Thrust Vectoring Effect on Rate of Turn
 

Hi everyone

I just got a question after reading my old POF book. the Eqaution for rate of turn: ROT = 1,091 x tangent of the bank angle / airspeed

How would that change when the airplane is equipped with Thrust Vectoring?? like the F22 og SU35???? Does anyone has an equation for that??

Works on the Harrier.

Actually, on the Harrier it increased the pitch rate, which gave an instantaneous increase in rate of turn BUT this caused a loss of airspeed with the resultant reduction in rate of turn.

Usefull to generate angles and force a fly-through but at the cost of a massive loss of energy.

Happy days!

Mog

Loss of airspeed would result in a higher rate of turn...

I think Mogwi might know quite a lot about this, having been one of the very few pilots to have actual air combat experience on the type! In 1982....

It’s a bold move to attempt to correct someone offering the benefit of their experience with a statement that belies so little knowledge, especially when that person is a combat-experienced Harrier pilot! Your quote above is only true at a fixed bank angle at zero pitch rate above stalling speed; you might consider taking some elementary training in steep turning. In combat, turning is achieved by rolling to point the lift vector in the desired direction and pulling to attain maximum lift (and therefore turning performance). Maximum attainable turn rate rises with airspeed until Vc or ‘corner speed’ because the increase in available lift (a factor of V-squared and part of the numerator in a turn rate equation) dominates the calculation (in which the denominator is a factor of V). Turn rate decreases above Vc as structural load limits prevent additional lift from being applied.

1091VTAN Phi is dependent only based upon bank the bank angle, no matter how that particular bank angle was achieved, including a thrust vector component to the bank angle.

Wow! That's pretty arrogant arguing with a Combat Experienced Harrier Jock!

It doesn't contravene him at all...just a slight addition.

It still doesn't mean he is correct :)

I have 10 combat tours under my belt but I still have stuff to learn from others, to think otherwise is a tad arrogant.

p.s. I have no idea who is correct.

The Harrier has been around a while so I'm sure what Mogwi said is correct. But lets examine a hypothetical future aircraft.

The Harrier needs all of its thrust to sustain a high g turn. If you use some of this thrust to temporarily increase pitch rate then speed falls off. A future aircraft might be able to sustain limiting g at less than full power. If this is the case and it is capable of thrust vectoring, then the forward component of thrust may still be able to sustain limiting g whilst leaving some thrust spare to generate nose angle. Obviously its a little more complicated than that in that alpha is also likely to increase which will have a significant effect on drag and there may no longer be 'spare' power available to generate nose angle.

Just to add: kinematics is the study of motion independent of the applied forces that cause the motion. 1091VTAN Phi is a kinematic relation:)

In such a situation the ‘upward’ component of thrust would be added to the lift vector. This would instantaneously reduce the alpha (as the flight path would be changed ‘upwards’ with respect to the oncoming airflow) which would reduce lift (and turn rate), requiring additional nose-up control input to regain and then maintain the target alpha. The ability to make this control input manifests as “nose pointing” capability but in the hypothetical sustainable case it can also be seen as increasing the turn rate. In turn, the additional nose-up control input requires the thrust vector to be rotated slightly aft to sustain forward speed. A complex equilibrium would eventually be reached with not quite as much benefit as a ‘static’ analysis of the available excess thrust vector might suggest.

Other “nose pointing” effects may result from the thrust vector producing a rotational moment around the CofG. This is what I think you were getting at with your reference to increasing alpha, but it is actually an installation-specific effect.

2 items
.1 Your simplified calcilation is valid only for a level turn.
2. Vectoring adds an additonal force to that only from bank angle

FDR if you please, may you elaborate? I don't think that I'm understanding you correctly for The gTan Phi limitations

Yes, From your our primary training.... remember how during slow flight when the turn coordinator showed a high rate turn? If you choose to turn in the first place gotta give her slightly higher power setting and then you can safely commence that turn with a five or ten degree bank angle...just for general interest, the turn radius at the stall is infinite. :)

Surely if an aircraft is turning at the limiting G, increasing the pitch rate, no matter how you do it will exceed the limiting G?

Wouldn't that only be true if the lift at the stall was zero in a stable environment without thrust vectoring?

I was going to say something about that in one of my earlier posts but they got a bit long. The short answer is yes. However the 'upwards' component of the thrust vector contributes to the increase in pitch rate without increasing wing loading (lift is constant if speed and alpha are maintained) so the increase in load factor only applies to other parts of the airframe. So such a system would only be useful if structural limits were based on the wings or their attachment points.

PA,


there are various methods of TV, and TVC applied.

