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    Su-57 Stealth Fighter: News #6

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    PeregrineFalcon

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    Post  PeregrineFalcon Yesterday at 12:01 am

    LMFS wrote:Welcome!

    Thanks!

    LMFS wrote:Good find! I had not seen this before, thought it had been argued they could be used to pitch the nose up. 99% per cent of the time LEVCONs are scheduled like the LE flaps and only bend downwards, but you managed to find a moment when they go upwards, even when only very slightly, way less than normal canards. It makes sense, since this is an additional trim option the plane has. I stand corrected.

    BTW, during take off the plane seems to need really minimal deflection of the surface controls to pitch up the nose and take off. It seems like the plane has substantial longitudinal instability, how do you think this should work in supersonic flight? What do you think is the design point of the airframe, that is, the speed at which it needs the least trim?

    I share your opinion about planes instability levels that actually dictate such a minimal upward LEVCON deflection. Since the PAK FA aerodynamic design represents blended wing-body sheme, in a sense it is like a big wing [cranked-arrow wing] with mixed airfoil cross sections, LEVCON's can be used for trimming purposes in the way that they can be deflected upward by a few degrees to reduce camber drag penalties during supersonic flight, and since they are placed in front of planes CoG they create lift and reduce drag compared to horizontal tails that are placed behind the planes CoG and need negative deflection.
    Of course, this is my theory, but coupled with the TVC, there is no doubt that the plane has ability to significantly reduce trim drag during supersonic flight, and for example, there are calculations that only by using TVC for trimming purposes there could be increase in turning rate by 14% for sustained supersonic turn!

    LMFS wrote:The interview is good, from the old days when they actually said interesting things about the plane, thanks. I am not seeing all those claims there, if you find the video it will be great.

    Don't get me wrong, I expect that the Su-57 surpasses the Su-35 in most regards, but in absence of proof is good to be cautious.

    BTW I found your argumentation very well built, my main comment was the lack of evidence for some claims as they were posted here. Of course I am happy to learn new things about the Su-57, it is getting harder this days almost nothing is disclosed

    Thanks!
    I always try to base my argumentation on the facts as much as possible, but sometimes we need to use our brains to fill the holes the best way we know in the absence of hard factual data Wink


    Last edited by PeregrineFalcon on Mon Nov 23, 2020 11:24 am; edited 1 time in total

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    Post  GarryB Yesterday at 12:07 am

    At supersonic speeds the center of lift moves backwards, that means, the nose tends to fall.

    That is correct.

    To compensate for that your nozzles need to point upwards, that is, create a downward force, to lift the nose.

    That is not correct.

    They don't actually point upwards they might shift angle a few degrees which will compensate for the shift in cg, but they will maintain the angle of the aircraft into the incoming airflow to maintain attitude without having to shift a horizontal tail surface or canard foreplane to do the same... a few degrees shift in engine nozzle will not create drag or increase RCS... a shift in canard or tail surface will.

    That force needs to be compensated with more lift to keep level flight.

    There is no change in lift, just a shift in cg that is compensated by a slight change in thrust line.

    Think of the V-22 or the Harrier... but instead of using vectored thrust to take off vertically you just use it like a normal aircraft only moving thrust angle by 10 degrees up or down at most to change the angle of attack of the aircraft to optimise the efficiency of the wings... so you can remove the horizontal tail because you are using the engine for up and down force.

    It would optimise drag even better because the tail surface would essentially have zero drag and RCS which wont increase in turns or at different flight speeds.

    The one thing in common with all drawings of supposed 6 gen fighters is a main wing and no vertical or horizontal tail or canards... less weight, less drag, less RCS.
    Yet with TVC you get all the turn authority any physical surface could provide at any flight speed including zero where normal control surfaces are dead weight.

    A downwards force needs to be compensated, not differently to tail deflection.

    But a downward force from a control surface is not the same as a downward force from an engine nozzle... you are trying to say that if there is enough downward force from a tail surface that the plane will fall out of the sky because the downward force will exceed the lift of the wing and the total downforce will make the plane lose lift and crash, but that is bullshit... if the downward force from the tail or the upward force from a canard at the front is too much that does not directly cause the plane to go up or down... it does what it is supposed to do and causes the aircraft to rotate around its cg to compensate for the shift in cg.

