f-15 vs su-27 flight characteristics

[steve2112]

i'm mostly interested in aerobatics and i've been flying a lot in both f-15 and su-27.

 

i notice that the su-27 is a lot harder to fly, constantly needs trimming and is a lot harder to maintain speed and heading.

 

the f-15 is a breeze by comparison, i haven't even bothered to put the trim controls on my stick.

 

i wonder, it that just the simulation or is that the real planes?

 

i doubt too many people have flown both in real life, but maybe someone has studded it.

 

 

also, in terms of aerobatic performance, high g turns, low speed flight, etc, who would be the winner?

[Stuge]

F-15 has built in automatic trim, that's why the carefree handling.

 

As for performance, I'll list winner for some attributes:

Instantaneous turn rate: Su-27

Sustained turn rate(horizontal): Depends a lot on fuel weight

Slow speed and high angle of attack flight and maneuvering: Su-27

Acceleration and climb: F-15

Top speed: F-15, especially at high altitude

[steve2112]

thanks, for super tight turns in the su-27, what the best airspeed and should i use flaps? (assuming no weapons and half full tanks)?

 

thx

Edited January 6, 2016 by steve2112

[pr1malr8ge]

thanks, for super tight turns in the su-27, what the best airspeed and should i use flaps? (assuming on weapons and half full tanks)?

 

thx

 

flaps are dependant on speed not weight or armament.

Leading edge flaps on the su-27 are computer controlled and no user input is needed for them to activate [other then moving the stick in normal operations]

 

Flaps and I'm not sure about the su27 but most likely have an auto retract above XXX amount of speed. In the f15 flaps will retract at 250kts. If you leave the switch to flaps down and drop below 250kts they will auto extend and again will auto retract above 250kts. I don't suggest doing this combat situations as if you're in a turning fight and they deploy you will slow down even faster. Same thing if you're flying HIGH and slow down and they auto extend you will slow down even faster and can stall, since in thin air the flaps will only hurt and will not provide any additional lift that will out weigh the drag created.

 

I'm pretty sure the su27 is in the same boat as far as how the flaps auto retract/extend. With that being said, they should be used in almost every landing as soon as they can be extended. How ever in strong gusting cross winds or in emergency over weight landings then they can be left up to prevent unwanted speed bleed.

Edited January 5, 2016 by pr1malr8ge

[Sweep]

The F-15 doesn't quite have auto-trim...

 

Every time I fire a few slammers this is apparent to me.

[pr1malr8ge]

The F-15 doesn't quite have auto-trim...

 

Every time I fire a few slammers this is apparent to me.

 

it does have "auto-trim" but it only trims to 1g flight. So if you're rolling to the heavy side and do not exceed 1g it is in trim at least according to the CAS! :P

[Svend_Dellepude]

The F-15 doesn't quite have auto-trim...

 

Every time I fire a few slammers this is apparent to me.

 

There are quite a few posts about this in the F-15 FAQ thread.

[ineth]

thanks, for super tight turns in the su-27, what the best airspeed and should i use flaps? (assuming on weapons and half full tanks)?

 

thx

 

Flaps are only here to provide extra lift and will not in any way help you in dogfights or aeronautics in any way i can think off.

 

Fly safe!

[codefox]

Flaps are only here to provide extra lift and will not in any way help you in dogfights or aeronautics in any way i can think off.

 

Fly safe!

 

They can lower your AoA for some aerobatics tho. Useful for slow formation flights.

[Stuge]

thanks, for super tight turns in the su-27, what the best airspeed and should i use flaps? (assuming on weapons and half full tanks)?

 

thx

 

No, don't use the flaps, they are only useful for takeoff/landing. In maneuvers, the flaps move automatically to some extent, and that is enough :)

 

I can recommend some airspeeds and other things for different maneuvers in Su-27. Note that during some maneuvers I recommend using the direct control mode (keyboard S key to toggle by default, bind this to your joystick!)

 

These speeds are for sea level:

 

Sustained horizontal turn: 600-800 km/h (direct control OFF for stability).

 

Maximum performance instantaneous turn: 1050 km/h, pull hard straight up (fully vertical), then direct control ON once joystick pull is established. This maneuver is very extreme and can result in blackout or loss of control.

 

Maximum performance energy-conserving turn: 1050 km/h, hold down wheel brake button (W key by default), pull hard up, or slightly to the side (oblique). For more horizontal turns, use slower starting speed. Once airspeed falls below 800 km/h you can release wheel brake button and continue with a turn of your choice. This maneuver risks blackout!!

 

Slow speed turns and loops over the top(<400 km/h): direct control ON for extra nose authority, unless sustained turn is desired in which case maintain airspeed or slowly accelerate to 600+ km/h and direct control OFF. Note that during vertical maneuvering using direct control can provide an extreme turn rate boost if airspeeds are lower than optimal for sustained turn.

 

Whenever extra nose authority is needed to take a shot(missile or gun): direct control ON.

