Rudder effectiveness

[cichlidfan]

The Sabre is such a stable aircraft that you generally need pretty much zero rudder to stay coordinated.

 

It sounds to me like someone is trying to use one effect to justify the other.

[derelor]

Nothing substantive, but it feels like they'd mention if the plane was extraordinarily insensitive to rudder turns.

I agree

[effte]

The rudder allows the pilot to control the beta (slip) angle of the aircraft. You can either remove unwanted slip, such as adverse yaw due to aileron use or yaw due to asymmetrical stores, or add slip e g to perform a side slip to steepen the angle of descent when coming in to land*.

 

This, the rudder of the BST F-86 does splendidly.

 

When you fly with a beta (slip) angle, the forces acting on the aircraft are no longer symmetrical. First off, you get a lateral component of the thrust of the engine. Secondly, you set the fuselage up at an angle to the free stream which tends to induce a further lateral force in the same direction. The combined lateral force sets up an acceleration towards the 'leeward' side. An acceleration results in a curved path, in this case with a wings level side slip a flat turn.

 

This does not seem to happen in the F-86. In other words, the rudder is effective and performs its primary function quite allright. However, the effects of a side slip on thrust components and aerodynamic forces on the fuselage seem to be largely missing. It's not exactly disasterous, as you'll never see this when flying by the book, but I'd say something was left out of the FM.

 

It'd be interesting to see if the increased drag due to a side slip is there. Easy enough to test.

 

Cheers,

/Fred

 

*) Less common when flying swept wings, but that's for another thread.

Edited October 5, 2014 by effte
Wrong company...

[Flamin_Squirrel]

edit: Just tested it. The plane deflects in either direction only about 8° and as soon as the rudder is set to neutral it jumps back zo 0° deflection. The plane actually keeps turning veeery slowly when full rudder is applied.

 

That's normal. It's an aeroplane, not a boat.

 

I tried an F-86 landing with a 10m/s crosswind*, and there was sufficient rudder authority to straighten the nose before touchdown. Seems absolutely fine to me.

 

The F-15 however may be bugged. Set up a strong wind, then go to F2 view and watch the rudders. They're straight if you're flying with or against the wind, but deflected to the side when flying crosswind.

 

This may be a feature of the fly by wire, but I doubt it.

 

*edited

Edited October 5, 2014 by Flamin_Squirrel

[Foxhound_vva]

Guys, it is necessary to perform the next flight. Refuelling of the aircraft must be less than 50%., two full tank under one wing, better right, so it's easier on the landing. Need to go to the landing compensating slide only the rudder. Rudder is enough. Touchdown runs perfectly. This is the main criterion for the effectiveness of the rudder. Only the speed should be kept a little higher to touch the runway. and the flaps do not let.

[Viersbovsky]

That's normal. It's an aeroplane, not a boat.

 

I tried an F-86 landing with a 10m/s crosswind*, and there was sufficient rudder authority to straighten the nose before touchdown. Seems absolutely fine to me.

 

The F-15 however may be bugged. Set up a strong wind, then go to F2 view and watch the rudders. They're straight if you're flying with or against the wind, but deflected to the side when flying crosswind.

 

This may be a feature of the fly by wire, but I doubt it.

 

*edited

 

Ah, sorry, was talking about the F-15.

It still seems strange that it turns only ever so slowly (about 1/3° per secoond). Indeed, it is no boat, so the effect of the AoA of the fuselage is less, but this still seems somewhat extreme to me.

As I said, I believe the forces generated trying to straighten the aircraft out just completely overpower the forces which would actually turn the aircaft.

[Foxhound_vva]

Derelor, do the following experiment: with the A-10C, while flying at a moderate speed, gradually apply full rudder, using the ailerons to prevent the aircraft from banking. The aircraft, slowly but surely will perform a flat turn. This kind of behavior is to be expected from pretty much any aircraft in the world.

A-10C has a straight wing. Aircraft with swept wings has another the aerodynamic. This is obvious.

[Foxhound_vva]

Ah, sorry, was talking about the F-15.

It still seems strange that it turns only ever so slowly (about 1/3° per secoond). Indeed, it is no boat, so the effect of the AoA of the fuselage is less, but this still seems somewhat extreme to me.

As I said, I believe the forces generated trying to straighten the aircraft out just completely overpower the forces which would actually turn the aircaft.

Indeed, we recalculated the lateral forces .. I think it will be better after the update. Eagle will do "knife" as the airshow.

[Viersbovsky]

Indeed, we recalculated the lateral forces .. I think it will be better after the update. Eagle will do "knife" as the airshow.

 

:thumbup:

That is good to hear!

[effte]

A-10C has a straight wing. Aircraft with swept wings has another the aerodynamic. This is obvious.

 

The planform is pretty much irrelevant in this respect though.

 

However, a big, flattish fuselage with loads of side area will contribute more to a boat turn than a slender oval or round fuselage. High speed will also reduce the resulting turn rate. I'd expect to see a significantly lower turn rate for an F-86 than for an A-10... but not no turn rate.

[Fox One]

In my previous post I said the F-86 when applying rudder and keeping the wings level it will fly straight in this yawed state. I stand corrected – the aircraft actually performs a flat turn, but it is so slow, it is barely perceptible.

 

I did some more testing and found another thing. I accelerated the plane at low level using military power, it accelerated to 1130 Km/h in external view. Then I slowly applied full rudder, while keeping the wings level. You’ll never believe what happened (or maybe you will…) The speed remained 1130!!!!!!!!! How on earth could that be realistic?