    The F-14 has small surfaces in the wing leading edge that deploy in some flight modes to prevent cg going to dangerous places at low speed where conventional controls might not have enough effect to compensate... it does not have thrust vectoring engines obviously.

    The downwards or upwards force of the engine doesn't make the aircraft descend... what it does is as you say... stops the nose from pitching down ... that is all.

    It doesn't make the aircraft fly faster or higher or lower... and it does so without increasing drag like moving a canard or horizontal tail surface would... like I am saying.

    It is better to have lift in front of the CoG than downforce behind it.

    Surely it makes no difference at all where the lift is as long as it is kept near the cg, which is what deflecting canard or tail or engine nozzle does... the latter does so without increasing drag.

    Yes I know. The point you seem to be missing is that pitching the nose down is done by creating additional lift with the TVC while pitching it up demands the nozzles to press the tail down.

    And what you seem to be missing is that travelling through that transonic area of flight speed causes the cg of the aircraft to move and the shifting of TVC angle doesn't increase AOA... it maintains it while the cg is moving and then stabilising... once past the transonic speed the cg settles down and does not move at any other flight speed so temporarily shifting engine nozzle trim to compensate maintains AOA it does not change it.

    Using canards or tail surfaces also maintains AOA but generates drag and effects RCS with it moving to compensate and then moving back as you accelerate further through to higher speeds.

    So one way it creates lift, the other reduces it.

    No. They both essentially do the same thing they are turning the aircraft around its cg to maintain AOA... it is the lift from the main wing and the fuselage lift that keeps the aircraft in the air most of the time the canard or tail are neutral... when they are not their job is still not to keep the aircraft in the air... their job is to control the AOA of the aircraft... which can increase or decrease drag and lift.... Canards and or tail surfaces maintaining an AOA of 120 degrees without TVC engines not only does not create lift but will lead to a super stall on most conventional aircraft almost immediately.

    That was the whole rationale behind canards on longitudinally stable planes BTW, so instead of a tail reducing lift all the time to avoid nose falling, you have a foreplane increasing it.

    The only real advantage of the canard arrangement is making the aircraft stall proof where the canard stalls before the main wing and forces the nose to drop before the main wing stalls and control of the aircraft is lost.

    Interestingly the An-2 has similar properties... if your engine fails you just pull back hard on the stick and hold it there... the aircraft will slow down until the flaps and slats deploy and the plane slowly sinks to the ground...

    This seems legit, dont know if it was posted before, just seen it on another forum.

    The missiles are not right, but the obvious question is why would it be landing or taking off with missile bays open... except as a nice photo op... in which case I would expect a much better quality photo...

    Interesting though...


    Compared to deals like France/India, I think Russia gave Algeria as good of a deal as they could.

    They would need to give Algeria the coatings needed to maintain stealth, and the technology associated to apply and maintain them on the aircraft, plus likely simulators and of course new weapons to go with them too...

    Perhaps 20-40 million dollars worth of stuff for a 100 million dollar aircraft... the Rafale was 222 million but it was new.... they had never operated that aircraft before so that would include a pool of spare parts and training and support equipment and machinery to get them working and keep them going till their next purchase... would be the same for the Su-57.

    Wonder if this will lead to renewed interest from India... or even Turkey.
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    Post  PeregrineFalcon Yesterday at 12:19 am

    Backman wrote:No way ! Welcome Fastestbird. Make sure you introduce yourself in the intro page. Its a requirement on the forum.

    Many thanks for the warm welcome!!!
    LMFS
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    Post  LMFS Yesterday at 1:38 am

    PeregrineFalcon wrote:I share your opinion about planes instability levels that actually dictate such a minimal upward LEVCON deflection. Since the PAK FA aerodynamic design represents blended wing-body sheme, in a sense it is like a big wing [cranked-arrow wing] with mixed airfoil cross sections, LEVCON's can be used for trimming purposes in the way that they can be deflected upward by a few degrees to reduce camber drag penalties during supersonic flight, and since they are placed in front of planes CoG they create lift and reduce drag compared to horizontal tails that are place behind the planes CoG and need negative deflection.