 

Cobra from straight and level flight (aerobatic): 350-400 km/h, throttle idle, then simultaneously pull hard and direct control ON. Once nose starts to settle back down, direct control OFF, increase throttle and gently pull stick back to stop nose from falling below horizon.

 

Cautions when using direct control:

 

-Don't enable direct control in straight flight, but only after positive G pull has been established. Otherwise nose-down attitude and subsequent negative G may lead to an unrecoverable flight state. If straight flight is required, set direct control OFF.

 

-Be very cautious when using direct control with full internal fuel tank. The aircraft is prone to becoming uncontrollable at high positive angle of attack. With half or less fuel, this problem disappears.

 

-If aircraft has taken damage and feels unstable, set direct control OFF for more stability.

 

-In case of single engine failure during hard maneuvering, immediately cut throttle as much as possible, set direct control OFF, and recover only once sufficient airspeed is built up. Otherwise you risk an extremely dangerous spin due to asymmetric engine thrust. But then again, most likely cause for single engine failure is damage from a gunshot, so you are probably at great risk anyway :D

 

Sorry if too much info :):):)

[steve2112]

we seem to have got off on a discussion about flaps, but i'd really like to know what the optimum airspeed for tightest radius turn in the su-27?

 

right now i'm finding that 500kph and jamming the throttle on full power at the start of the turn is working well for me, but i haven't done extensive testing yet.

[Stuge]

we seem to have got off on a discussion about flaps, but i'd really like to know what the optimum airspeed for tightest radius turn in the su-27?

 

right now i'm finding that 500kph and jamming the throttle on full power at the start of the turn is working well for me, but i haven't done extensive testing yet.

 

For minimum radius turn, direct control ON and pull as hard a turn as you can. As long as you don't completely stall by jerking the stick too hard, but instead ease into the turn swiftly but in a controlled manner, there really is almost no minimum speed, you can go extremely slow! About 250 km/h in the horizontal, and perhaps 100 km/h or less when coming over the top with gravity assist.

[steve2112]

250 km/h, that's slow, not sure i can keep it in the air, but i'll try.

 

just to be clear, what exactly do you mean by 'direct control ON'?

[SinusoidDelta]

There are quite a few posts about this in the F-15 FAQ thread.

 

Yeah, there was some mention about lateral store asymmetries causing a persistent roll, not so much about pitch. The Pitch Trim Controller automatically trims the aircraft by adding or subtracting collective stabilator to maintain G at a constant stick position. I see this refered to as "series trim" because the PTC adjustments (obviously) result in zero stick deflection. The roll axis is more augmented than auto trimming. Roll CAS commands differential stabilator to counter uncommanded roll.

 

In almost every circumstance the neutral stick position and constant rate response of the F-15 is an advantage. I don't envy Flanker drivers who basically have to churn butter with the stick, especially in a kill or be killed enviornment.

[DarkFire]

F-15 has built in automatic trim, that's why the carefree handling.

 

As for performance, I'll list winner for some attributes:

Instantaneous turn rate: Su-27

Sustained turn rate(horizontal): Depends a lot on fuel weight

Slow speed and high angle of attack flight and maneuvering: Su-27

Acceleration and climb: F-15

Top speed: F-15, especially at high altitude

 

Absolutely agree on every point, with the possible exception of the top speed. I don't for a second doubt your knowledge on this, but at the moment the Su-27 PFM appears to be bugged in that at around 13,500m altitude if left in AB for long enough a clean Flanker will actually achieve Mach 2.6 on a "standard" DCS day. I think the published maximum for the Su-27 is 2.35 so in the sim as it is now the Flanker is quite a bit faster than it ought to be at maximum thrust.

 

That being said, I don't fly the Eagle so maybe that also goes faster than it ought to :)

 

On a general note, I'd agree with others in that the Flanker has significantly better ultimate nose authority and marginally better turn rate below ~6,000m altitude. Of course, it's better turn rate is amplified by the abilities of the R-73 combined with the helmet mounted sight. From what I've experienced the F-15 has significantly better climb performance, though with the changes made to the Su-27 engine temperature profile (I think this happened in DCS 1.5 hot fix no.1) the gap has narrowed slightly with the F-15.

 

One other important consideration: fuel capacity. The Su-27 has nearly exactly 50% extra fuel over the F-15 (not counting bags of course) which can help but can also hurt: fully loaded the Su-27 has a range and loiter time that the Eagle can only dream of, however with a full 9,400 Kg fuel load it performs comparatively sluggishly, especially at higher altitudes.

 

Pilot skill being equal, in a WVR / dogfight situation the Flanker will generally be victorious. By contrast the F-15 is the undisputed absolute king of BVR engagements. For pure BVR combat I doubt we'll get anything even close to being as capable until we get the F-14, F-18 (maybe) and EF-2000.

 

Best case situation for the Su-27: WVR battle below ~6,000m altitude.

Best case situation for the F-15: high altitude, high speed BVR engagement.

[DarkFire]

250 km/h, that's slow, not sure i can keep it in the air, but i'll try.

 

just to be clear, what exactly do you mean by 'direct control ON'?

