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Too Hot and too High?

#2
UK AH do fly during the day. I dont want to drum down too much on here but hot and high has a marked deterioration on engine performance and hence aircraft performance.

Crews can make up for this by lightening the load, maybe by carrying less ordnance or fuel or pax for SH.

As the summer wains and winter approaches the aircraft performance will better. So therefore my commando friend you'll be treated to a much longer service.
 
#4
Muttley said:
UK AH do fly during the day. I dont want to drum down too much on here but hot and high has a marked deterioration on engine performance and hence aircraft performance.

Crews can make up for this by lightening the load, maybe by carrying less ordnance or fuel or pax for SH.

As the summer wains and winter approaches the aircraft performance will better. So therefore my commando friend you'll be treated to a much longer service.
So is this a fundamental limit to rotor-wing performance?

(BTW dont be fooled by the name I'm not a commando. It's Aussie slang for a reservist a cut lunch being what we'd know as a packed lunch in the UK. I suppose a suitable translation would be PackedLunchPara.)
 
#6
CLC, dont want to go into POF too much but engine performance is reduced leading to poorer rotary performance therefore leading to a lower maximum all up mass of the aircraft.

However the UK has been practising operating in these conditions regularly and we are therefore, quite used to it.
 
#7
'Tis a combination of engine performance and aerodynamics. As you get higher the air becomes less dense and this is worsened with heat. Therefore, even at sea level, some helicopters performance can be affected if it is plus 40 or so but then put them at 6,000ft and the aircraft performance thinks it is at 10,000ft. Conversley, at 10,000ft but minus 40, the helicopter will perform admirably as the air is that much denser. It affects weapon performance also, ToF for missiles is greater in colder conditions because the thing is trying to force its way through treacle. Basically, there is less air for the engines to suck in and for the rotor blades to bite on.
 
#8
Sloppy Link said:
'Tis a combination of engine performance and aerodynamics. As you get higher the air becomes less dense and this is worsened with heat. Therefore, even at sea level, some helicopters performance can be affected if it is plus 40 or so but then put them at 6,000ft and the aircraft performance thinks it is at 10,000ft. Conversley, at 10,000ft but minus 40, the helicopter will perform admirably as the air is that much denser. It affects weapon performance also, ToF for missiles is greater in colder conditions because the thing is trying to force its way through treacle. Basically, there is less air for the engines to suck in and for the rotor blades to bite on.
So how is it that a AS350 (single engined light heli) can fly to and land on the summit of Everest? Surely the air is quite thin up there isn't it?

http://www.eurocopter.com/everest/


Further reading,
http://www.mounteverest.net/story/Wheretheeaglesfly-ChopperlandsontopofEverestMay62005.shtml


It affects weapon performance also, ToF for missiles is greater in colder conditions because the thing is trying to force its way through treacle.
But surely according to Newton, this would also mean that there is equally, a greater force on which the rocket motor can push against!

:study:
 
#9
SilsoeSid said:
But surely according to Newton, this would also mean that there is equally, a greater force on which the rocket motor can push against!
The gas coming out of the back of the missile pushes the missile. This is why rockets work in the vacuum of space (See where I came over all sci fi there). Air density does influence the performance of the rocket motor (which are designed with different altitudes in mind, look at the shape of the F1 Motors on the bottom of Saturn V compared with the shape of the Service Model motor in the film Apollo 13 for instance) but the density of the air has a far greater effect in creating extra drag. A character in Ice Station Zebra quotes the effect of denser colder arctic air on the trajectory of a rifle round.

Edit: Does no one have any knowledge of the performance of Soviet Helos in Afghanistan?
 
#10
Ah, SS, always guaranteed to hunt for an argument where there isn't one. IYR, the original question was from a laymans point of view and with that in mind, that is the way that both Mutley and I explained it to the original poster. An AS350 on Everest is an example of what can be done however, AS350 does not have a warload of 6 ton to contend with or 30 troops down the back. Our description was a generalisation in the same way that you can generalise about a bullet's performance or a motorcycle's. Whilst on the subject of projectiles, the motor thrust is only in the first 1.5sec of flight, thereafter the missile is coasting, it follows that if it is coasting in treacle it will slow down quicker that if it were coasting in gin. To travel 6km in treacle takes longer than travelling 6km in gin but, the missile still will travel that 6km and smoke your sorry ass!
 
#11
Thank you SL, in with both feet!
Perhaps then it would be wise to follow in the footsteps of our Danish friends.

"The AS 555 Fennec single-engine helicopter. Shown here in service with the Royal Danish Army."


 
#12
The Mil Mi-6 heavy-lift helicopter was widely used by the Soviet forces in Afghanistan to transport heavy equipment.

It should not come as a surprise, therefore, that the Soviet military came to Afghanistan in 1979 with a huge helicopter force. Combat helicopters played primary roles in all aspects of this military operation and achieved significant success.

At the same time there were a number of serious drawbacks - most of them known but some unexpected - uncovered in the design and performance of military helicopters.

One of the first difficulties encountered by the Soviet helicopter pilots in Afghanistan was the requirement for high-altitude take-offs and landings: attacking from the northern mountainous regions of Afghanistan the Soviet helicopter force operated from a number of airfields constructed high in the mountains.


Despite of many operational difficulties the Mi-8 was the primary transport helicopter of the Soviet army in Afghanistan. This type accounts for most of the helicopter losses sustained by the USSR in the 1979-1989 Afghan War.

