Hot high & Humid

[MILO]

I have not flown for some 4 weeks (Duxford Airshow got in the way). Just in conversation with 2 trike pilots over the last couple of days they have noticed the longer take off runs. This invoked some lively discussion and we concluded, nice warm mornings & evenings, up on the highveld, throw in some humidity and wet grass (on the runway! ) and an engine sucking in less oxygen and more moisture (less horse power) and we can have a recipe for disaster. This whole density altitude thing is back on the table again. Reading through my microlighters handbook there is some interesting theory on why air is less dense when moist than dry (opposite to the uninitiated individuals view). The message here is be careful, throw in MTOW and a downwind takeoff and the fun will be on!

[Morph]

So true Milo, but the term "Hot, High and Humid" is actually relating to the risks of carb icing.

You are refering to density altitude problems here

Be carefull, at 25degC your trike will take significantly longer to take off than at 15deg C. If your airfield was at 5500 ft 25deg C QNH 1022 and dew point at say 15degC then your plane would perform as if it was at 7832feet.

check this site out for a density altitude calculator

http://wahiduddin.net/calc/calc_da_em.htm

[JACO]

Hi All.

Sorry if i'm being very "stupid" now, , But as i understand it, this whole density alt is supposed to tell me when i can fly and when not, ,
but i don'nt understand how it works , can you help ? I allso had a scare on sunday when my 582 took longer to take off than normal, and i think the higher temp and high humidity had something to do with it.

Thanx all
Jaco

[Morph]

Simple,

your plane doesn't know what altitude it is at. It can't read a dial. It does have wings and they are dependent on the molecules of air to generate enough lift in order to take off. Now on a cool day you would only need to get to say 40mph ground speed before the wings have enough air molecules flowing across them to lift off. On a hot day, the air becomes thinner. The molecules move apart, but the plane doesn't know this. All it knows is it will lift off when you get to that happy number of molecules across the wing. This means the plane needs to run faster on the ground until the wings get enough lift. In order to get a higher ground speed you need to run further i.e. more runway required.

NOTE: your airspeed indicator also reads molecules of air and it too doesn't know that the air is thinner. It too will only get to 40mph when the right amount of wind is flowing into the tube. If you use a GPS for ground speed mesurements you will see that on a hot day your ground speed will be higher than the speed on the ASI compared to a cool day.

This is why it seems to take forever to get up to the takeoff speed, you are traveling faster than normal but the ASI is not telling you this.

When you add humidity you now have water taking up space in the air, further reducing the amount of air molecules.

Density Altitude is a calculation that tells you what altitude the aircraft thinks it's at instead of really at because the air is thinner. In other words the plane will perform as if it is at that higher altitude. As in the calculation above you are at 5500ft but because the air is thin it thinks it's at 7800ft, a big difference.

Here's a test if you have a VSI (Vertical Speed Indicator). Take off from your field on a cool morning and give yourself the best rate of climb. (check the airspeed and the VSI) lets say it is at 1000fpm at 45mph. Now climb 2500ft, level out at 45mph, climb again at best rate of climb and see what your climb is now. The performance of the plane drops as the air get's thinner.

[MILO]

Cool, so we can throw carb ieshing on finals into the mix as well! I love this sport.

[Mogas]

Jaco
Density altitude, or the effect of warmer and less dense air on your airplane is perhaps more easily understood if you consider the effects on engine performance.
The engine needs oxygen to operate, the more the better. If the air is less dense (hot) there is less oxeygen present in a given volume, for example the amount that can be sucked in on the intake stroke of your engine. This intake stroke volume remains the same every time, regardless of the RPM.
Humid air has the same effect on the oxygen present in a given volume. Introduce water molecules (humidity) and the other elements have to give way, including oxygen.
So if you consider both of these together it is a double whammy, the oxygen content suffers both ways when it is hot and humid.
When flying off the highveld you are already at a disadvantage when it comes to oxygen content, add in hot and humid and you will definitely notice a decrease in engine performance.
Thats why turbocharging, supercharging and nitrous oxide injection were invented. All increase the oxygen content on the intake stroke.
Fly safe
Mogas

[DieselFan]

Not to mention the propeller efficiency decrease

1. Wing
2. Engine
3. Prop

When you add it all up it can really be dangerous. Remember that the prop is a "wing" too and as Morph explained the wing needs more speed to get same amount of lift so, as is, the engine makes less power and now your asking it to turn the prop faster to get same thrust.

