Hot or high?

[chornedsnorkack]

The performance of aircraft deteriorates in thin air - in "hot-and-high" conditions. Wings have less air for lift, and engines have less air for thrust.

But is there any difference between hot and high? Do aircraft perform differently in low-but-hot conditions and cold-but-high conditions? Or is the air density all that matters, with temperature and pressure being irrelevant for performance at a given air density?

[Mad (Flt) Scientist]

The performance of aircraft deteriorates in thin air - in "hot-and-high" conditions. Wings have less air for lift, and engines have less air for thrust.
But is there any difference between hot and high? Do aircraft perform differently in low-but-hot conditions and cold-but-high conditions? Or is the air density all that matters, with temperature and pressure being irrelevant for performance at a given air density?

No, temperature also matters. Because often your engine thrust deteriorates significantly at high temperatures, due to internal temperature limits in the engine. Even at the same density/pressure altitude.

So a plane taking off at SL at ISA and one at SL at ISA+30 may have very different performance, because the thrust available is much lower, even though the aircraft aerodynamic conditions are identical.

Hot AND high is also worse than just hot OR high, for similar reasons.

[gearpins]

...:confused: but at ISA+30 the density would be lower is it not?
so is density the fundemental factor??

[Mad (Flt) Scientist]

No.

Even at the same density, with a higher temp you'll hit ITT limits, for example, sooner. So the engine will produce less thrust, so the performance will be worse on a hot day, even if everything else were the same.

There is no single factor you can reduce it to; it really is both "hot" AND "high" that counts

[Chesty Morgan]

For the purpose of understanding. Say you have an engine that is flat rated at ISA+30 and therefore can produce full power without becoming temperature limited, the worst case scenario would then be the "High" situation, as the air density is less than in the "Low" situation.

There are, basicaly, two different but coincident factors.

Or have I got the wrong end of the stick?:}

[gearpins]

considering 2 identical a/cs:
if we assume all other parameters contant and
for a/c1:decreasing density
for a/c2:increasing temp
who suffers most thrust loss?:confused:
hypothetically ofcourse....my bets on density..at least for now:O

[N1 Vibes]

I always equate an engine to the human body, and it's not far wrong. On a hot day you can run at the same speed as a cold day, but will succombe to heat exhaustion (overtemp) sooner. But, running at a higher altitude has an immediate affect on your running performance, less dense air, less oxygen, less power (thrust).

Hope this helps :)

[barit1]

Hot: High TAS (inertial speed) is required for same IAS, and
Reduced thrust available if OAT above flat rating

High: Lower OAT, but engine may reach N1 or EPR limit, so SL thrust may
not be maintained, and
Less dense air means high GS (inertial speed) is required for same IAS

In each case, the higher inertial speed required means the momentum added by the engines (= mass x velocity) is greater than the SL Std Day case. At same TOGW, same thrust, the TO roll is thus longer. If thrust is reduced, even MORE TO roll is needed.

[chornedsnorkack]

OK, let´s imagine a comparison between the ecact same airplane in two conditions. Say, Phoenix in Arizona, at about 1000 feet above sea level - and the air temperature happens to be 52 Celsius or 125 Fahrenheit. Which should be about ISA+40 - or is it called ISA+72... Anyway, the density altitude should be about 6000 feet.

Now, compare it with conditions at actual altitude of 6000 feet, say around Denver, and in ISA conditions, which means 5 Celsius or 41 Fahrenheit there.

Airplane at 1000 feet and 52 Celsius and airplane at 6000 feet and 5 Celsius should encounter exact same air density. They ought to stall at exact same TAS and therefore need exact same TAS and ground speed to unstick.

The fans ought to meet the same mass flow, and therefore ought to produce equal thrust at equal N1. Correct?

So, the difference should be that the mass flow entering the engine at 1000 feet and 52 Celsius would be less effective in cooling the turbine? And therefore, the engine would have to operate at lower rpm and thrust to prevent overheating - thus requiring longer takeoff roll to reach the same Vr?

Correct?

[josemarb]

N1 Vibes, great equation!

[barit1]

In the Denver ISA case, the engine probably is "altitude bumped" (N1 or EPR) to create approximately the same thrust as at SL - That's usually the case with airliner engine ratings. (Disclaimer - I cannot speak for GA turbine engines!)

Thus at same TOGW, the aircraft acceleration (Kts GS/sec) should be about the same. (A = F/M and all that) However - to reach V1, Vr etc. (IAS) the ground speed will be higher, thus the time (and runway) to rotate is longer!

