Negative G Question
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1.0 (positive 1 G, no pressure on control stick)
0.9 (less drag)
0.8 (less drag)
0.7 (less drag)
0.6 (less drag)
0.5 (less drag)
0.4 (less drag)
0.3 (less drag)
0.2 (less drag)
0.1 (less drag)
0.0 (zero G)
-0.1 (increased drag)
-0.2 (increased drag)
-0.3 (increased drag)
-0.4 (increased drag)
-0.5 (increased drag)
-0.6 (increased drag)
-0.7 (increased drag)
-0.8 (increased drag)
-0.9 (increased drag)
-1.0 (negative 1 G)As you start pushing forward on the control stick, you go to 0.9, 0.8, 0.7, etc. As you do drag decreases. Once a fighter gets to zero-G, it is free from the the drag caused by producing lift with its wings. As you continue to apply forward pressure, you begin going -0.1, -0.2, -0.3, etc. As you do, drag increases. So I was wondering, once you reach -1.0, will you have just about the same amount of drag that you had when you were at 1.0.
Starfighter
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It depends on many other things. We have a case of chaos theory here, and would have to use something called poisson statistics. Too many variables. But in theory without accounting for the relationship between the mass creating gravity (Earth), other forces such as force and it’s vector (i.e. thrust) it can be seen as equal, because the variables you left out are the variables which in reality would make them not equal. If in an controled STP experiment with a symmetrical A/C the drag would be the same. In reality they are most often not the same for many reasons. The reason it is even a question is because of all the variables we are not taking into account here.
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For +1 or -1 acceleration there must be an equal magnitude force. This force is lift and the lift comes from AOA. AOA has an associated increase in drag. The AOA for +1 and -1 is probably not exactly equal as the craft isn’t physical symmetrical but first-order it should be close. The drag-per-unit-AOA is probably not identical either, close. I hazard a very tentative guess that there’s more drag per unit in the negative direction but that might not be so. So the drag at +1 and -1 are functions of their respective required AOAs and how much each of those AOAs induce drag. Those 4 numbers are probably all different.
Qualitatively the graph of drag v load factor would be a U-shape between +1 and -1 with a minimum at 0.
It’s commonly said that 0g gives the best acceleration and this is not strictly true long term. 0g suggests minimum induced drag but it neglects potential energy effects. Pointing the nose toward the floor also matters. Between 0 and -90 pitch you get a whole 1G longitudinal acceleration from gravity. Thus it might be worth it to suffer from induced drag for negative G in order to rotate the pitch down to enhance the gravitational accelerative effects.
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Frederf,
Thanks for your reply, and thanks to everyone else who has replied to my post.
Speaking of pointing the nose straight down toward the earth, should a fighter jet be able to go inverted while at zero-G, precisely at 0.0? Or should that not be possible, to push up against gravity and go inverted at 0.0? Would the fighter be stuck in a 90 degree nose-down attitude? Would the pilot have to push some negative G, -0.2, -0.3, in order to get the fighter to start going inverted?
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Speaking of pointing the nose straight down toward the earth, should a fighter jet be able to go inverted while at zero-G, precisely at 0.0? Or should that not be possible, to push up against gravity and go inverted at 0.0? Would the fighter be stuck in a 90 degree nose-down attitude? Would the pilot have to push some negative G, -0.2, -0.3, in order to get the fighter to start going inverted?
Yes, with 0g, the aircaft behaves ballistically, so no going inverted from wings level.
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I’m not sure I understand the last question, or Cruse’s response…
Are you talking physically or theoretically? When you say “inverted” do you mean can you apply rotational force to the roll axis? 0G and 90 degree nose down have nothing to do with each other per se, the previous example was just an example for illustration. You can get 0G without 90 degrees nose down, and vice versa. So I’m a little confused about your question.
Are you asking A: Can you maneuver the aircraft in a roll at 0G, or B: Can you roll the aircraft when you’re 90 degrees nose down. The answer to both is yes, however, you won’t “technically” maintain a 0G state when you start to induce roll into the entire system. You have to remember that “G” is a term relevant to Acceleration, not speed - which means technically you can achieve a “0G State” at any speed or attitude, so long as the acceleration toward the ground is the same as the acceleration of gravity… And lift/drag are both functions of Speed. You can create the required force to effect the control surfaces at any “G”, so long as that “G” doesn’t occur in a vacuum.
Roll? Not sure why you inject it in that thread.
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Frederf, that makes sense. It made me think about how initiating a -G condition likely creates more drag from the control surfaces because the A/C is designed to pitch up more efficiently. I guess it would be a bad design to make an A/C more efficient at -1.0 G than at 1.0 G because the A/C is at positive G’s much more. So, I see the practical answer being that if you want to pitch away from gravity you of course want to pull back on the stick, and if you want to pitch towards gravity there is likely a point at which going inverted would be best. Of course you can only hold negative g’s for so long unless you are inverted. Eventually you would end up inverted going the other direction. If I have that correct.
