Effect of altitude on flying characteristics

[robinsm]

Hi, I recently flew from Goulburn (2140' amsl) to Wollongong (0' - 100' amsl, I use a rotax 582 grey head with brolga prop. I was interested to note that the aircraft seemed to float furthur on landing and when taking off I noticed a signifigant increse in clinb out rate (500 - 600 ft/min at YGLB, and 650 - 750 ft/min ay YWOL. Is this normal? Seemed funny doing circuits at 1000' amsl instead of 3200' amsl.

 

Cheers

 

Maynard

 

 

[facthunter]

Effect of altitude.

 

Your actual take-off & landing speed is considerably higher so you need more runway. Also as you approach your absolute ceiling, your climb rate (roc) drops off to nothing. Your engine develops less power also as your manifold pressure decreases with altitude. Your indicated stall and approach speeds can read the same as you normally use. Nev.

 

 

[Guest Decca]

Pressure & Density Altitude

 

Hi Maynard. Just as well to find out about “Pressure Altitude” by flying from a high elevation to a lower one, then back, on the same day.

 

Please excuse the long reply in getting to the point - I’m trying to use a Human Factors approach.

 

edit: it was a long reply, Nev got in over an hour before me.

 

I suspect you are not the only person to have just discovered, or about to discover, this cruel reality of our environment. I suspect that for most of us, it’s a forgotten element of our training.

 

Why? Possibly because over time we take for granted that aircraft performance has been the same or close to it day after day. We may not even notice a slight increase or decrease in rpm during takeoff when we’ve had to set a correspondingly slightly higher or lower barometric pressure on our altimeters to get proper airport elevation.

 

So to the point; Anyone who has forgotten how to make allowances for changes in Pressure Altitude, refer to your RAAus CD, look up “tutorials”, find “Safety during Takeoff & Landing”, and read 9.1 “Pressure Altitude” and 9.2 “Density Altitude” under the heading “Pressure and Density Altitude”

 

They are very brief paragraphs, and further reading may be required, but reading these will get you on track to a safe way of thinking how your environment is going to affect your aircraft’s performance today.

 

For every HectaPascal (millibar) reduction below 1013 you effectively lose 30feet (increase) of altitude in performance. So at 1003 Hectapascals your pressure altitude is 300ft higher than your airfield height above sea level.

 

With each incremental decrease in pressure (hence increase in effective altitude) your aircraft (engine, propeller, and wing) will lose some performance.

 

A far more sinister performance killer is the effect that an increasing OAT - Outside Air Temperature - will have on Altitude and therefore performance.

 

Corrections made to Pressure Altitude using temperature variations above or below 15deg C (and to a lesser degree how humid the air is) will lead you to Density Altitude. An increase of 1deg C will increase your effective altitude by 120' - quite significant.

 

Hope this helps,

 

Regards, Decca.

 

 

[shags_j]

Jesus, talk about long winded.

 

How about something a bit shorter (and I hope is correct).

 

Lift equation includes Rho (air pressure) as a factor (albeit 1/2 Rho). At higher altitudes the air is less dense therefore...

 

 

[Guest Qwerty]

Gday,

 

I think that you will find that Rho is density not pressure. the two are,however, intimately related. The long winded guy is correct, that is the hotter and higher (AMSL) and higher the QNH, the worse preformance in all respects you can expect from your aircraft due to reduced engine performance, reduced propellor performance and reduced wing performance.

 

 

[eastmeg2]

So it was you I saw south of Goulburn heading towards Braidwood, Sunday morning I think.

 

Grabbed the binoculars to have a look and was able to see your yellow aircraft and narrow it down to being a Thruster or X-Air.

 

 

[slartibartfast]

Maynard has been spreading his wings and extending his range. He also flew to Bathurst recently. Great to see.

 

How about we appreciate the effort Derek went to, rather than dismiss him as "the long-winded guy".

 

As you can see, shorter is not necessarily better when dealing with an important subject such as this. In our neck of the woods, you'd better have a very good understanding of this topic before you attempt to use an airstrip at 5000' on a 38 degree day with your 120kg friend on board. On a wet runway. Uphill.

 

 

[shags_j]

Sorry you misunderstand. I do appreciate the effort.

 

 

[robinsm]

Hi Eastmeg, yep that was the demented bumble bee.

 

Cheers

 

Maynard

 

 

[Guest Decca]

Thanks for your support qwerty, but I did not write this; "and higher the QNH, the worse preformance" & don't believe this is the case.

 

Regards, Decca.

 

 

[motzartmerv]

Yes, that info is not correct...A higher QNH means more air pressure and better performance (baring other factors) Back to the books qwerty ...

 

 

[Simonflyer]

The simple rule of thumb i was taught(apart from the actual theory)is that on a hot day and at high altitude the performance will be generally worse of than a cooler day at sea level..

 

 

[poteroo]

Keglsugl in PNG at 8300 amsl makes for interesting performances out of any aircraft. When you pull less than 60% power for t/o in a normally aspirated C185/206 - the fall off into the Chimbu valley is the only thing that gets you airborne.

 

Was in Flagstaff,Arizona at 7000 amsl a few years back in a normally aspirated C206, with 4 pob plus half fuel...and that was another case of gravity helping.

 

happy days,

 

 

[Guest pelorus32]

This is a pretty important part of aircraft performance and I reckon that you better know what the effect of pressure altitude and temperature are on your aircraft.

 

Many RAAus aircraft have insufficient data in their POH to determine performance at higher density altitudes. Some do however and it's startling to see the impact.

 

Even on an ISA day at 5000' the air pressure will only be around 850hPa or 84% of what it was at sea level. Add some temperature increase into the equation and you have seriously degraded performance.

