Forum: Modern Military Aircraft

The F-35 wing loading thread went off topic but had lots of interesting stuff in it.

I wanted to make a new thread for that kind of thing.

The original thread involved debate over body lift and the design of the F-15 vs the Su-27. High speed vs low speed etc.

I wanted to get a good thread going and I thought I would post some questions to start it:

1. Is the canard or a LERX a better overall solution to getting high alpha performance but also supersonic and transonic cruise?

2. Why did the Typhoon pick the foreplane over the close coupled canard?

3. How limited in performance is the Hornet series because of what seems like an aged design?

4. How effective would a tailless design with a double delta or large LERX and highly swept wing be in various areas?

5. What about the relative merits of different designs like Typhoon vs Rafale? Is the close coupled canard and foreplane really a big difference?

6. How is it that the F-15 can beat the F-16 at higher speeds when it seems the F-16 has a more advanced design?

7. How would an F-23 perform compared to the F-22?

I know that's a lot but the F-35 thread was going OT and I have lots of questions and I like to read the answers to questions like the above.

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The YF-23 had better stealth and speed than the YF-22, and apparently only lost out in terms of post-stall maneuverability (from the -22's TV) and price. Considering high-off-bore-sight missiles and helmet mounted target designation, and the fact that price estimates went out the window, the F-23 may well have been the better choice.

I also look forward to see an answer to the first question because canards are totally and foreplanes are to some extent meaningless to me.

Canards can not be mounted at the nose section of the a/c
1. Because of torsion they create during roll maneuvers
2. Also because the nose section is narrow which results in a short moment arm for rolling.
On the other hand in a Rafale style mounting of canards, this time the moment arm for pitching becomes shorter.

Therefore between the two (canards vs foreplanes) I support the Typhoon design.

I don't claim to be a super expert as I'm only a junior in Aero engineering and as such am just now "getting to the interesting stuff" but I will attempt to answer at least one of the questions.

1. It's difficult to compare canards directly to LERX as canards serve a lot more purposes than LERX than just simply increasing high-alpha performance through vortex lift. Canards provide far more efficient lift generation subsonically due to having essentially a much higher aspect ratio than LERX. That and LERX doesn't really work worth jack in cruising flight anyway IIRC. It isn't until high alphas are performed that LERX becomes valuable. Canards are a control surface, a lifting surface, a combination of both, etc. You can't really compare canards to LERX due to the limitations on the purpose of what LERX is supposed to do.

I'd select canards though for transonic and supersonic cruise. As has been found in the attached Virginia tech paper canards tend to provide much better supersonic trim drag reduction qualities due to load sharing between canard and wing as well as a "smaller increase in stability from compressibility" when compared to conventional tails. Also with canards the aft end of the fuselage can be made clean which can help reduce wave drag due to providing a favorable cross-sectional area distribution on said aft-end. Of course the issue with canards is developing a good control scheme for them. They can flood everything aft of them with turbulent airflow which can harm performance of everything aft of them including the wing itself so placement of canards as well as sizing of canards is a VERY sensitive task.

http://www.dept.aoe.vt.edu/~mason/Mason ... rdsS03.pdf

For high alpha maneuvering I'd imagine the LERX would be better for keeping flow attached to the upper surface of the wing and energizing the flow circulation of the upper surface of the wing airfoil thanks to stronger vortices coming off of them. Canards would likely not be designed to generate such strong vortices off of the tips in part due to the necessity of being able to be efficient at their jobs of serving as a control surface, lifting surface, etc. As such canards would be more designed to discourage spanwise vortex flow over the tips if high efficiency of lift generation is to be maintained.

r2d2 wrote:

I also look forward to see an answer to the first question because canards are totally and foreplanes are to some extent meaningless to me.

Canards can not be mounted at the nose section of the a/c
1. Because of torsion they create during roll maneuvers
2. Also because the nose section is narrow which results in a short moment arm for rolling.
On the other hand in a Rafale style mounting of canards, this time the moment arm for pitching becomes shorter.

Therefore between the two (canards vs foreplanes) I support the Typhoon design.

I think the moment arm is important because it lowers the amount of lift needed from the canard/foreplane as what is being produced is a torque. Less lift, less drag.

The reasons given for not mounting canards on the nose are not valid. Nose structure can carry torsional loads as well as any other, provided proper load paths are provided. If you are still concerned about the torsion, a little clever scheduling of the canards during rolls can totally eliminate the torsional load. The moment arm is indeed short for the canards to contribute much roll moment, so don't bother using them for rolls. Wings and tails have provided roll power for a long time.

Thanks for the answers. I would like to dig it up a little more but first I must ask this:

I thought the term 'foreplane' is used for symmetric moving control surfaces and the term 'canard' is used for control surfaces that can move independently from each other.