It was neither safe nor easy to get a standard transport helicopter, such as an Mi-8, loaded with cargo and fuel, airborne in the rarified air of mountainous Afghanistan. The Mi-6 superheavy cargo helicopter and the Mi-24 assault helicopter both had a significant advantage under such conditions in the form of stubby wings on the lower fuselage. These wings generated up to a quarter of the total lift in flight at cruise speed.

During take-offs from mountainous airfields these winged helicopters would accelerate down a runway for additional lift from its wings to complement the lift generated by the main rotor. This made it possible to get a heavily loaded helicopter airborne even from the high-altitude airfields, which, in turn, reduced the number of required flights and lowered potential losses.

Thanks to its large cargo compartment the Mi-24, which, in essence, is an attack helicopter, was able to perform various transport duties, when other - wingless - helicopters could not be operated safely. The Mi-24's wings, of course, were also used as weapons stations.

The role of the wings on a helicopter during take-offs was a positive side effect. However, the primary purpose of these wings was fuel economy.

http://www.aeronautics.ru/news/news002/news058.htm
google "soviet helicopters performance in Afghanistan"
350,000 answers in 0.33 secs
:wink:
 
#14
feels like being back in the Theory of Flight class again...... or was it Theory of Fright??
Any hows, you have pretty much the definitive answers to why hot and high is not good for aircraft performance, humidity does no favours either. Pilots will talk about density altitude and so on, but if you are hot and high, air is less dense, and your flying machine, be it rotary wing or fixed wing will not want to fly so well. So you will be limited on your take off mass (fuel, pax, payload) and if you are fixed wing, your take off run is much longer, and your climb performance is much less. As pointed out above, having wings made the Soviet Helicopters able to operate in hot and high.

As for the AS350 (single engined light heli) which they flew over Everest, well with the right support and alterations you could pretty much get any aircraft over Everest when it is previously not designed for that purpose. Oh on that note, over Everest... therefore its cold, so colder air is more dense, which might have helped them a little to offset the high altitude.
 
#15
gearupflapup said:
So you will be limited on your take off mass (fuel, pax, payload) and if you are fixed wing, your take off run is much longer,

As for the AS350 (single engined light heli) which they flew over Everest, well with the right support and alterations you could pretty much get any aircraft over Everest when it is previously not designed for that purpose. Oh on that note, over Everest... therefore its cold, so colder air is more dense, which might have helped them a little to offset the high altitude.
I guess that is why they didn't try and land a fixed wing on the summit then. :lol: (But if the wind was in excess of the a/c stall speed......!)

As for the 350 they landed on Everest, a few years ago there was a request by the someone in the AAC, name eludes me at the mo, to fly a Gazelle over the summit. It was refused by both the AAC and the relevant authorities IIRC. Not sure if they wanted to try and land there though.
(still not sure if the 350 had permission to do it) :?

If a hanglider can do it with a piston engine, surely most helicopters could do Everest anyway!
Mt Kenya was high enough for me!! 8)

I'm sure the DA on Everest is still greater than its height. :?
 
#16
Just as an aside, but still on the vague topic of performance in a high densty altitude environment, the Everest trip turned out not to be just a record event.

Having just read through 'Rotor Journal' the Eurocopter News Magazine, (Summer 2006 special edition), I see that " At the request of the Nepalise Authorities, the aircraft rescued two mountain climbers who were in difficulty".

An account of a similar event in 1996 (with a 350 B2) is told by Dr. Beck Weathers, one of those saved from 18,000ft.
 
#17
why is humidity bad for a jet engined aircraft? SHC of water would keep the engines cooler, provide more mass in the engines for thrust (as per the Harrier). Sounds pretty good to me!
 
#18
In the case of UK aircraft (especially military) it's less to do with what the aircraft CAN do, more what they are CLEARED to do.
 
#19
quickstop said:
why is humidity bad for a jet engined aircraft? SHC of water would keep the engines cooler, provide more mass in the engines for thrust (as per the Harrier). Sounds pretty good to me!
Okay, you want the text book answer on why Humidity is bad..... Theory of Flight flash backs again.....

Okays, humidity is the water vapour saturated in the air..... so the air is less dense as air particles are displaced by water particles, so the air is less dense in high humidity that if there were less water vapour in the air, low humidity. So less dense air means poorer aircraft preformance, both fixed and roatary wing.
And to add the point my Theory of Flight instructor drumed in to us, "you can't fly on water"

Added to that, air holds more water the hotter it is, "humidity saturation ratio" so the hotter the air is, the more able it is to hold water vapour, so you are likely to be hot where you find high humidity, so again, aircraft performance is impaired.

Hope that is useful to you!
 
#20
quickstop said:
why is humidity bad for a jet engined aircraft? SHC of water would keep the engines cooler, provide more mass in the engines for thrust (as per the Harrier). Sounds pretty good to me!
If I remember my enginer modules well enough:

Thrust is based basically on the change in mass/volume of input against output. For high thrust you want nothing going in and huge amounts coming out. inserting water at the mid point (for example Harrier) will increase the output mass/volume but has no effect on the input. humid air has an increased input mass which is bad.

Also cooling an enginer is not generally a good thing, you want the highest temperatures difference between input and output air you can have (due to mass/volume needs) The harrier needs the water to cool the engine otherwise it will melt in the hover. if you are lucky the input of water may balance out the loss of temperature by a gain in volume, but not generally.

humid air will also play havoc with ignition and burn processes, not something you really want.

S_R

p.s. it's been nearly 6 years since I did my degree in this and haven't used it since so may be way off the mark.
 

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