Su-27 - Comparing flight model with real life numbers

[Fox One]

The first thing I noticed about the flight model – it is too hard to rotate the aircraft during takeoff.

 

See the diagram below. For a weight of 25 tonne, taking off in afterburner the aircraft should lift off at 280 Km/h.

 

My experiment (track attached) – with 25 tonne weight (95% fuel and no stores) in standard conditions, switched on full AB, at 200 Km/h pulled the stick fully aft and kept it like that until the aircraft was airborne. The nose wheel lifted at 300, the main wheels lifted at 330 Km/h. That’s 50 Km/h more than what the diagram shows! How can I be airborne at 280 when the aircraft doesn’t want to lift nose wheel until the speed is 300?

 

I did some more similar experiments at lower weights – it seems to me like the lower the weight, the closer it gets to what the diagram shows.

27TO.trk

[AtaliaA1]

I believe those factors are attributed to the "Manufacturers" Biased production runs.

 

They want those numbers to look as good as possible so during the manufacturing process and engineering (No weapons or pods) they put together their "Pamphlet" on what their product could do.

 

Add a variable subtract a variable and your graph or pie chart will give the information you want. just don't mention the "Variables" in the ad..

[King_Hrothgar]

I don't think it's intentional. You're right that a fully loaded plane will handle differently from the usual pamphlet specs, but even if the bird won't lift off it should be able to get the nose up. Ground handling in general in DCS is a bit off I find. The ground has always struck me as strangely sticky for lack of a better term. Planes don't roll freely with the brakes off, wheels stick to the ground when they shouldn't and then there is the off road handling of planes.

[Hummingbird]

AFAIK the aircraft should actually lift off by itself at 280 km/h, whilst you can force a take off at speeds as low as 240-250 km/h using full AB - in real life mind you.

 

Haven't tried the beta yet, so can't comment on that.

Edited November 9, 2014 by Hummingbird
Making clear I was talking about the RL properties

[SUBS17]

What fuel weight is ideal for the Cobra? Because I found that with to much weight the aircraft departs! Very nice FM BTW.:thumbup:

[nhowell01]

The first thing I noticed about the flight model – it is too hard to rotate the aircraft during takeoff.

 

See the diagram below. For a weight of 25 tonne, taking off in afterburner the aircraft should lift off at 280 Km/h.

 

My experiment (track attached) – with 25 tonne weight (95% fuel and no stores) in standard conditions, switched on full AB, at 200 Km/h pulled the stick fully aft and kept it like that until the aircraft was airborne. The nose wheel lifted at 300, the main wheels lifted at 330 Km/h. That’s 50 Km/h more than what the diagram shows! How can I be airborne at 280 when the aircraft doesn’t want to lift nose wheel until the speed is 300?

 

I did some more similar experiments at lower weights – it seems to me like the lower the weight, the closer it gets to what the diagram shows.

 

By starting your rotation early you are extending the amount of take off roll needed to get airborne. Start your rotation later and the jet comes off the ground right around 280 fully loaded with fuel and weapons

[Robin_Hood]

I have the same problem. The real-life manual (Su-27SK, so probably not the exact same variant, I grant you) says : pull the stick at 200 km/h and nosewheel should come off the ground at 230-280 km/h (depending on the mass).

 

By the way, this is from an operating manual, not an advertisement brochure.

 

I am not saying the PFM is wrong, just wondering. I must admit I haven't done a lot of testing, either.

 

PS: Incidentally, I tend to see the same thing in the MiG-21, ie. nosewheel comes off the ground at a higher speed than mentioned in the manual.

[Fri13]

I believe those factors are attributed to the "Manufacturers" Biased production runs.

 

They want those numbers to look as good as possible so during the manufacturing process and engineering (No weapons or pods) they put together their "Pamphlet" on what their product could do.

 

Add a variable subtract a variable and your graph or pie chart will give the information you want. just don't mention the "Variables" in the ad..

 

Have you checked how does that diagram work for the airfields and dirt airstrips length?

 

Of course it is difficulty to know what different variants really need, but Su-27 and Su-33 difference can't be dramatically huge (even when requiring a special version for carriers).

 

 

As I have heard, read and find out, Soviets typically underestimated the values instead overestimated.

Like example in armor piercing, westerns made values so that it is required to have 50% change to penetrate so hole can be visually confirmed to be done (so hole can be just fraction of millimeters) but soviets had requirement that over 75% of the time shell did penetrate the armor fully so the shell had enough kinetic/chemical energy/mass to cause damage to otherside of armor, instead just possible causing small hole (of course spalling etc but..)

 

I don't know how soviet/russia air force has metered these typically but if it goes similar way, you want to make sure that when pioneers are given a task to build a dirt airstrip or prepare a road airstrip, they know what is the take of speed and landing speed so they can calculate distances and find suitable location for field operators.

[Fri13]

I have the same problem. The real-life manual (Su-27SK, so probably not the exact same variant, I grant you) says : pull the stick at 200 km/h and nosewheel should come off the ground at 230-280 km/h (depending on the mass).

 

By the way, this is from an operating manual, not an advertisement brochure.

 

I am not saying the PFM is wrong, just wondering. I must admit I haven't done a lot of testing, either.

 

PS: Incidentally, I tend to see the same thing in the MiG-21, ie. nosewheel comes off the ground at a higher speed than mentioned in the manual.

 

Have you tried different temperatures in DCS? Like -15 and +30 degree is there difference for take off speed?