 

I made a screenshot from above and from it measured the beta angle – it was approx. 4 deg. At such a speed (M0.93) at low level and 4 deg beta angle the sideforce generated by the fuselage side facing forward can’t possibly be a close to a negligible amount. And there is also the sideways component of engine thrust, an elementary calculation shows this only should amount to close to 200 Kgf. But IMO the sideforce produced by the fuselage in this particular case should amount to maybe even more. I remain convinced the almost imperceptible flat turn the plane does in this situation is much too slow.

 

I also noticed in my experiment that I was able to deflect the rudder fully. The rudder is not hydraulically actuated. At 1130 Km/h at low level the force necessary to deflect the rudder fully is measured in hundreds of Kgf. How could possibly a human deflect the rudder fully? In other planes without hydraulically boosted controls (P-51, Fw 190) if you try to deflect rudder or ailerons at high speed they will not deflect fully – the maximum force a human can exert on the controls is obviously simulated there. Nevermind that if the rudder was actually hydraulically boosted, deflecting it FULLY at low level and M0.93 would probably brake the vertical fin. Most planes with hydraulically boosted rudder actually have some rudder limiter, at hight speed full rudder is not available (or is available while exerting a very high force on rudder pedals to overpower the limiter).

 

And since the subject here is F-86 rudder – when using rudder trim the trim itself in outside view will deflect correctly, however the rudder will remain centered (in flight, of course) and there is no rudder effect noticeable.

[9.JG27 DavidRed]

yeah, this issue seems to be connected with the problem to stay on the runway at crosswind landings. i already assumed that something is off after the first few days of flying it.

cant say im too impressed with this module so far.so it stays in the hangar until some improvements are noticeable.

[Flamin_Squirrel]

yeah, this issue seems to be connected with the problem to stay on the runway at crosswind landings. i already assumed that something is off after the first few days of flying it.

cant say im too impressed with this module so far.so it stays in the hangar until some improvements are noticeable.

 

You're doing something wrong then because it lands fine in a crosswind.

[9.JG27 DavidRed]

im speaking about high wind speeds

back then when i last tried it, it just went off the runway, no matter what, even with full opposite rudder and brake...

possible, that something changed in regards of crosswind landings since the release.as i said, havent flown it for quite some time.

 

to your assumption....no, i know how to do crosswind landings.

[Flamin_Squirrel]

Like I said, I've done a landing with 10m/s (~20kts) crosswind and landed fine. It's perfectly capable of doing it.

[9.JG27 DavidRed]

and like i said, i was speaking of higher wind speeds.10m/s is no problem i know that myself.

[effte]

How much higher? 12.86 m/s? 13? 15?

[bkthunder]

I don't really get this "hard evidence" thing. I am no FM expert but shouldn't BST be able to calculate the correct forces and moments of inertia based on the shape of the tail and fuselage?

I also thought that's what the AFM did: you break up the aircraft into "sections" and given the right parameters the sim calculates local physics. (At least that's how it's advertised on the dcs website)...

[Flamin_Squirrel]

and like i said, i was speaking of higher wind speeds.10m/s is no problem i know that myself.

 

I've tried at 15m/s (~30kts) and managed fine. That's likely around the maximum a plane like the Sabre is capable of anyway, so still don't see why you think there's a problem.

[Corrigan]

To be honest, anecdotes are not really useful.

[effte]

It would seem our resident crosswind expert has withdrawn from the thread and does not wish to quantify the "higher wind speeds" used.

 

25 knots is the crosswind component the handbook states is manageable, with the suggestion to find another runway if there's more xwind. That'd probably be a demonstrated xwind component today, meaning that an experienced pilot who is on top of his game may well get away with a few knots more, but even skilled pilots could get in trouble below said speed on a bad day.

 

That's 12.86 m/s, so already at 10 m/s you're getting close to what the aircraft can handle. Chances are the "higher wind speeds" were simply in excess of what the aircraft can handle and thus the loss of directional control experienced was as it should be.

 

Edit: At 14 m/s (27 knots) it's quite manageable, but there's not much room for error. Sounds about right to me.

 

Cheers,

/Fred

Edited October 7, 2014 by effte

[Corrigan]

Get out of here with your facts and and experience, please.

[effte]

My apologies. I'll shut up now. :D

 

:pilotfly:

[CoBlue]

As a side note, the F15C's (& some other models) IAS is way way off when flying in an headwind.

Check out the track furthest down here:http://forums.eagle.ru/showthread.php?t=122387 , F15C starting on rwy with 20m/s=40kts headwind the IAS shows 80kts on ground & then at rotation drops suddenly. Also I'm flying at 60kts without stalling?

Anyway just showing that the SFD & AFD still needs working in DCS & if the IAS shows wrong at a "simple" 40kts headwind...who knows what else is wrong?

 

Just checked the F-86 & it seems right in the situation above

[bkthunder]

Also I'm flying at 60kts without stalling?

Anyway just showing that the SFD & AFD still needs working in DCS & if the IAS shows wrong at a "simple" 40kts headwind...who knows what else is wrong?

 

 

And that's what pisses me off in a monumental way. We're being sold a PFM and then you find out these MASSIVE mistakes. The problem is not so much the one particular issue, but the fact that it puts everything else in doubt (FM wise).

btw, the F-15 is a freakshow, but let's keep this on topic.