Extending Flaps at High Speed
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Just a question regarding how this should be modelled in a flight sim.
Let’s say you are flying a fighter in a sim, and the fighter has take-off flaps that can only be deployed below 250 KIAS. If the fighter is flying at, say, 400 KIAS, and you lower the take-off flaps, what should happen in the sim? In real life what would probably happen to the flaps? Would they rip off? Could the aircraft become unstable and could the pilot lose control?
Starfighter
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Most aircrafts has safety systems that close flaps when its maximun allowed speed is reached
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in tactical fighters they won’t extend, in most computer aided systems they will not extend, this is NOW true of thrust reversers too, although tragically that was not always the case. I know of at least two total losses of life and aircraft from accidental in flight deployment of spoilers, reverses, or flaps. Just a few years ago someone in a cirrus got a go around because of wind, didn’t flap in quite fast enough wing stall asymmetric spin right into the bed of a truck in the airport parking lot. sad times for all.
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You need to know the reason for the limit. In a little Texetron 172 the flaps are electrically operated. Faster than around 85 knots (110 to 115 for the first stage, in later models), the flaps are not to be operated.
The reason in that case is that the electric motor isnt that powerful, and trying to force the flaps down against the relative airflow is going to damage the motor. Even at 90 or 100 knots it would probably still work, though. There is always a healthy safety margin applied, because there is always going to be someone that insists on operating right up to the limit.
Flaps increase the camber of the wing, and increase the stalling angle of attack. They also increase lift, so by deploying flaps you will (in a conventional aircraft) need to lower the nose and probably the airspeed also. They increase drag, so you will probably find it relatively easy to slow down.
Going out on a limb here, as Im not a qualified engineer nor an aircraft designer. Seems to me that if you lower the flaps in your fighter at 400 knots, then either the flaps will lower or there will be a system failure. Depends how strong your hydraulic system is I guess. At 400 knots, there is going to be a great deal of pressure on the flaps deployed surface, and that is going to translate into a great deal of force on the actuator. If the hydraulic system can supply sufficient pressure on the actuator to generate the required force, then the flaps should deploy. If the real aircraft is only supposed to deploy flaps below 250 knots, there is a reason for that. Could be the hydraulic system cannot supply enough pressure to extend the flaps under conditions much faster, say at 286 knots in smooth air perhaps the flaps dont deploy safely. Perhaps the flaps have locks to prevent them from moving with small changes in hydraulic pressure, and the locks cant lock correctly with too much force. Without knowing more about the system in question, all you can do is guess, and try to make it an educated guess. My guess would be the hydraulic system cant supply an infinite amount of pressure, and that at 400 knots it probably cant fully extend the flaps against the wind force.
So lets look at the other side of it, what happens if the hydraulic system can supply the required pressure? what happens if the flaps do lower? Well, we know that flaps increase camber of the wing, and that by doing so they increase the coefficient of lift (which conveniently enough also increases lift). Additionally, by increasing camber we can (broadly speaking) increase the stalling angle of attack, increasing the max lift we can generate. They also increase drag.
Lift and drag are pretty key forces here. As you might be aware already, both forces are proportional to the square of your true airspeed. That is, if you double your true airspeed, and keep all the other factors the same, you also multiply your lift and drag by 4 times (rather than by two times). In flight, when you double your airspeed, you also lower your angle of attack, so as to avoid climbing from that excess lift. Hence that part about keeping all other factors the same.
So if we deploy the flaps at 400 knots, we already know the wings are going to generate a lot more lift, and a lot more drag. So the immediate effect is going to be an increase in load factor, and you will probably start slowing down. Now, depending on a LOT of factors is what happens to the nose position. For a jet, its probably safe to assume that deploying flaps is going to cause some nose movement, and that its going to be essentially just pitch movement. The question is whether there is going to be more torque lowering the nose, or more torque raising the nose. I would assume (theres that lovely word again) that the nose is going to lower, but it depends on a great deal, especially where the flaps and wings are located in relation to the centre of gravity, and the net change to the airflow around the aircraft caused by lowering the flaps.
From that immediate effect of the increase in load factor, theres the possibility of structural damage. That depends on how much of an increase there is, and on how fast the flaps are deployed. If the wings go from 1G to 11G, theres probably great potential for that. That would be a good reason to avoid lowering the flaps above the rated airspeed, wouldnt it? Immediately following the increase in load factor, the load factor is going to reduce again. It comes from the flaps being deployed, but the flight path of the aircraft will pitch up to follow the load factor, which in turn will decrease the load factor (it being caused by the reaction on the aircraft of the aircrafts action of turning the flow of the air its in. If that was a difficult sentence to parse, Im sorry - it was annoying to type, also).
In the case of the F-16, its obviously quite simple. The pilot doesnt operate the flaps, the computer does so - efficiently and smoothly as required. In the case of say an F-15, we are back to guesswork and assumptions (hopefully good ones). At any given speed, either it will work, or it wont. If it works, there will be a momentary increase in load factor, flight path will increase, and there will (probably) be some nose movement, which may be either a pitch increase or a pitch decrease. If it doesnt work, it may damage components of the hydraulic system, or it may simply extend the flaps only partially against the force of the wind.