[USARStarkey]

Have you checked how does that diagram work for the airfields and dirt airstrips length?

 

Of course it is difficulty to know what different variants really need, but Su-27 and Su-33 difference can't be dramatically huge (even when requiring a special version for carriers).

 

 

As I have heard, read and find out, Soviets typically underestimated the values instead overestimated.

Like example in armor piercing, westerns made values so that it is required to have 50% change to penetrate so hole can be visually confirmed to be done (so hole can be just fraction of millimeters) but soviets had requirement that over 75% of the time shell did penetrate the armor fully so the shell had enough kinetic/chemical energy/mass to cause damage to otherside of armor, instead just possible causing small hole (of course spalling etc but..)

 

I don't know how soviet/russia air force has metered these typically but if it goes similar way, you want to make sure that when pioneers are given a task to build a dirt airstrip or prepare a road airstrip, they know what is the take of speed and landing speed so they can calculate distances and find suitable location for field operators.

 

The specs you are quoting are for WW2 not modern times, and even then there is more to it than that. Bottom line, those requirements have nothing to do with today----and they did not make a very big difference back then either for a variety of reasons. The penetration tables of major ww2 nations are more or less directly comparable. The american testing during ww2 required that a "significant portion" of the projectile passed through the plate 50% of the time. So, you'd be doing plenty of damage on the other side. It is unlikely that a complete shelll would make it though a plate in one piece anyhow. What is more, this is just the definition of what the base requirement was when tabulating what was considered a pass or a fail. Its impossible to say exactly how much of the shell penetrated for a specific range test, just that it was a "signiificant portion" 50% of the time at a MINIMUM.

[Fri13]

The specs you are quoting are for WW2 not modern times, and even then there is more to it than that. Bottom line, those requirements have nothing to do with today----and they did not make a very big difference back then either for a variety of reasons. The penetration tables of major ww2 nations are more or less directly comparable. The american testing during ww2 required that a "significant portion" of the projectile passed through the plate 50% of the time. So, you'd be doing plenty of damage on the other side. It is unlikely that a complete shelll would make it though a plate in one piece anyhow. What is more, this is just the definition of what the base requirement was when tabulating what was considered a pass or a fail. Its impossible to say exactly how much of the shell penetrated for a specific range test, just that it was a "signiificant portion" 50% of the time at a MINIMUM.

 

As I said... soviets. Meaning over 20 years ago at least.

 

There has been huge trend still among western and eastern side how tests has been done.

I hope there isn't even need to quote Colonel James G. Burton book about tests in even modern projects like F-16, A-10 and F-15?

 

And as I said, 50% was the minimum change, not 45% or 20%. And to that if you shot 100 rounds and 50 rounds penetrated, 35 making 0.1mm hole and 10 making 1mm hole and 5 making 2cm hole, all were counted as penetration. Because someone told the specs, and when specs were met, it was enough. Because then later you can charge more for new project that improve the results and sell that for high price.

 

The point isn't specific old historical event, but how different can be the information that is given about war product.

 

Like here said already, the official specific Su-27 manual gives same speed values, it isn't about show or for sales. It would be interesting to know how it is today in Russia (as I said, I don't know much from there) with PAK-FA project (that indians call catastrophic) but same time on west F-35 requirements are lowered so it will pass the Pentagon tests.

 

As simple sounding static about take off speed can sound "promotional", but it can be required to be very accurate as there is 100% change that the aircraft is taking off and landing. While there isn't 100% change that armor is taking a hit from optimal situation.

 

I just got Su-27 beta installed so maybe tonight I have time to check that myself too.

[Fox One]

Start your rotation later and the jet comes off the ground right around 280 fully loaded with fuel and weapons

 

If you can do that please post a track, because I surely can't do it.

 

*cough* Trim *cough*

 

What has trim to do with what's being discussed here? The manual for real aircraft says trim in all 3 axis must be neutral before takeoff. You can apply before takeoff all trim you want, if you think that will magically increase the stabilator max deflection angle available you're completely wrong.

[nhowell01]

If you can do that please post a track, because I surely can't do it.

 

 

I think this is close. The load out was 4xR27ER, 4x73, 2xR27ET, and 100% fuel.

su-27 beta takeoff.trk

[Fox One]

^^^ The aircraft got airborne at 300 Km/h (and you had no missiles).

 

I had an idea for another test. In first post in this thread I posted a diagram that shows for a weight of 25 tonne (95% fuel and no stores in DCS) the liftoff speed is 280 Km/h.

 

See attached track for the test. Took off with 100% fuel, burned 5% fuel, landed the plane, aerobraked for a while, then engaged full afterburner, maintaining a pitch angle of 10 deg (as recommended for takeoff by real flight manual). The aircraft got airborne at approx. 275 Km/h. This is very close to what the diagram shows. For an AoA of 10 deg the lift plus vertical component of engines thrust equaled 25 tonne at 275 Km/h. This is great, it shows the aerodynamic characteristics of the flight model in takeoff configuration is pretty much spot on.

 

The real problem here is that, for whatever reason the aircraft can’t be rotated soon enough (or with a pitch rate high enough) to be able at 25 tonne weight to attain a pitch angle of 10 deg NO LATER than when the speed is 280 Km/h. This is the only problem here. The lift produced by aircraft in said condition appears very accurate.

27TO1.trk

[Yo-Yo]

Thank you for the nice test and right conclusions. :)