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Frederf,
Thanks for your reply, and thanks to everyone else who has replied to my post.
Speaking of pointing the nose straight down toward the earth, should a fighter jet be able to go inverted while at zero-G, precisely at 0.0? Or should that not be possible, to push up against gravity and go inverted at 0.0? Would the fighter be stuck in a 90 degree nose-down attitude? Would the pilot have to push some negative G, -0.2, -0.3, in order to get the fighter to start going inverted?
If I understand your question, in the condition of -90 pitch is it possible to continue a negative load and pass -90 pitch? Yes, it is possible to complete any amount of “nose down” pitch change even so far as to arrive at the opposite horizon inverted and beyond. A complete loop (or more) with only nose down pitch rate is called an outside loop.
Frederf, that makes sense. It made me think about how initiating a -G condition likely creates more drag from the control surfaces because the A/C is designed to pitch up more efficiently. I guess it would be a bad design to make an A/C more efficient at -1.0 G than at 1.0 G because the A/C is at positive G’s much more. So, I see the practical answer being that if you want to pitch away from gravity you of course want to pull back on the stick, and if you want to pitch towards gravity there is likely a point at which going inverted would be best. Of course you can only hold negative g’s for so long unless you are inverted. Eventually you would end up inverted going the other direction. If I have that correct.
It’s more obvious if you consider a world which a mass of air but there’s no gravity. The zero-lift (ballistic) path is obvious: a straight line. To go down you’d have to apply negative lift and to go up positive lift. On Earth with gravity the ballistic path is a downward curve and all paths with more downward curvature require negative lift while all curves with less downward curvature require positive lift.
“Negative G” is not a direction in relation to gravity. It’s simply load factor expressed in units of G. If a weight on a spring in the cockpit stretches toward the floor that’s positive load factor and if it stretches toward the canopy it’s negative load factor. It’s theoretically possible to do endless loops of negative load factor.
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It’s theoretically possible to do endless loops of negative load factor.
Actually, what I was saying is that you cannot pitch down towards Earth and stay at -1.0 G’s endlessly. Once you gain inertia in the direction of gravity you are stuck unless you deviate from -1.0 G. You will either hit the ground or you will have to pull more than -1.0 G in order to level off. Of course you can go in endless loops but with gravity once you get to the point you are trying to fly straight and level, you will have to not only overcome gravity but your inertia towards the ground, hence passing -1.0 G’s. I was talking about how you can endlessly do -1.0 G’s and exactly -1.0 G’s by flying inverted and pitching away from gravity, but you cannot do that when pitching towards the Earth, because eventually you will hit the ground or have to pull more than -1.0 G. Maybe I didn’t explain it well enough. You can do horizontal loops. But not vertical while staying at -1.0 G’s. The effect of gravity makes it impossible. It’s like if you bungee jump or sky dive. In order to decelerate while falling, you have to pull more than 1 G; or if you jump off a bench or something. It’s not the falling that kills you, it’s the sudden stop.
Negative G can be a direction in relation to Gravity. It is a metric of force. I think you misunderstood what I posted because I never posited anything about negative G being a direction. It is a force, and the Earth’s gravity is in the direction of Earth. When accelerating towards or away from Earth we experience G forces due to the direction we take in relation to the center of mass. Everything you have ever done in your entire life is in relation to the direction of gravity. Unless you have been out in space, you have never experienced gravity not having a direction. You just overcome it with an opposite force which creates G forces.
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Thanks very much to everyone who has replied to my post.
I just wanted to clarify my question.
If a jet fighter pilot was in a 90 degree nose-down attitude – heading straight down to the earth – if he pushed the stick forward just enough so that the G-meter read exactly 0.0 G, would he start going inverted? Or would he have to push the stick a little more to get at least, say, -0.3, -0.4, or -0.5 in order to begin going inverted?
Thanks,
Starfighter
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Thanks very much to everyone who has replied to my post.
I just wanted to clarify my question.
If a jet fighter pilot was in a 90 degree nose-down attitude – heading straight down to the earth – if he pushed the stick forward just enough so that the G-meter read exactly 0.0 G, would he start going inverted? Or would he have to push the stick a little more to get at least, say, -0.3, -0.4, or -0.5 in order to begin going inverted?
Thanks,
Starfighter
Going inverted means flying upside down, to me. As well clarify that ASAP
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Going inverted means flying upside down, to me. As well clarify that ASAP
Actually, in air/space, upside down is 360 degrees. The only difference is at which degree you are fighting more or less gravity!!!
C9
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@Cloud:
Actually, in air/space, upside down is 360 degrees. The only difference is at which degree you are fighting more or less gravity!!!
C9
Language problem now. OP should clarify, just MHO.
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Thanks very much to everyone who has replied to my post.
I just wanted to clarify my question.
If a jet fighter pilot was in a 90 degree nose-down attitude – heading straight down to the earth – if he pushed the stick forward just enough so that the G-meter read exactly 0.0 G, would he start going inverted? Or would he have to push the stick a little more to get at least, say, -0.3, -0.4, or -0.5 in order to begin going inverted?