I cant think of any case where deploying flaps would cause the aircraft to become unstable, either in the technical definition or in the more general case. But then, Im not a designer. And for modern fighter aircraft, which are unstable from takeoff to landing, and flown by the computer, the question may be significant (although one would hope the computer would prevent an issue with the aircraft configuration).
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Lowering flaps increases the nose up pitching moment, so in the case of an F-16 I should think it could possibly make the jet un-trimable at some high speed. As always, it depends on the jet and it’s design.
In RL, if you could force the actuators to extend you’d most probably blow out the flap hinges/points - that’s the weakest structural point in the system in general, and the most likely point of failure…in which case the flaps will rip off of the jet once that failure occurs. After that, if you try and extend the gear to slow down you’ll rip the landing gear doors off too…which is where the 250 knot ROT for PA really comes from, as I recall.
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Ive not tested it in an F-16 obviously, but every low wing aircraft Ive flown so far (and granted, its a relatively short list) pitches down when you add flap. The additional drag located below the CoG in these cases increases the nose down torque, or moment.
Wind tunnel testing would be the way to go for finding out, and that data might be in TP 1538? My initial guess would have been to expect a nose down reaction from putting flaps in on an F-16, just based on where the TEFs are located relative to where I assume the aircraft CoG would be. But then again, for the F-16 in particular, this is not so useful knowledge. Seems like OPs question would relate to conventionally controlled aircraft that actually have a flap lever. In which case, it would as you say depend on the jet and its particular design.
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I seem to recall that when I was flying the Turbo Arrow (my favorite) that I had to push the yoke forward when I put the flaps down…in general, if you increase the camber of an airfoil (by extending flaps) it produces an increase in pitching moment for that airfoil…it’s quite possible that the Arrow pitches up because it’s a high power airplane, dunno. Pretty sure I recall the Dakota doing the same, and that the effect is more notable with a higher powered airplane. Given that you can actually feel the flap loads through the Johnson bar in a Piper, yeah…but the real limiting factor is still the flap hinge moment - load transfer from the flap to the hinge…the hinge is the weakest link, and what will break first.
Something I’ve been wanting to play with is the LE FLAP LOCK switch on the Flight Controls panel…I’m interested in if they lock to where they are when you throw the switch or if they lock into some preferred position. And what - if anything - that’s good or bad for…
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can the F-16 ever be parked with flat & not deployed flaps? I couldn’t do it in F4 BMS but I saw only one picture where the F-16 was parked and flaps were not down.
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Its possible to pin the landing gear in place. If you pin the landing gear, then press and hold the DN LOCK REL button (located immediately left of the gear handle), then raise the gear handle, the TEFs should retract to the streamline positions.
This is beneficial if its necessary to fly the aircraft for an extended distance with the LG extended, as retracting the LEFs and TEFs significantly improves fuel consumption.
Something I’ve been wanting to play with is the LE FLAP LOCK switch on the Flight Controls panel…I’m interested in if they lock to where they are when you throw the switch or if they lock into some preferred position. And what - if anything - that’s good or bad for…
Should lock in their current position. Good if you have a failure of the LEFs to track to the commanded position, but they end up in the 2° up position, and you can then lock them there.
If they become asymmetric, they should automatically lock, but you can manually command lock if you want. The -1 has a section on handling characteristics, and a heading in that section is dedicated to flight with locked LEFs. You could read that if you wanted an idea of what it should look like.
LEADING EDGE FLAPS LOCKED (SYMMETRIC)
Flight characteristics for landing and low AOA maneuvering are not significantly affected by locked LEFs. At high air-speeds, LEFs locked down cause increased buffet. At high AOA, LEFs locked up reduce stability, increase departure susceptibility significantly, and cause increased buffet. Above 16-18 degrees AOA, an abrupt yaw departure may occur, producing an uncommanded roll with little or no forewarning. Do not exceed 12 degrees AOA with the LEFs inoperative. Locked down LEFs significantly reduce cruise range. During landing, floating may also be noticeable if LEFs are locked at or near full down. The aircraft may float, sink rate may decrease, and a slight forward stick pressure may be needed to fly through the ground effect. -
Flaps encountered up/down post landing can depend on then the picture was taken - if the flaps are hyd powered they may be up at shutdown but then bleed down as hyd pressure decays…if they are electrically actuated they should stay put…in theory.
Failure modes are understandable, but my wanting to play with locking the LEFs has to do with some gouge I got from a RL fighter jock concerning using the AOA indexer in the T-38 up and away…he caveated it with thinking it only holds for a hard wing, but playing around with BMS I think it may hold with variable camber too, maybe even to greater extent. I’d like to do some test flying and compare observations - but I need the gear up.
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Well, thats pretty easy to do. Increase your angle of attack, either by increasing Nz or decreasing airspeed. Once your LEFs are extended sufficiently, lock them into place, then perform your testing as desired. Perhaps considering unlocking them before landing.
