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).
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.
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
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.
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).
I wasn’t really looking for that. I started out trying to find videos of Abrams tanks, but it turns out there aren’t many at all.
I then started looking for missle videos, as my brother’s FIL worked on the Javelin. I found that site, and as I’m one of the uberdorks who read the first SD book about 5 times, I remembered that question, and searched out the thread that I figured someone would have made.
But, yeah, I’m awesome.
Tex says “Chandelle” but a chandelle is not a roll. Tex performs a barrel roll.
Also. It is commonly said that a barrel roll is a 1G manoeuvre, it is not. It is a positive G manoeuvre which involves the aircraft pitching and rolling simultaneously. Any time an aircraft is pitching nose up against gravity (as happens during parts of a barrel roll) it is pulling more than 1G. A well executed barrel roll will often involve upto 3Gs or more during the initial pullup and final recovery. A gentle barrel roll intended to avoid stressing the aircraft or alarming passengers–hehe–can be done with maybe 1.5G however it is always more than 1G during those phases of the manoeuvre.
give an account of a Boeing 747 that was inadvertantly rolled and pulled to over 5g when the pilot lost control in the cruise. The aircraft lost 30 000 ft in a very short time and is thought to have gone supersonic briefly, but there were few serious injuries and the pilot was able to make a safe emergency landing at SFO.
“On 19 Feb 1985 it was at FL410 over the Pacific Ocean on its way from Taipei to LA, when it lost power on engine nr.4. After disconnecting the autopilot it went into an uncontrolled dive, made a full 360 degrees aileron roll, and became supersonic for a short time. It recovered at 9000 feet. In the process it lost 11 feet of the left stabilizer, the whole elevator and 5 feet from the right stabiliser, also the APU separated from its mount.”
Yup. It’s an absolute testament to Boeing’s overbuilt design. I can’t see a 757 or a 767 standing up to this kind of treatment. They were also lucky it happened so high up as they had time to sort it out before ploughing into the ocean.
IIRC, the captain’s first transmission was something like “We are emergency, we are niner thousand feet”
Whih must have been surprising to the ATC staff which had seen him at FL410 just a very few minutes before. So that’s a third strand of luck; they were lucky there was nothing immediately below them.
Can someone explain to me what it means to loose a “whole elevator”? How would you maintain the pitch of the aircraft? or are we talking about loosing one of the horizontal stabilizers?
I don’t interpret it this way. In conventional parlance, a plane like the 747 has just one elevator. It is in two parts (left and right) but it’s generally thought of as a single control surface. The word “the” in the phrase “the whole elevator” seems to conform this.
As it’s the primary means of pitch control, its loss is a very serious issue - fatal in many aircraft. Since it wasn’t so in this case, it may be that the 747 accomplishes pitch trim by means of small adjustments to the angle of the horizontal stabilizers (which should still be possible after losing the elevator). Perhaps someone who flies these machines will be along to clarify.
Or by unstable, do you mean stalling the wing e.g. snap roll entry? If that’s the case then you botched the loop.