[Morph]

Cool, so we can throw carb ieshing on finals into the mix as well! I love this sport.

hey stop stirring up the cocktails

[JACO]

Thanx Morph, Mogas, and everybody els,

I'm with you now. So how do i use the calculated density alt to determine
how my aerie will react ? Is there a table or graph that i can consult ?
Or some more calculations using my engine performance, Wing area,
weight on board, ect, to calculate my takeoff run, rate of climb ext ?
Or do you calculate the density and say anything above for example
8000 ft, i stay on the ground and clean the hanger !?

I've got a VSI and will do the test Morph described, should be interresting

Thanx again to all ...
Jaco

[KFA]

Jaco, kick your instructor. You are supposed to know these things. There is a rule of thump for calculating this. I am not sure of the exact figures but it goes something like this : take your normal take-off distance and add 10% for hot, 5% per 1000ft amsl for high, 10% for humid. add 10-20%% for grass(depending on length) and another 10% for wet grass. This is just a rough guide I use, maybe Brian can help. You will see that by being hot high humid and at gross weight on a grass strip you could easily end up using double or more the normal run.

[grubsner]

I use the basic but effective way to calculate the DA before I fly. It does not include humd or baro pressure but still is conservative enough.

There is some prework required: The international standard temperature at sea level is 15 deg C and the assumed temp lapse rate is 2 deg C for every 1000 ft above MSL.

Calculate your field's ISA then subtract the field ISA from the current Ambient temp and multiply the answer with 120 ft. That will give you a conservative DA for the ground ambient temp.

Example: Field at 5000 ft and Amb Temp = 25 deg C what is the DA?

First calculate the ISA for the field: 5 x 2 = 10 deg C. The ISA for field atl is 15 deg C - 10 deg C = 5 deg C. This value will remain unchanged unless the field elevation changes

Now, the Amb temp is 25 deg C. 25 deg C - 5 deg C (ISA) = 20 deg C.
Mutliply 20 x 120 ft = 2400 ft.

DA = 5000 ft (field alt) + 2400 (DA ft) = 7 400 ft

If the amb temp = field ISA temp the DA = field alt. You can use 100 ft instead of 120 ft if you have to do a quick mental calc! Just add 500 ft for a safety margin.

I hope this helps.

[grostek]

Found this

Things that are handy to know

Altimeter rules of thumb

• For each 10 °C that the outside air temperature is warmer than ISA standard, increase the indicated altitude by 4% to give true altitude. Conversely for each 10 °C cooler decrease indicated altitude by 4%. (10/273 approximates 4%; refer Charles' law)

• When flying from higher to lower pressure conditions, without altering QNH, the altimeter will over-read ( indicate higher than actual altitude) by about 30 feet – below 10 000 feet – for each one hPa pressure change.

• When flying from lower to higher pressure conditions, without altering QNH, the altimeter will under-read ( indicate lower than actual altitude) by about 30 feet, if below 10 000 feet, for each one hPa pressure change.

• If the altimeter sub-scale setting is less than QNH the altimeter will over-read. Conversely it will under-read if the setting is greater than QNH.

• Air density decreases by about 1% for each:
— 10 hPa fall in pressure, or
— 300 feet increase in height, or
— 3 °C increase in temperature, from the msl standard

Hope it helps somebody

Kind regards

Gunter Rostek

[grostek]

Hi

Also this about engine performance loss at altitude.

