinated at 1G (one gravity) and don’t really stress any airframe part.
Actually, that wasn’t asserted. In both the barrel roll and the loop that Cecil was talking about, it was clear (to me) that he was saying that the wings in those two situations produce no force that keeps the plane off the ground. You may define “lift” to mean that it’s always in a direction relative to the plane’s attitude, but Cecil was talking about lift away from the ground.