Isn't that correct?

shingen wrote:

3. How limited in performance is the Hornet series because of what seems like an aged design?

Only about as much as any other plane of its era. What made you wonder about that one in particular? The fact that it's still in production and thus sometimes presented as a cheap alternative to F-35? The biggest problem or design-goal "failure" I know of to associate with Big Bugs in particular is that part of the reason why they made them bigger than the original Bugs was for range, but they ended up not really gaining much range when carrying a load. But this is not due to the design's age; it was the case even when the plane was new. It was because some unanticipated aerodynamic effect made bombs turn inward and hit each other when released, which had to be prevented by mounting them turned outward, thus increasing drag; it was too late in the plane's development to reshape it much to change the aerodynamic cause of the situation.

shingen wrote:

6. How is it that the F-15 can beat the F-16 at higher speeds when it seems the F-16 has a more advanced design?

I don't know of any way F-16 is more advanced. It came along at about the same time as F-15 and part of its design goal was actually to be simpler and thus cheaper. Their advantages and disadvantages against each other are not just a matter of advancedness.

F-16 is lighter. Lighter planes, all else being equal, can be more maneuverable, because of a geometric scaling effect. Cut the length, width, and height in half, and you cut the control surfaces down to a fourth because they're two-dimensional (measured in square units), whereas you cut the weight to and eighth because it's related to volume, which is three-dimensional (measured in cubic units)... which means you end up doubling the ratio of control surface area to weight. That was part of why they made F-16 small, and it did work; they are indeed more nimble than their bigger brothers. (This is also part of why missiles are more maneuverable than planes. It's just easier to get a light object to turn than a heavy one.)

But large size retains other advantages. Fuel storage is a cubic function whereas drag is square one, so the ratio of fuel to drag is higher for a bigger plane, which gives it more range. That matters not only for simple consideration of how far away from your base you can fly, but also for some tactics you could use in a dogfight. If you don't like the position you're in, you are more free to loiter until it improves, or go a long way around to get where you want to be, or disengage and come back again, or make more liberal use of your afterburner(s), without having to worry about creating a low-fuel problem for yourself.

A bigger plane also can carry a bigger radar and more electronic gadgets of other kinds, and is less affected by the weight or drag of the same weapons.

One of the biggest factors is that a bigger plane is more likely to have two engines instead of one. Although I used doubling or halving above as a simple mathematical example of geometric scaling issues, a heavy fighter usually isn't quite twice as heavy as a light one, but, all else being equal, two engines will be twice as powerful as one. F-15 and F-16 are a perfect example of this, because they did indeed have exactly the same engine (at least at first), but F-16 didn't weigh half as much; it weighed slightly more than half as much, which made its thrust-to-weight ratio (TWR) lower. The plane with a higher TWR can climb faster and accelerate harder. Climb rate is useful in dogfights for countering the tendency to lose altitude with intense maneuvering. Having greater acceleration lets you reach higher speeds, which is good both for chasing enemies if they try to get away, and for getting away yourself (even if only to come back moments later), and putting more engine power into your turns helps with turning rate, thus reducing the scaling-related maneuverability gap I described above with the ratio of control surfaces to weight.

What all of that does is give the F-15 pilot some ways to try to make up for the smaller plane's greater nimbleness and/or render it moot by understanding their differences and putting them to tactical use: not getting pulled into the F-16's tight-maneuvering game, but keeping his distance (possibly by opening with a long-range shot made possible by the bigger more powerful radar so there's not even a dogfight at all), playing for altitude, and holding open the option of disengaging and reengaging. These kinds of things are how F-4 pilots learned to beat the smaller, more acrobatic MiGs of its era.

Since your question specified that we're talking about a fight at high speeds, I'll also add that the twin-engined F-15's higher TWR and top speed mean that, at any given speed, it's not as close to its limits as an F-16 would be at that same speed, so it still has more room to momentarily accelerate and/or go against gravity while the F-16 is under more strain just to be flying at that speed at all.

shingen wrote:

7. How would an F-23 perform compared to the F-22?

Simple maneuverability was not the only reason it was rejected. The maneuverability gap was especially drastic at high altitudes and supersonic speeds, which are areas of the "flight envelope" where control surfaces lose effectiveness but thrust vectoring doesn't. This was exactly the part of the flight envelope which the Air Force was most interested in preserving maneuverability in. Also, YF-23's engine outlet tunnels were lined with heat tiles which were a maintenance headache, and the outlet placement and shape meant that its infrared signature reduction was better from below but didn't exist from above, whereas YF-22's was consistent above and below. And it carried fewer missiles in a complicated rack system which wasn't working at all yet at competition time, couldn't be adapted to carry air-to-ground weapons like YF-22's could, and could result in one missile being blocked from use if another one jammed. (There could also have been differences in their electronic packages which we still don't know about because they're still secret.)

Quote:

What all of that does is give the F-15 pilot some ways to try to make up for the smaller plane's greater nimbleness and/or render it moot by understanding their differences and putting them to tactical use: not getting pulled into the F-16's tight-maneuvering game, but keeping his distance (possibly by opening with a long-range shot made possible by the bigger more powerful radar so there's not even a dogfight at all), playing for altitude, and holding open the option of disengaging and reengaging. These kinds of things are how F-4 pilots learned to beat the smaller, more acrobatic MiGs of its era.