Altitude and Engine Performance

Sea Level: Units of Pressure

--------------------------------------------------------------------------------

Inches of Mercury = (in. Hg.)
Atmospheres = (atm)
Kilopascal = (kpa)
Millibars = (mb)

Pressure equivalents to: 1.0 atm = 29.9 in. Hg. = 760 mm Hg. = 101.3 kPa = 1013.25 mb

Atmospheric Pressure = 14.7 psi & 13 cubic feet of air = 1 pound

Power Loss due to Altitude
Air Density decreases at a rate of 2.9% - 3.0% for each 1000 ft. of elevation above Sea Level. See Standard Atmosphere below for background information.

Naturally Aspirated: Atmospheric Pressure 14.5 psi (It's hard to ride at sea level 14.7 psi)
Atmospheric Pressure @ 9000 feet = 10.5 psi
Pressure Loss = (14.5 - 10.5) = 4.0 (4.0/14.5) = 27.58 % @ 9,000 feet


Does a Turbo lose power with altitude? Yes!
Atmospheric Pressure = 14.5 psi, Boost = 10 psi, Total Pressure = 24.5
Atmospheric Pressure @ 9000 feet = 10.5 psi + Boost of 10 psi = Total 20.5 psi

Approximate Pressure Loss = (24.5 - 20.5) = 4.0 (4.0/24.5) = 16.32 % @ 9,000 feet
The power loss due to altitude is much less with the Turbo. The critical difference is that you can flip the switch on the Turbo to 15 lbs boost and get your sea level HP!!

Turbo considerations: As altitude is increased the turbo fan must increase rpm to maintain a constant boost pressure. With large displacement engines (read 1000cc 4-strokes) the turbo fan may have to spin faster than is efficient. The result is slower acceleration. The cure is a larger turbo or lower elevation.

Hope this helps,

Gunter Rostek

[DieselFan]

Grostek thanks for the post, however I have a few comments regarding the actual content, so nothing personal as I know you're just relaying .

The info from http://www.2-stroke-porting.com/altiden.htm
seems to create an unrealistic figure for Turbo applications

VW claimed a few years ago that at upto 10000 ft their TDI only lost 5% MAX, 3% at 7000. The link also doesn't take turbo maps, air density, air temp into account let alone the effects of inter or aftercoolers. Having a higher boost won't always give more power - airtemp.

Most aircraft turbo applications will provide SEA LEVEL rated power till about 12000 feet. Some car turbos have even been used and rated till 18k.

Some of Isuzu's more recent bakkies have "mini turbos" or called altitude compensators by the salesmen. They are supposed to provide SEA level specs till 10000 feet.

We use 0.2-0.6% / 1000 ft for Turbo engines and around 4.5% per 1000 ft for NA engines. I've seen some aircraft manu's go upto 5%.

Although the theory of the above link is partially true - It's misleading. Even for NA installations

[DieselFan]

http://www.canyonflying.com/lightenup.html
This one is worth checking as it has many of the formulas mentioned here for DA, but also makes mention of the PROP, WING and Engine issues It's also the most apt for the thread


http://www.planeandpilotmag.com/content ... power.html

"The maximum height at which a turbocharger can deliver sea level MP is known as the turbo’s critical altitude. Most turbos deliver sea level power to 16,000 through 18,000 feet. Some of the stronger turbochargers extend that height to nearly 25,000 feet"

http://joecessna.com/Arrow.htm
"With the same power available at, say, 12,000 feet as at sea level, an airplane is going to feel pretty much the same at 12,000 feet as at sea level."

http://www.turbodriven.com/en/turbofact ... tages.aspx
"power loss of a naturally aspirated engine is considerable. In contrast, the performance of the turbine improves at altitude as a result of the greater pressure difference between the virtually constant pressure upstream of the turbine and the lower ambient pressure at outlet. The lower air density at the compressor inlet is largely equalized. Hence, the engine has barely any power loss."