Just keep in mind that part of the variable camber comes from the ability of the LEFs to move. And that unless Im mistaken about how BMS gathered lift data for the F-16, what you see in BMS with locked LEFs may not represent what you would see in the real aircraft. Im not familiar enough with the source data for the aerodynamic simulation, but unless the test figures include the effects of LEF positions being outside the FLCS commanded values, then there may be some unsimulated effects.
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Its possible to pin the landing gear in place. If you pin the landing gear, then press and hold the DN LOCK REL button (located immediately left of the gear handle), then raise the gear handle, the TEFs should retract to the streamline positions.
This is beneficial if its necessary to fly the aircraft for an extended distance with the LG extended, as retracting the LEFs and TEFs significantly improves fuel consumption.
but the DN LOCK REL button is not functional in BMS where you answering for the real jet? because my question was about BMS
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Ah, it wasnt clear to me that you were asking specifically about in BMS. So far as I know its not possible in BMS?
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Well, thats pretty easy to do. Increase your angle of attack, either by increasing Nz or decreasing airspeed. Once your LEFs are extended sufficiently, lock them into place, then perform your testing as desired. Perhaps considering unlocking them before landing.
Just keep in mind that part of the variable camber comes from the ability of the LEFs to move. And that unless Im mistaken about how BMS gathered lift data for the F-16, what you see in BMS with locked LEFs may not represent what you would see in the real aircraft. Im not familiar enough with the source data for the aerodynamic simulation, but unless the test figures include the effects of LEF positions being outside the FLCS commanded values, then there may be some unsimulated effects.
Yes, but AOA really doesn’t have anything to do with what I want to do other than that I want to fly to maintain on-speed using the Indexer up and away…what AOA that turns out to be is going to vary with GWT, altitude, etc. I’m not sure about the TE flaps on the T-38, but I’m thinking they don’t move up and away? The USN A-7 used to have “maneuvering flaps”…at least I think it was the A-7…meaning that one could employ them during ACM. I used to hear older RL jocks lamenting not having them in more modern jets…but I have no idea if the T-38 did/does anything similar.
So…what I’d really like to do in BMS is to lock both the LEF and TEF (you’re correct in that unless I do both I’ve still got variable camber) up, and then do my maneuvering…and see what I get compared to one and/or both in full authority. I’m expecting similar results, but at differing levels…I’d just like to know to which side of “better” in each available case. Just curious.
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Yes, but AOA really doesn’t have anything to do with what I want to do…
Well, the reason I suggest increasing AOA is to set the LEFs to the angle you desire, for testing. If you lock them at high speed, low AOA, they will be in the 2° up position. If you want to test with them down, you will need to lock them during a high G turn.
Im hopeful that Mav-JP will chime in here, and comment on whether BMS will model the aerodynamic qualities of the wing accurately with the LEFs and TEFs not in the FLCS scheduled positions?
… other than that I want to fly to maintain on-speed using the Indexer up and away…what AOA that turns out to be is going to vary with GWT, altitude, etc. I’m not sure about the TE flaps on the T-38, but I’m thinking they don’t move up and away? The USN A-7 used to have “maneuvering flaps”…at least I think it was the A-7…meaning that one could employ them during ACM. I used to hear older RL jocks lamenting not having them in more modern jets…but I have no idea if the T-38 did/does anything similar.
So…what I’d really like to do in BMS is to lock both the LEF and TEF (you’re correct in that unless I do both I’ve still got variable camber) up, and then do my maneuvering…and see what I get compared to one and/or both in full authority. I’m expecting similar results, but at differing levels…I’d just like to know to which side of “better” in each available case. Just curious.
Do keep in mind that if you lock the TEFs, you dont have much control over the roll attitude of the aircraft. Actually, Im not even sure you can lock the TEFs? You can tell the FLCS to deploy them or handle them automatically, I dont think there is an option to lock them. There isnt a good reason to lock them anyway, because if the TEFs cant move, you only have very limited roll authority (through differential movement of the horizontal tails).
As far as the A-7, I dont know much about it, but I do know that it had flaps as well as ailerons. The fact that the F-16 doesnt have separate ailerons makes locking the TEFs a dicey proposition, at best.
Im a big fan of curiousity, generally. In this case, Im suspicious that the aircraft performance might not match the real thing, although Im hoping Im mistaken on that. Depends how BMS models the aerodynamic performance of the aircraft, something I dont know enough about.
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Hmmnn…agreed on the TEFs in the F-16 case. Hadn’t really considered that, but that’s still something I’d expect the old Navy dogs would lament. I think I have to just accept them doing what they do…which I don’t think will impact my test given the design of the airframe.
I’d like to lock the LEFs at zero for what I want to try…so I guess I’d need to unload? And I’m not interested in the effect on/of G either - on-speed will be on-speed and whatever G that turns out to result in will just be what I get and that will become part of the observation. Even if what BMS does doesn’t mirror RL exactly, I should see something close to what I’m expecting…I think. Or at least some differences between configurations, which is more what I’m interested in - degree and extent of any difference. The results will at least be consistent within the limitations of the BMS aero model because I’ll be using the same aero model. I’m more curious about what BMS does vice the real jet, in this case.