    Great explanation thumbsup

    Of course, this is my theory, but coupled with the TVC, there is no doubt that the plane has ability to significantly reduce trim drag during supersonic flight, and for example, there are calculations that only by using TVC for trimming purposes there could be increase in turning rate by 14% for sustained supersonic turn!

    Maybe you have seen I am discussing with GarryB precisely about TVC and trimming. So, as the CoL moves backwards, it may eventually get behind the CoG and actually give the plane the longitudinal stability it does not have in subsonic regime. At this point, my understanding is that you need lift in front of the CoG as you explained above for the LEVCONS. I understand that in such situation where trim requires pitching up the nose, TVC, being after CoG, will need to deflect upwards and push the tail down to correct the AoA. Minus de added drag that deflecting tails would imply, this use of TVC would be equally detrimental to the plane's overall lift, do you agree? From your statement above it seems TVC could be actually used for supersonic trim purposes. The plane being unstable I would agree, but having turned stable by the higher flight speed I can't see how it would help. That 14% improvement would be compatible with the reduction in overall lift that negative deflection of the tails would mean, I don't see why creating the same effect with the TVC would not result in the same overall reduction in lift and hence STR.

    I always try to base my argumentation on the facts as much as possible, but sometimes we need to use our brains to fill the holes the bast way we know in the absence of hard factual data Wink

    Yeah, in the case of Su-57 that is 90% of the time, so sometimes we get a bit wild speculating, but there is no way around it scratch

    GarryB wrote:That is not correct.

    They don't actually point upwards they might shift angle a few degrees which will compensate for the shift in cg,

    My opinion in this regard is clear. The tails or TVC push the tail down to compensate the nose's downwards moment, rotating it upwards around the CoG. The result is an effective reduction of overall available lift as explained. Maybe I am missing something, I don't design planes for a living, but as far as I see it is pretty simple. It is not clear to me what do you mean by "shift angle a few degrees": if you mean shifting "a few degrees" upwards, then it is the situation I am describing and there will be a vertical component of the force pointing downwards, I don't know how to get around that dunno

    But a downward force from a control surface is not the same as a downward force from an engine nozzle... you are trying to say that if there is enough downward force from a tail surface that the plane will fall out of the sky because the downward force will exceed the lift of the wing and the total downforce will make the plane lose lift and crash, but that is bullshit...

    Downforce is downforce, a force vector pointing down in the vertical axis, no matter what device created it. The tails have added drag, the TVC not so much, but drag is not what I am talking about.

    The plane will not "fall out of the sky", because the penalty is small compared to the overall lift and will be compensated by means of increased AoA, but of course for a plane that tries to have a very high service ceiling it can be very counterproductive, since generating lift at high altitudes is quite difficult. And of course STR would suffer quite a bit too. Those are all issues a plane specifically designed to fly fast and high with high manoeuvrability and for extended periods should address from the onset.
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    Post  PeregrineFalcon Yesterday at 3:20 am

    LMFS wrote:
    Maybe you have seen I am discussing with GarryB precisely about TVC and trimming.

    Yes, when I finished with my post I have seen that you two are having an interesting debate, and in my humble opinion both of you have some good and some bad, or should I say somewhat wrong assumptions/conclusions.

    I will try to be short this time, but maybe later, if I find more time I can join more detailed discussion.

    LMFS wrote:So, as the CoL moves backwards, it may eventually get behind the CoG and actually give the plane the longitudinal stability it does not have in subsonic regime. At this point, my understanding is that you need lift in front of the CoG as you explained above for the LEVCON's.

    You can create that lift in a few ways in order to pitch the nose up.
    One way is by using the LEVCONS as explained above, the other way is by using the horizontal tail where we apply the downforce on the back of the plane to increase AoA which consequently increases the overall lift needed to pitch the nose up, but at a cost of creating more drag and less overall lift compared to the LEVCON's [by deflecting the tails down we don't create the lift, unlike the LEVCON's].
    The third way for example is by using the TVC.