 

Direct control de-couples the automatic control system (fly by wire) from the controls, giving you direct proportional control over the aircraft's control surfaces. Non-direct control is the normal situation where the flight control system interprets your control inputs before acting on the control surfaces. For a much better description, read pages 110-117 of the Su-27 flight manual.

 

Turning direct control ON, i.e. turning OFF the FBW system, is done by pressing the "S" key. BE WARNED: at any airspeed (and the effect is worse as speed increases) at neutral control position the nose will pitch violently downwards which, unless you're prepared for it, instantly induces negative G lock at best and at worst kills you (the virtual pilot). To counter the violent pitch-down, if you must use direct control mode then pull back on the stick before hitting the S key.

 

Until you're very accustomed to how it behaves, the Su-27 becomes extremely twitchy in pitch when in direct control mode. At first it will seem unflyable but you can get used to it and it can under certain circumstances be useful. You also run the risk in direct control mode of entering the dreaded inverted-shuttle-cock-of-death situation where the Flanker will enter a departed state inverted and will actually stabilise in that condition. Under the best of circumstances it's difficult to get out of at anything under 5,000m altitude.

 

There's a reason most people consider the S key to be the instant death key :D

[steve2112]

Direct control de-couples the automatic control system (fly by wire) from the controls, giving you direct proportional control over the aircraft's control surfaces. Non-direct control is the normal situation where the flight control system interprets your control inputs before acting on the control surfaces. For a much better description, read pages 110-117 of the Su-27 flight manual.

 

Turning direct control ON, i.e. turning OFF the FBW system, is done by pressing the "S" key. BE WARNED: at any airspeed (and the effect is worse as speed increases) at neutral control position the nose will pitch violently downwards which, unless you're prepared for it, instantly induces negative G lock at best and at worst kills you (the virtual pilot). To counter the violent pitch-down, if you must use direct control mode then pull back on the stick before hitting the S key.

 

Until you're very accustomed to how it behaves, the Su-27 becomes extremely twitchy in pitch when in direct control mode. At first it will seem unflyable but you can get used to it and it can under certain circumstances be useful. You also run the risk in direct control mode of entering the dreaded inverted-shuttle-cock-of-death situation where the Flanker will enter a departed state inverted and will actually stabilise in that condition. Under the best of circumstances it's difficult to get out of at anything under 5,000m altitude.

 

There's a reason most people consider the S key to be the instant death key :D

 

thanks for explaining, i had no idea, sounds like something to avoid at low AGL flying (unless you are really good)

[ineth]

Maximum performance energy-conserving turn: 1050 km/h, hold down wheel brake button (W key by default), pull hard up, or slightly to the side (oblique).

 

OK.

I am fresh to DCS and the Su27 so i might be missing smt here.

 

Why press the wheel brake (w) during a high speed aerial maneuver?

[Stuge]

OK.

I am fresh to DCS and the Su27 so i might be missing smt here.

 

Why press the wheel brake (w) during a high speed aerial maneuver?

 

Holding wheel brake button allows the joystick to move a bit more back, providing about 1 G extra in high speed turns. It simulates pulling the stick beyond the "normal" limit. But why it is simulated like this, don't ask.. but it works, try it!

[Sweep]

^Yep, at lower speeds and high AOA holding the wheel brake button will allow full pitch and rudder movement...but it doesn't disable the roll limits... :(

[ineth]

Holding wheel brake button allows the joystick to move a bit more back, providing about 1 G extra in high speed turns. It simulates pulling the stick beyond the "normal" limit. But why it is simulated like this, don't ask.. but it works, try it!

 

OK Thanks man. +1

[Stuge]

^Yep, at lower speeds and high AOA holding the wheel brake button will allow full pitch and rudder movement...but it doesn't disable the roll limits... :(

 

Hmm, I never tried using the wheel brake button for slow speed turns, but it may work while providing good stability. I guess I've been perfectly satisfied with the performance of direct control in that regard.. gun shots from cobra are the best! :D

[steve2112]

is the wheel brake key the angle of attack limiter disengage?

 

i was reading about cobra, interesting

https://en.wikipedia.org/wiki/Pugachev%27s_Cobra

[Sweep]

^No, it doesn't disable the FCS and AOA limiter; That would be the 'S' key.

 

Wheel brake just allows for maximum stick/rudder travel (minus roll, that is limited) at high AOA/G.

 

Although pulling maximum structural G (like 12-15+) should break the plane and kill you...but it doesn't yet, so guys like Stuge'll keep pulling off their crazy 25g 45 dps vertical 1c merges. :D

[Stuge]

^No, it doesn't disable the FCS and AOA limiter; That would be the 'S' key.

 

Wheel brake just allows for maximum stick/rudder travel (minus roll, that is limited) at high AOA/G.

 

Although pulling maximum structural G (like 12-15+) should break the plane and kill you...but it doesn't yet, so guys like Stuge'll keep pulling off their crazy 25g 45 dps vertical 1c merges. :D

 

A true warrior always tries to use every method available to achieve victory!