1. Harrier: 2-D split cold/hot exhaust, vectoring angles are downward relative to the aircraft longitudinal axis, giving a variable from thrust, or positive force normal to the longitudinal axis, or a combination of that or similar with a deceleration component. Viffing adds a vertical force that decouples the turning rate from the angle of the bank partially, still requires a vector in the desired turn, but the total normal force is a combination of lift and any thrust vectoring. Comes with some costs in EM state.

2. Sukhoi, limited axisymmetrical TVC. Gives increased agility, adds control authority to extend instantaneous rates to point nose. Has a force component for pitch rates, but that is also partially opposite the normal force from lift... for a pitch up by vectoring exhaust nozzles upwards, total lift force from the wing and body has to be increased to counter the negative component of the TVC vector. Not a problem if you have big wing areas and can generate reasonable CL/AOA. Instantaneous rate of turn is the benefit. Normal vectoring angles are around +-20 degrees from the longitudinal axis. Provides roll as well as yaw potential.

Note: any pitch TVC will increase wing bending load for a given g limit, the thrust counters the aerodynamic force required to achieve the total normal force to get the g... so TVC does come with a structural penalty, but great offsets.

3. F-22. 2-D, low RCS TVC. drops the yaw ability. Could do roll augment.

4. F-35: 2-D has a dedicated cold fan lift for TO/LDG, but does have the potential for TVC but is not indicated to be implemented. If it was it could be 2D or axisymmetric, with pitch and yaw, no roll. The design of intake suggests that it is not intended to do the Harrier Viffing deal.

5. J10B TVC: axisymmetrical TVC demo'd in 2018.

6. USSR, Yaks etc... dedicated lift engines, and always had a potential for pitch augmentation.

The F-22 has a Type III TVC nozzle which is in keeping with low RCS, all of the others, have type I nozzles.

Re g limits, only the harrier type TVC can unload the wing while achieving a given g loading. The g load limit may have a constraint on the flight control systems or fuselage itself but it is usually the wing bending load that determines the limit. Mixed design bag, the weight saved in the structure is offset by the weight of the cold exhausts and the total power plant installation weight. The AV8B had lots of potential to improve TVC ability... still a neat plane.

Schneider (1988) was mainly talking about the AV8 type design, and stated:

"the results indicate that the use of vectored thrust to supplement the aircraft's lift by directing the thrust into the turn can substantially reduce turning times and increase in-flight maneuverability".

That is a valid statement for an in-flight lift design, which most current designs are not, they instead have gone to the pitch augmentation which gives instantaneous rate enghancement, but detracts from available lift for a given AOA (yes, it also gives more AOA capability, so other than the buckets of drag... etc... structurally, the wing has to be stronger than a non pitch augmenting TVC design.

The AV8 would have had intersting potential for extreme agility with a bi 'o mixing of the nozzles, and throw in an ACM mix of RCS as well...The F-35 not implementing TVC is curious, even a TYpe III nozzle wouldn be worthwhile or a Type IV fluidic on any exhust would be entertaining for instantaneous rates.. In inpinging flow would also direct the exhaust, there was some interesting work in htat area in the early 90s, but was not directed to giving improved agility to aircraft. (did a bit of that on a Learjet exhaust, which was working on acoustic supression, ended up getting an effect similar to exhaust wedges, but without the EGT rise, was all going well until it didnt, and then we got to remake the devices and redo it, and repaint the nacelle).





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good reading at:

Snow, B. (1990). Thrust Vectoring Control Concepts and Issues. SAE Transactions,99, 1488-1499.

Lee, P.H., Lan, C. E., (2012) Effect of thrust vectoring on level-turn performance. AIAA J. Aircraft, vol 29 (3) Engineering Notes

Schneider, G.L Watt, G.W., (1988) Minimum-Time Turns Using Vectored Thrust . AIAA J. Guidance,

Victoria, R., Gatlin, D., Kempel, R., Matheny, N., The F-18 High Alpha Research Vehicle: A High-Angle-of-Attack Testbed Aircraft, NASA TM-104253, Sept. 1992.

Canter, D., "X-31 Post-Stall Envelope Expansion and Tactical Utility Testing," Fourth NASA High Alpha Conference, NASA CP-10143, vol. 2, July 1994.

Kidman, D. S., Vickers, J.E., Olson, B.E., and Gerzanics, M.A., Evaluation of the F-16 MultiAxis Thrust Vectoring Aircraft, AFFTC-TR-95-12, Sept. 1995

Orme, J. S., Hathaway, R., and Ferguson, M.D., Initial Flight Test Evaluation of the F-15 ACTIVE Axisymmetric Vectoring Nozzle Performance, NASA TM-206558, July 1998