    LMFS wrote:I understand that in such situation where trim requires pitching up the nose, TVC, being after CoG, will need to deflect upwards and push the tail down to correct the AoA. Minus de added drag that deflecting tails would imply, this use of TVC would be equally detrimental to the plane's overall lift, do you agree? From your statement above it seems TVC could be actually used for supersonic trim purposes. The plane being unstable I would agree, but having turned stable by the higher flight speed I can't see how it would help. That 14% improvement would be compatible with the reduction in overall lift that negative deflection of the tails would mean, I don't see why creating the same effect with the TVC would not result in the same overall reduction in lift and hence STR

    By using the TVC for supersonic trimming purposes we get better lift to drag ratio compared to classic aerodynamic surfaces. The principle of action is the same when we talk about TVC and horizontal tails [or elevon in delta wing etc.], but the difference in drag is significant. Protruding elevators create much higher amount of drag at supersonic speed compared to just a few degrees that are needed for the TVC to trim the plane, and that is the main difference.
    In other words, for the same AoA you create less drag and more lift with the TVC because you are not reducing the overall lift by greater downward deflection of the control surface [you do not need that negative deflection of the control surface when you have TVC], and you do not create additional drag by the higher protruding surface.
    Since the amount of the lift is the crucial thing for the pitch up of the nose, your plane with the TVC will generally need less AoA to create the same amount of the lift compared to the plane with the classic aerodynamic surfaces that needs higher AoA [which will aditionaly worsen the drag] because that plane will inherently have worse lift to drag ratio.

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    Post  LMFS Yesterday at 11:44 pm

    PeregrineFalcon wrote:I will try to be short this time, but maybe later, if I find more time I can join more detailed discussion.

    No problem, if anytime you have the opportunity to explain further, it will be welcome.

    You can create that lift in a few ways in order to pitch the nose up.
    One way is by using the LEVCONS as explained above, the other way is by using the horizontal tail where we apply the downforce on the back of the plane to increase AoA which consequently increases the overall lift needed to pitch the nose up, but at a cost of creating more drag and less overall lift compared to the LEVCON's [by deflecting the tails down we don't create the lift, unlike the LEVCON's].
    The third way for example is by using the TVC.

    Yes, no questions here.

    By using the TVC for supersonic trimming purposes we get better lift to drag ratio compared to classic aerodynamic surfaces. The principle of action is the same when we talk about TVC and horizontal tails [or elevon in delta wing etc.], but the difference in drag is significant. Protruding elevators create much higher amount of drag at supersonic speed compared to just a few degrees that are needed for the TVC to trim the plane, and that is the main difference.

    Agree here too..

    In other words, for the same AoA you create less drag and more lift with the TVC because you are not reducing the overall lift by greater downward deflection of the control surface [you do not need that negative deflection of the control surface when you have TVC], and you do not create additional drag by the higher protruding surface.

    Since the amount of the lift is the crucial thing for the pitch up of the nose, your plane with the TVC will generally need less AoA to create the same amount of the lift compared to the plane with the classic aerodynamic surfaces that needs higher AoA [which will aditionaly worsen the drag] because that plane will inherently have worse lift to drag ratio.

    Wait a second, I will make an example to see where I am getting lost, if you don't mind. Let us say at a given moment of the supersonic flight you need say 20 "points" of net downforce on the tail to trim the plane. Let us assume the horizontal tails in neutral position at that AoA would create 30 points of lift. So if you use them for the trimming, you deflect them negatively and change 30 points of lift for 20 of downforce (I am ignoring effects of the extra drag here). Now with the TVC, you would need to create 50 points of downforce to negate those 30 points of lift behind the CoG plus the amount needed for the trimming? Wouldn't it be better to adjust the horizontal tail for zero lift at that AoA and produce just 20 points downforce with the TVC? That means, no matter what way you are generating that downforce, you end up losing the same amount of lift and the only difference is that TVC is not creating big additional drag. Because all the lift you generate after the CoG is just further pushing the nose down in the end...
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    Post  GarryB Today at 12:10 pm

    The tails or TVC push the tail down to compensate the nose's downwards moment, rotating it upwards around the CoG.

    That is correct, but the tail or engine nozzles weren't developing any lift to begin with... it is just generating a force to maintain aircraft body trim...

    The result is an effective reduction of overall available lift as explained.

