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On the web, on several sites, the A-10 is listed with a ceiling of 14 km, the Su-25 with 7 km, although there is data that puts the height for the Su-25 at 10 km.

Nevertheless, why would the Su-25 fly only half as high as the A-10 knowing that the Su-25 has more engine power and is much lighter?

Federico
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Vido
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    The data that puts Su-25 ceiling at 10 km seem to only have appeared when Russia tried to blame it for the MH-17 incident. The manufacturer (and it's Russian) lists 7 km. – Jan Hudec Oct 15 '14 at 06:17
  • TWR of a U-2 is about 30% higher than an A-10 but it flies another 7 km higher (the air at 21 km is about 3x as tenuous vs 14 km) – Nick T Oct 15 '14 at 07:25
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    The data that puts the su 25 at 10 km pre dates the MH 17 incident by decades. http://www.enemyforces.net/aircraft/su25.htm http://www.military-today.com/aircraft/sukhoi_su25_frogfoot.htm – Vido Oct 15 '14 at 10:19
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    Because wing shape is quite different. Important parameters are aerodynamics. The engine performance at high altitude is important but not as important as aerodynamics. – Manu H Oct 15 '14 at 14:50
  • I am with Manu on this one. The goal of the A-10 was to be able to able to make quick turns to bring that big gun back on target after each strafing run, long loiter time over the target, and pilot protection and survivability. It is design goals like this that drive the differences in aircraft design, and thus the performance characteristics. – JerryKur Oct 15 '14 at 20:15
  • in this video you can see in the cockpit the altitude indicating 8900 meter https://www.youtube.com/watch?v=GY_2NoHdNso – Vido Oct 17 '14 at 12:20
  • @Vido where in the video? Is 8900m the same as 10,000m? Does the plane have weapons on it? Easy questions if you think beyond the Russian propaganda ministry. – Evil Washing Machine Oct 18 '14 at 10:12

3 Answers3

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Just look at the wing loading: The Su-25 has between 585 kg/m² (regular Su-25) and 691 kg/m² (Su-25TK) when fully loaded, whereas the A-10 has a wing loading of 316 kg/m². This alone should explain why the Su-25 cannot climb as high.

Even when flying at minimum mass, the Su-25 has still 309 kg/m² (345 kg/m² for the Su-25TK), and the A-10 is almost a glider by comparison, having just 208 kg/m² at minimum mass.

Since both are subsonic designs, they have a strict limit on maximum speed. With that, wing loading and the airfoil determine their ceiling if thrust is sufficient.

Another limiting factor is the cockpit: Since the Su-25 was designed for ground attack, it has an unpressurized cockpit. This detail limits the certified envelope, even though a lightly loaded Su-25 can climb higher than 7000 m.

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Peter Kämpf
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I may be mistaken, but you seem to be using the same logic one would use to assess automobile performance. With a car, power to weight affects one of its major performance characteristics: acceleration.

With a plane, power to weight still affects acceleration performance and (ultimately) top speed. But it has less to do with how high the plane can fly. Altitude is determined by the design of your wing and at what altitudes the engines can function.

Remember, as you go higher there is less air. Less air means that the wing produces less lift, and it also means that the engines will often produce less power. You can compensate for this by other design means (like adding a turbo to the engine for example), but if you don't eventually you'll get to an altitude that you can't get past.

To be fair, both of these aircraft are designed for low altitude flying (ground attacks, mostly). So it would be mostly pointless to fit a bunch of stuff to allow the craft to fly much higher than a couple miles or so. Thus I'm willing to bet the engineers just skipped out on that stuff to save weight for other things (like guns, missiles, armor, etc).

This, of course, all assumes that the information being given out on the craft is accurate. I'm sure it's all top secret, and I doubt Russia or the US are going to just hand out hard numbers.

jwenting commented that, in reality, the difference between high altitude and low altitude design generally has to do with optimization, not adding "stuff". This is a great point, for some reason my head was thinking "turbo/supercharger" (which doesn't even make sense with jets). I would maintain that, in many cases, you are adding stuff to get to a higher altitude (pressurization stuff, for example).

But, and this is jwentwings main point I think, when you look at the aerodynamics of an aircraft the main difference is a matter of optimization. A wing that has good lift, good maneuverability and good stability at low altitude/speed, is not going to be so great at high altitude/speed.

nobody
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Jae Carr
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    given that the Su-25 was exported to several nations outside the USSR there's the option for independent verification of its performance characteristics. – jwenting Oct 15 '14 at 06:24
  • The main problem with getting something suitable for both high and low altitude optimisation isn't adding "stuff", it's that the "stuff" you need for one is going to be less than optimal for the other. Can't very well fit 2 pairs of wings for example (to overcome that you can use variable geometry wings, but those have their own problems, they're heavy, difficult to maintain, and the wing box is large). – jwenting Oct 15 '14 at 06:26
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    There might be some point in classifying performance data for newest aircraft like F-22, but A-10 and Su-25 are both quite old, sold to dozen or more countries and have been seen in many battles, so any potential adversary has many sources to verify their information. The published data are most likely correct. – Jan Hudec Oct 15 '14 at 07:18
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    @jwenting: With regards to wings, it just so happens that the A10 has wings that's optimal for both high altitude flight and low speed, ground hugging flight: large, high aspect ratio wings. So sometimes it can be done. The SU25 on the other hand gets away with having delta wings due to its low weight. The delta wings probably gives it an edge with maneuverability so like you said, it's a tradeoff. Which is always the case with aircraft. – slebetman Oct 15 '14 at 08:13
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    @slebetman the Su-25 has no delta wings, it has slightly swept wings, of far smaller area than those of the A-10... You're confusing it with the Su-15. – jwenting Oct 15 '14 at 08:42
  • @jwenting: http://upload.wikimedia.org/wikipedia/commons/thumb/3/32/Suchoi_Su-25.svg/280px-Suchoi_Su-25.svg.png - that's a delta wing. Albeit a high aspect ratio delta. The trailing edge is straight, not swept - hence delta – slebetman Oct 15 '14 at 08:44
  • @jwenting: Even at high aspect ratio as in the SU25, a delta wing will have characteristics of delta wings - ability to retain aileron controls at high angles of attack, resistance to tip stalls on slow banking turns etc - this is due to leading edge vortex shedding which disappears when you sweep the trailing edge as well. – slebetman Oct 15 '14 at 08:47
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    @slebetman get your terms right. It's not a delta wing... For a delta wing, see the Su-15, the Mirage III, the F-106. – jwenting Oct 16 '14 at 06:51
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Engines have different performance at different altitudes first of all. Additionally, I would suspect (I can't remember) that the SU-25 doesn't have pressurization equipment. Additionally, the wings on the SU-25 are smaller. An aircraft generating less lift can't fly as high, and this is likely the main reason.

Bassinator
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