Since your question specified that we're talking about a fight at high speeds, I'll also add that the twin-engined F-15's higher TWR and top speed mean that, at any given speed, it's not as close to its limits as an F-16 would be at that same speed, so it still has more room to momentarily accelerate and/or go against gravity while the F-16 is under more strain just to be flying at that speed at all.

Thats right. The F-16 will get the edge if he can get the F-15 in close and make it bleed energy to the point where it KIAS get bellow a certain figure.

And believe me, this do happend in DACT exercise between the two.. Sometimes the F-16 do get a kill.

The Term you are looking for is a "Defensive Manuvere", which consist of the F-15 doing a wide turn, thus keeping its speed/energy intact. The F-16 may turn inside the F-15 wide turn, but it wont do much good for F-16 due to the higher F-15 speed. A defensive manuvere could also be the F-15 do a climb at the same time as i mention above.

shingen wrote:

1. Is the canard or a LERX a better overall solution to getting high alpha performance but also supersonic and transonic cruise?

None, LERX or canard is not a solution by itself.
For good high alpha (high angular limit and stall flutter characteristics) and wave drag performance (trans-supersonic cruise), you need low aspect (long chord) wing and that's delta wing, or some delta's hybrid (like F22's wing).

Quote:

6. How is it that the F-15 can beat the F-16 at higher speeds when it seems the F-16 has a more advanced design?

By exercising superior SEP in certain parts of the envelope.
Same as with Su27, that we had in that other thread.
It's perfectly possible to win with inferior SEP (particularly if armed with all aspect missiles), but is more demanding and thus takes more work by pilot and plane and each error or misjudgment, is more difficult to correct.

Fighter engines often get overlooked, since they don't have visible features like aerodynamics I guess, but they are just as important.
If you got motors powerful enough, you can literally make a brick fly...it won't fly any good, but will fly never the less.

About the Su-27 vs F-15.
The Flanker do have an edge in climb rate, and i do mean with weapons attached.
So while the F-15 can acellerate faster in the 450-600Kn range, in doing a climb the Flanker will nullify or even exceed the F-15 acelleration.
This is due the Flanker higher lift.

I do hope you challange me in this as well Cola, cause this is a given. Devil

delvo is correct about F-23 vs. F-22. Aerodynamic control surfaces lose effectiveness at high altitudes due to low density of air and supersonic flow is not very kind to the effectiveness of aerodynamic control surfaces either. As such it would be nice to have something de-coupled from traditional aerodynamic forces such as lift and drag to generate moments to control the aircraft at high speeds and the F-22's thrust vectoring afforded just the solution. Also I would imagine that thrust vectoring would allow trim drag reduction by way of keeping the stabilators more rigid and frozen in place and then relying on the TV nozzles for pitch trim. TVC also augments the F-22's roll rates as well and might also be helpful in providing lateral roll stability without causing unnecessary drag from ailerons.

I'm not so sure about upper surface of F-23 being that big of an issue IR wise. I seem to recall seeing a photo from SecretProjects.co.uk that showed a special second upper shroud of the YF-23 PAV-1's SERN nozzle that served to shield the YF-119 core exhaust's IR emissions by serving as a duct for the engine core bypass flow and using said bypass flow as a make-shift coolant. This cooling method isn't at all on par with what was utilized on the bottom side of the engine exhaust but I'm sure it helped a little. Here's what I'm talking about. The images are at the top half of the page showing the PAV-1's YF-119 engine.

http://www.yf-23.net/Galleries/trough.html

delvo,
I have no argument at all with most your post comparing F-15/F-16, but you know as well as I do that acceleration is not determined by the nominal TWR, for two reasons. First, the thrust in your TWR is sea level static, not the true thrust at the flight condition of interest. Yes, thrust at condition bears some relation to sea level static, but there could be significant differences in the thrust loss or gain between two different airplanes, even with the same engine. Secondly, the equation is (thrust - drag) / weight, not thrust / weight. F-15 drag could possibly be more than twice that of the F-16. None of that changes the outcome of a drag race between the two airplanes, true enough.

The F-16 is certainly more advanced than the F-15 in electronic flight controls, relaxed static stability, and scheduled LEF and TEF.

One other point, the larger airplane does not always have longer range or endurance. For example the F-16A had more range and endurance than the F-15A, due to 7000 lb fuel vs. 10000 lb fuel. Somehow, MAC found room for about 3000 lb more fuel in the F-16C, thus eliminating that embarrassment.

haavarla wrote:

I do hope you challange me in this as well Cola, cause this is a given. Devil

I'm not here to challenge you, but it looks more like the other way around...
Again according to Russians, F15 has the edge in climb up to, at least 23kft.

Look, get a grasp on the subject before trying to make a confrontation, or go bother someone else...ok?

The YF-23 also had some spece-effeciency problems (due to area ruling and stealth considerations), but Northrop did have plans to fit 9+2 (!!) AA missiles internally in the production version.

Forum: Modern Military Aircraft

Aicraft design choices