Thanks,
Starfighter
I see what you mean. For any dive angle the g-meter reading which continues a straight path is the cosine of that dive angle. For a straight line -60 dive the g-meter reads 0.5. For a straight line -90 dive the g-meter reads 0.0. If the a airplane is at -90 and the g-meter is 0.0 then the airplane will continue in a straight line. A dart doesn’t pull G. It’s always at 0g and we know a dart will approach -90 pitch and never go beyond that.
Actually, what I was saying is that you cannot pitch down towards Earth and stay at -1.0 G’s endlessly. Once you gain inertia in the direction of gravity you are stuck unless you deviate from -1.0 G. You will either hit the ground or you will have to pull more than -1.0 G in order to level off. Of course you can go in endless loops but with gravity once you get to the point you are trying to fly straight and level, you will have to not only overcome gravity but your inertia towards the ground, hence passing -1.0 G’s. I was talking about how you can endlessly do -1.0 G’s and exactly -1.0 G’s by flying inverted and pitching away from gravity, but you cannot do that when pitching towards the Earth, because eventually you will hit the ground or have to pull more than -1.0 G. Maybe I didn’t explain it well enough. You can do horizontal loops. But not vertical while staying at -1.0 G’s. The effect of gravity makes it impossible. It’s like if you bungee jump or sky dive. In order to decelerate while falling, you have to pull more than 1 G; or if you jump off a bench or something. It’s not the falling that kills you, it’s the sudden stop.
The cockpit instrument only measures acceleration in the vertical (meaning top of airplane) direction. It does not measure lateral or longitudinal. An F-16 suspended from a crane with its nose pointing exactly up or down or even rolled 90 degrees either way would have its meter read 0.0 despite gravity being 1g toward the tail or nose or right or left wingtip. Given enough altitude a constant -1.0g load factor would have a radial acceleration of -2 from level to -1 pitched down and 0 approaching inverted level (the last result taking infinite time). A -1LF loop is not a closed maneuver because the radius of curvature would become infinite half way through it. But any LF more negative than -1 is a closed maneuver.
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So if you’re in level flight, and you push the stick forward until the G-meter says exactly 0.0, the nose of the fighter will start to pitch down. If you keep holding the stick at 0.0, the fighter’s nose will eventually be pointing straight down to earth, and will not go inverted until you push the stick forward some more, pushing some negative G.
Starfighter
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that is correct.
Its worth noting that the stick movements for this are so simple due entirely to the G-command system the FLCS uses in the F-16. Also worth noting is that you should technically never reach directly downward. As the nose drops, you will note the pitch rate decreases as the dive angle steepens, if you hold constant 0.0g in the dive. Three other factors affect the final dive angle; due to drag you can get essentially all the way down given enough time, then given sufficiently slow speed and high angels, you can get to a very nose low attitude - essentially 90 degree dive.
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I read the following from a publication called “Interceptor” by USAF ADC:
“Zero G is a ‘best exercise’ when you find you’re in a nose up, bleeding airspeed situation. But delaying initiation of a zero G recovery can place the airplane in a low speed situation so delicate that zero G cannot be maintained practically.”
What is the minimum airspeed required to push forward on the control stick and enter 0.0 G?
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@Red:
if you want to accelerate quickly, go zero G
that’s where you make a difference.I did a week of aerobatic flying in an Extra 300 with Patty Wagstaff down in Florida a couple of months ago and I asked her about this. Specifically, whether or not unloading and maintaining zero G would yield a greater speed build up versus nosing over with negative G and getting helped by gravity. She leaned toward negative G + gravity being a faster method, but was interested at the premise.
We were going to do some tests on our last flight… but ended up forgetting until the end and I wanted to spend the last bit of fuel in our acro tanks on seeing how high I could get the Gs in a loop entry. (7.2Gs! The two seat Extra is rated for +8/-8.)
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The cockpit instrument only measures acceleration in the vertical (meaning top of airplane) direction. It does not measure lateral or longitudinal. An F-16 suspended from a crane with its nose pointing exactly up or down or even rolled 90 degrees either way would have its meter read 0.0 despite gravity being 1g toward the tail or nose or right or left wingtip. Given enough altitude a constant -1.0g load factor would have a radial acceleration of -2 from level to -1 pitched down and 0 approaching inverted level (the last result taking infinite time). A -1LF loop is not a closed maneuver because the radius of curvature would become infinite half way through it. But any LF more negative than -1 is a closed maneuver.
This gives more clarity to the lazy way I was trying to inform how to best test the drag at -1.0 and 1.0 G. Doing a loop would not be an accurate test because of the relationship to the Earth’s mass and the limits of the A/C’s G sensor. The reason I even mentioned this is because testing drag would be best with no vertical speed and inverted for -1.0 G. The 1.0 G test could be done in standard level flight. My point was that going into a loop would be a bad way to test drag. I didn’t really make that clear when I mentioned it. I kind of let my thoughts spill out onto the post without clarifying.