    A couple of problems with this... aircraft lift is not a zero sum game... the consequences of not using tail or engine nozzles to raise the nose will generate a negative angle of attack which will have a much more dramatic effect on lift than any trimming effect they do have to rotate the aircraft on its new cg.

    The lift that keeps an aircraft in the air comes from the wing and the fuselage for those aircraft with lifting bodies. The forces applied by control surfaces and moving engine nozzles create very specific forces to turn the aircraft in various ways including roll, yaw, and pitch, and in that sense they often generate a little extra drag, but they are airflow deflectors... not lift generating devices as such.

    If your aircraft can only maintain flight with the help of canards lifting at the front you have a shit aircraft design.

    Maybe I am missing something, I don't design planes for a living, but as far as I see it is pretty simple.

    I don't design them either, but ripping the horizontal tail surfaces or canards off a plane does not lead to loss of lift and rapid descent... it more often leads to loss of control... because that is their primary function.

    It is not clear to me what do you mean by "shift angle a few degrees": if you mean shifting "a few degrees" upwards, then it is the situation I am describing and there will be a vertical component of the force pointing downwards, I don't know how to get around that

    It goes back to boats and trim. With the propeller horizontal the force it generates pushes the boat horizontally through the water, but water is a very high drag medium. With a very small boat with a big engine, what you can do is angle the propeller so the force does not just push the boat horizontally but it actually pushes down and lifts the boat out of the water. It does not make it fly, but it does make it skim or plane on the surface so there is a lot less drag and returning the prop to horizontal means the boat can go faster for a given amount of power because it does not have to push through as much water.

    WTF am I on about... a jet aircraft does not skim water.

    You don't design aircraft and neither do I but the shape of an aircraft is absolutely critical to move efficiently through the air. Changing the angle of attack will effect how that aircraft moves through the air... there will be a specific angle of attack for a given speed and altitude where the air flows over the wings and it generates the ideal amount of lift with the minimum level of drag... a 90 degree angle of attack to the flight airflow is essentially being an airbrake... no lift and all drag.

    Lets say the ideal AOA for aircraft x flying at airspeed y at altitude z is 8 degrees... you might need a canard deflection of 15 degrees to maintain that, or a tail surface deflection of 15 degrees to do the same. The Canard would be lifting the nose and the tail surface pushing the tail down, but the effect is the same because the engine thrust is angled down 8 degrees and the wing angle of 8 degrees creates the ideal level of lift for that aircraft in that configuration etc etc etc.

    As the aircraft approaches and then exceeds the speed of sound the balance of the aircraft changes... the cg moves which causes the nose to drop... if you do nothing you will have problems because you will start to lose altitude for a start... while you are flying through this trans sonic speed range the cg moves but as you keep accelerating it moves back, so the shift and also the countering of the effect is temporary anyway.

    During this period of instability you might increase canard deflection to 20 degrees up, or the tail surface to 20 degrees down to maintain the 8 degree aircraft pitch up attitude.

    What I am saying is that a 20 degree pitched up or 20 degree pitched down canard or tail surface increases drag and RCS when the aircraft is pitched up a 8 degrees that is an almost 30 degree pitch up angle for the canard, but only a 12 degree pitch down from horizontal for the tail surface, but the nozzles on a thrust vector jet engine have much more control authority than canards and tail surfaces so the ideal low drag angle of attack might not be 8 degrees with TVC... it might only be 4 degrees with the TVC nozzles pitched up 4 degrees in normal flight at that altitude and speed, but the shift in cg it might need a TVC nozzle shift of a further 4 degrees to 8 degrees upwards to counter cg shift until that speed gap is passed through and then it can return to a 4 degree angle up thrust line.

    What I am saying is that the engine nozzle pitch angles will be much less to get the same effect but also not really noticeable from the front anyway in terms of IR and RCS factors... and they do their job at any speed including zero forward speed and even negative forward speed in a tail slide.

    The canards and tail surfaces can be kept at low drag neutral positions...


    It is like talking about super sized HUDs offering the best opportunity for the pilot to keep his eyes outside the plane and not staring at screens in the cockpit, when the obvious solution is really a helmet mounted sight...


    Downforce is downforce, a force vector pointing down in the vertical axis, no matter what device created it.