 

Melbourne Tullamarine has an elevation of 430 feet. On the day of the big fires in Melbourne this year I calculated the DA at YMML is around 4,500 feet.

 

You can end up taking double or more the distance you usually expect to get airborne and as Nev pointed out the same thing happens when you land. This is because your ground speed (assume zero wind) will be greater.

 

Somewhere in the archives here is a thread about density altitude - I know I wrote a couple of posts and if I recall correctly that thread is worth a read.

 

Here's the other thread if you feel like it: http://www.recreationalflying.com/forum/navigation/9318-i-just-had-say.html#post46699

 

Regards

 

Mike

 

 

[Yenn]

In the good old days when I flew Victa Airtourers we had a terrible rate of climb on a hot day out of Whyalla and I believe they were eventually grounded when it got to 38deg.

 

 

[farri]

Anyone thinking that Decca has has made his post too long better think again .

 

To cover Density Altitude fully requires far more time and room than Decca has taken.

 

Cheers,

 

Frank.

 

 

[motzartmerv]

too right frank...:thumb_up:

 

 

[Guest pelorus32]

As an example from a POH.

 

Sea Level 20 degrees C take off distance over a 50 foot obstacle: 238 metres

 

3000 feet pressure altitude 40 degrees C over a 50 foot obstacle: 787 metres

 

Rate of Climb Sea Level 20 degrees C: 835 fpm

 

Rate of Climb 4000 feet PA 38 degrees C: 516 fpm

 

Landing Distance over 50 foot obstacle Sea Level 20 degrees C: 340 metres

 

Landing Distance over 50 foot obstacle 3000 feet 40 degrees C: 1,051 metres

 

Just an example aircraft but it gives you some idea of the dimensions of the changes.

 

Regards

 

Mike

 

 

[Guest Decca]

Hi Mike. Thanks for the "glaring" examples, and referring us back to last year's thread on this - I had forgotten about it, but they were still pumping chemo into me then so I reckon I've got an excuse:yuk:.

 

Regards Decca.

 

 

[facthunter]

More...

 

I haven't got any excuse Decca. DENSITY altitude is the whole picture. Everything works worse at high density altitudes. The density of air is a result of how many molecules are above it (what is pressing on it) and how hot it is. Your engine, your wings and your prop all suffer reduced efficiency as it reduces.

 

To climb ,or accelerate there has to be a surplus of energy left over above what you have been using to maintain what you were doing. At the extreme situation where you have reached the point where with full throttle and the most optimum angle of attack you can not climb any more, you have run out of surplus energy. let us call that the absolute ceiling. The closer you are to that point the more careful you must be, whatever you wish to do eg. take-off, climb or turn, or even enter turbulence. It would pay to have a good understanding of density altitude.. Nev.

 

 

[Guest ozzie]

The effects of altitude density on airframes and powerplants are often well discussed amongst pilots. Takeing off at high elevated airports or strips on hot days has caught many a pilot out. Even float plane pilots who have landed on some mountainous lake have had to leave the aircraft there could they could not get off again. Even cruising at the aircrafts ceiling requiers some thought about how true speeds and handeling are affected But the added effects on the human body into the equation are almost always overlooked. Humans suffer from the performance loss just as much as aircraft are. It is only after (maybe) long periods of exposure to the climate does the performance return to some degree. Brain functions and decision making are slower not to mention stamina. i had a reminder of this when paragliding in the Italian Alps with take offs around 8,000ft being common. I knew that the still wind launches would take a litttle more in both distance and speed, but the effect of the altitude on myself was greater. Nearly all previous take offs were below 1000ft and into 15 or more kts of wind. pretty effortless. however the takeoffs at altitude left me pretty drained. Then several minutes after takeoff I would be climbing higher. Weee. No wonder everyone says flying is a buzz. Just something to think about when you plot your density height and find it is getting up there.

 

Ozzie

 

 

[Guest pelorus32]

Great Post Ozzie!!!

 

We don't often think about that facet of DA.

 

Regards

 

Mike

 

 

[poteroo]

Altitude Effect on Engine Mixture

 

A consideration not discussed so far is the effect on mixture. The engine is very easily flooded at startup on high altitude strips - a situation you must avoid with fuel injection. As well, the engine will not develop maximum possible power for take-off unless some leaning is done - particularly once DA exceeds 5000ft. We used to lean off to around 100ROP EGT once the throttle was opened up fully. This used to roughly correspond to an altitude backscale on the fuel flow gage. We kept note of these for each strip we operated into.

 

Another way to obtain this setting was to record the fuel flow settings on climb when using wide open throttle, fine pitch and approx 100ROP on the EGT, at each DA step.

 

All of this refers to normally aspirated engines - with the turbos we obtained sea level pressure for most strips, and so leaning was not used on t/o.

 

How a Bing carby engine would work at the higher DA's, I don't know.

 

happy days,

 

 

[eastmeg2]

Ozzie's post just got me wondering whether float plane pilots look for mountain lakes with a waterfall leading into a valley just in case they need a bit of gravity assistance for take off ;-) Something similar was mentioned about airstrips high in the mountains in a similar thread somewhere . . .

 

 

[Yenn]

The Bing carby is supposedly automatically altitude compensated. If you run a Bing with EGT you should see if there is any great drop in EGT caused by a rich mixture at altitude. Normally aspirated Lycoming and continentals are usually leaned out once you get to about 3000' and at high airports such as Denver in USA they will need leaning for take off.

 

Once you start talking about leaning you uncover a great can of worms, where all that I was taught as a student in the 60's hs been superseeded, with a more sensible method, much to the disgust of a few old die hards.