Coming down to the Phoenix case, engine thrust (N1 or EPR) will be cut back for hot section protection in the high OAT. Thus aircraft acceleration is reduced, given the same TOGW. But - Ground Speed for a given V1, Vr etc. is higher than SL because of the TAS/IAS relationship at high OAT.

Thus high GS, and reduced acceleration, gives an even longer runway requirement at PHX.

These are general principles only, and different engine rating methods may affect the results. However, I believe most airliners will follow these guidelines.

[Mad (Flt) Scientist]

As an illustration, here's part of an AFM chart for Takeoff Field Length, showing how temperature and altitude interrelate - you enter the chart on the LHS with the ambient temp, read across to the pressure altitude, then downwards into another part of the chart (for weight, in fact). The further to the right you are when you go 'down', the longer the TOFL.

http://img242.imageshack.us/img242/6931/toflhotandhighnq1.th.jpg (http://img242.imageshack.us/my.php?image=toflhotandhighnq1.jpg)

The distinct non-linearity on the chart lines is associated with engine flat-rating limits coming into effect - you can see that above the flat rating, the effect is roughly one major division of distance per 5 deg C, and below the flat rating its more like 20-30 deg C for the same effect.

[GroundBound]

Perhaps I shoudn't be posting this, as my knowledge is probably vastly inferior to others on this forum. However ....

When I look at the chart posted by MfS, starting at 15C & SL we can derive the T/O distance at the bottom. If we stay at the same temp but move up 2000ft (right) there is an increase to the T/O distance.

Now, start at that T/O distance and look upwards until we reach S/L and find the temperature (about 38deg C).

So for the same T/O distance we have
15degC 2000ft above SL or
39degC at SL

The 2000ft change would represent notionally a 4degC temperature change, much less than the 23 degree change at SL.

From that, I would conclude that the drop in pressure with a small change in height had the same effect as the large rise in temperature (23 degrees).

Thus height has more effect than temp?

Am I wrong?

GB

[chornedsnorkack]

As an illustration, here's part of an AFM chart for Takeoff Field Length, showing how temperature and altitude interrelate - you enter the chart on the LHS with the ambient temp, read across to the pressure altitude, then downwards into another part of the chart (for weight, in fact). The further to the right you are when you go 'down', the longer the TOFL.
http://img242.imageshack.us/img242/6931/toflhotandhighnq1.th.jpg (http://img242.imageshack.us/my.php?image=toflhotandhighnq1.jpg)
The distinct non-linearity on the chart lines is associated with engine flat-rating limits coming into effect - you can see that above the flat rating, the effect is roughly one major division of distance per 5 deg C, and below the flat rating its more like 20-30 deg C for the same effect.

So, the engine is flat rated at low pressure altitude and low temperature. If the engine were to develop more than full rated thrust in low/cold conditions, the pressure and force might break something... If an airplane were to operate, say, from Yakutsk or Klondike or Yellowknife in midwinter, airport elevation near sea level, high pressure winter anticyclone and, say, -50 Celsius meaning ISA-65 conditions, the thrust would, because of flat rating, be exactly the same as under ISA conditions... probably because the engine N1 would be slowed down to prevent excessive thrust in dense air. Thus, the airplane would reach a given GS in exact same time and runway distance - but the wings are not and cannot be "flat rated" and therefore the plane would rotate at the same IAS, meaning at lower TAS and GS and after shorter time and runway distance than in warm weather. Correct?

[Mad (Flt) Scientist]

From that, I would conclude that the drop in pressure with a small change in height had the same effect as the large rise in temperature (23 degrees).
Thus height has more effect than temp?
Am I wrong?
GB

Correct for the case you worked out.

It's important to note that you chose a case which was essentially inside the engine flat rating - you can see from the kink in the SL line that the engine is flat rated to 35 degC at sea level.

If you did a similar exercise at, say, 6000ft and 24 degC (right at the 'kink' for that altitude) you'd find that a 2000ft increase in altitude was now the same as about 12 degC - in other words, once off the flat rating the temperature effect becomes twice as significant.

[Wizofoz]

The word we're grasping for here is DENSITY ALTITUDE.

ISA conditions corrected at the rate of 30' per MB pressure change and 120' per degree of temperature.

At 3000' Density altitude the aircraft will perform the same wether it is because it is at 3000' AMSL in ISA, or at sea level at 990 MB (= 690') and ISA + 20.(=2400').

[barit1]

I would judge from the chart MFS posted that this particular engine is not rated to maintain SL thrust up to any substantial aititude. I'm sure such rating methods must exist, but place an operator at at relative disadvantage at, say, DEN or MEX.

An engine maintaining SL thrust would compress the near-vertical lines much closer together, meaning the effect of airfield altitude is less.