    A down force that turns an aerodynamic structure like an aircraft to a nose up attitude increases the lifting performance of the main wing and makes the aircraft climb and not descend... why do you describe it as a loss of lift?

    The tails have added drag, the TVC not so much, but drag is not what I am talking about.

    Drag and lift are directly related and pretty damn critical if you want efficient long range super cruise.

    Having no tail and no canard benefits in terms of drag, but obviously creates risk in terms of loss of control if the TVC fails.

    Yes, when I finished with my post I have seen that you two are having an interesting debate, and in my humble opinion both of you have some good and some bad, or should I say somewhat wrong assumptions/conclusions.

    Don't hold back Smile ... I have been wrong about plenty of things in the past and I am discussing this not to be right but to compare my view with others to find the truth...
    (I am not an engineer so be gentle...)
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    Post  LMFS Today at 6:25 pm

    GarryB wrote:That is correct, but the tail or engine nozzles weren't developing any lift to begin with... it is just generating a force to maintain aircraft body trim...

    Tails and foreplanes are airfoils and can generate lift as wings do. In a plane with AoA 2deg. with tails at neutral position re. the fuselage, both wings and tails are generating lift. In fact the lift generated by the tails is crucial to calculate turning rates because on modern planes they do not need to be deflected negatively for the plane to turn (pitch up the nose with a banking angle) since they are unstable, and they add a sizeable amount of the total lift.

    aircraft lift is not a zero sum game...

    Not if it is produced at the CoG, if it is produced after or behind, it needs to be compensated or it will make the plane lose longitudinal control.

    but they are airflow deflectors... not lift generating devices as such.

    See above, they are airfoils and they create drag and lift by the same mechanisms the wings do.

    If your aircraft can only maintain flight with the help of canards lifting at the front you have a shit aircraft design.

    With traditional stable planes it is either canards or tail, and without them you simply crash.

    Lets say the ideal AOA for aircraft x flying at airspeed y at altitude z is 8 degrees... you might need a canard deflection of 15 degrees to maintain that, or a tail surface deflection of 15 degrees to do the same. The Canard would be lifting the nose and the tail surface pushing the tail down, but the effect is the same because the engine thrust is angled down 8 degrees and the wing angle of 8 degrees creates the ideal level of lift for that aircraft in that configuration etc etc  etc.

    Not really. If at a given point of the flight envelope with a certain AoA you have 100 points of lift produced by the wings and you use the canards to trim, which produce 20 points, you have 120 points of total lift available. If you instead use the tails to trim, creating 20 points of downforce, you have 80 points of total lift available. Obviously you will need either to reduce altitude or to increase AoA or to increase speed, but you will not be able to sustain level flight in the same conditions as the plane with canards. I recommend you to look for this on the many good sites about aerodynamics that you can find on the web, since I don't seem to be able to convince you.

    but the nozzles on a thrust vector jet engine have much more control authority than canards and tail surfaces

    It depends. Zero airspeed full AB at sea level is not the same as corner speed cruise throttle at high altitude. You would need to crunch some numbers there.

    What I am saying is that the engine nozzle pitch angles will be much less to get the same effect but also not really noticeable from the front anyway in terms of IR and RCS factors... and they do their job at any speed including zero forward speed and even negative forward speed in a tail slide.

    I am all for TVC, I am only saying on a plane that needs to pitch up the nose, it is better to generate lift in front of the CoG than downforce behind.

    The canards and tail surfaces can be kept at low drag neutral positions...

    Wings could also be kept low drag at neutral position, only the plane would not fly then... control surfaces contribute to the total lift generation. Of course deflections need to be much smaller than the values you were giving above, above 15 deg almost any airfoil is going to stall.

    A down force that turns an aerodynamic structure like an aircraft to a nose up attitude increases the lifting performance of the main wing and makes the aircraft climb and not descend... why do you describe it as a loss of lift?

    See above. Every loss of lift through the downforce needs to be compensated by higher AoA which creates more drag.

    Having no tail and no canard benefits in terms of drag, but obviously creates risk in terms of loss of control if the TVC fails.

    Without surface controls you cannot trim the plane and react to changes in CoG /CoL, there is no way around it. Even a flying wing needs surfaces to substitute tails.

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