WANT TO KNOW WHY GYRO'S HANDLE WIND AND TURBULENCE SO WELL?

[FO Gyro]

Attached is a spreadsheet I drew up some time ago where I compared the wing loading of various aircraft to compare how a gyro fares: a Trike, Cessna 172, Cessna 210, Boeing 747 very light at 220 000kg, and heavy at 360 000kg, a SU 27 military fighter, a F15, Learjet 35a, Dash 8 (SAX Airlines), Mirage F1, a Cessna 208 Caravan, and a Boeing 757.

The wing loading (or more correctly the blade loading) of a gyro it turns out is quite high, and is up there with the heavies. According to my spreadsheet, the wing loading is higher than a Cessna 210 and a 208 (Caravan), is higher than a Dash 8 (twin turboprop flown by South African Express Airlines), and just less than that of a Learjet 35a.

Also the tips are flying at around 600km/h, so the gyro behaves as though it is one of these heavy aircraft, flying at 600km/h!!! The wind doesn't stand a chance with a gyro!

Here's a the graph from the spreadsheet comparison.

[Gyronaut]

FO, I am keenly interested in your topic!

I have always held the belief that the wingtip speed in a Gyro (the lift generating end) is so much higher by several factors than wind gusts that wind gusts become insignificant. Of course, profile drag and induced drag exist as in most other aircraft so one is really only talking about wing reaction to adverse winds. Am I right or right off the track?

I am interested to know hou you calculated the wing-areas? Is this the wing area in a static position? i.e. length and width of both rotors in the case of a gyro? Should one not consider the disc area? Any aeronautical engineers amongst us?

Len

[FO Gyro]

Hi Jetranger.

I am certainly not a boff at aeronautical engineering here. In my example of the wing loading, I multiplied the rotor chord by the rotor length to get the static wing area. Not sure if the entire disc should be used, or on the other hand, maybe only the flying part of the rotor should be used, as some parts are in a stalled area close to the hub.

Any boffs out there...

[Low Level]

I'm not a boffin, but I'm thinking FO is hitting the nail on the head.

From what I know, the wind doesn't see a disc. It sees a wing travelling at between 600km/h and 120 km/h - tip to 1m from centre.

Say you fly in a 20 knot wind. At the tip, where moment on the plane is maximum, the wing sees a mere 10 % difference in windspeed between downwind and upwind rotation. It's like flying a fixwing in a 3 knot wind.

Wind coming from the side, just a slight pitch adjusment, from the front or back, just a slight roll adjustment.

Closer to the centre the effect of the wind is bigger, but the moment on the plane becomes minute.

Another obvious thing might be the total wing area effected by the wind.

Just calculated speculating.

[Condor]

It is so nice to have clever guys amongs us.

Next question, density altitude, calculations and effect please.

[Gyronaut]

Again, directly from the Rotorcraft Flying Handbook FAA-H-8083-21

Density Altitude—Pressure altitude corrected for nonstandard temperature
variations. Performance charts for many older aircraft are based
on this value.
Standard Atmosphere—At sea level, the standard atmosphere consists
of a barometric pressure of 29.92 inches of mercury (in. Hg.) or 1013.2
millibars, and a temperature of 15°C (59°F). Pressure and temperature
normally decrease as altitude increases. The standard lapse rate in the
lower atmosphere for each 1,000 feet of altitude is approximately 1 in.
Hg. and 2°C (3.5°F). For example, the standard pressure and temperature
at 3,000 feet mean sea level (MSL) is 26.92 in. Hg. (29.92 – 3) and
9°C (15°C – 6°C).
Pressure Altitude—The height above the standard pressure level of
29.92 in. Hg. It is obtained by setting 29.92 in the barometric pressure
window and reading the altimeter.
True Altitude—The actual height of an object above mean sea level.

[Low Level]

Hi Condor

My take on density altitude.

A wings lift is directly influenced by the density of air. The thicker (more dense) the air, the better the lift. Density is influenced by three variables - temperature, humidity and height above sea level - from there the saying in aviation - be careful of the three high H's - hot, high and humid.

Hot air and height to influence density is relative common knowledge. Watermolecules in air actually also makes air less dense, because it takes up less space than air molecules - thats another long story

Now, according to weather standards, temperature suppose to drop 2 C with every 1000 ft of altitude gained - therefore if it is 25 C in Durban, Joburg suppose to be 25- (5300 * 2/1000) i.e. 25 - 10,6 = 14,4 C. Now if this is the case the density altitude would roughly be the same as the altitude - 5 300 ft - if humidity is disregarded.

Fortunately for everyone - except pilots - this rarely happens that Gauteng is 11 C colder than the coast. This is now where density of air has to be recalculated.

The formula - taking humidity in account is quite immense, but because humidity does not have the same effect as altitude, a more simplified formula - not taking humidity in account is used:

DA=145 426*(1-(2,84*P/T) power of 0.235))
P - Air pressure at runway (Gauteng is approx 88 kPa, coast 101.35)
T - Ambient temp in kelvin - C+273)

One usually do not fly with a calculator, and the runway air pressure is also usually not known - therefore a thumbsuck formula has been derived.

To do a quick density altitude calculation for dry air:

International Standard temperature for the coast always to be used is 15 C
Say Brakpan is at 30 C at 5300 ft, but according to weather standards, at this altitude, it is suppose to be 15 C - (5300*2 C/1000) = 4.4 C

Difference is 30 C - 4.4 C = 25.6 C

Now add 120 ft per each degree C difference

That is 5 300 + 120*25.6 = 8 372 ft

Now this is density altitude - If one is going to fly on a 30 C day in Brakpan, the plane is going to act as if you were taking off at an altitude of 8 400 ft on a dry day. For a high relative humidity, another 10% must be added - that is 9 240 ft.

If your plane has a service ceiling of 10 000 ft - be very careful, you might not get off the ground.

[DieselFan]

Calc'd DA can quite easily be solved by using your cellphone's JAVA capability.

[Morph]

be careful of the three high H's - hot, high and humid.

Hot high and Humid is related to carb icing rather than Density Altitude

[Coyote]

Ldel - please explain to me why wind from the side translates into a pitch correction? is it because it it felt 90 deg from the occurance?

[Condor]

Thanks for the explnation guys.

Surely you would be able to carry less weight on a hot day than on a colder day, specially at altitude. That is why you do the calcs.

Are there tables, graphs avaialble for the Gyro's (by type if this should be different)?

[FO Gyro]

"Hot high and Humid is related to carb icing rather than Density Altitude"

Humidity also reduces the performance of aircraft. Very crudely speaking, when its humid, instead of air particles being evenly spread apart, water molecules replace some of the air molecules. Since there is slightly less air to burn with the fuel mixture, performance is reduced.

[Gyronaut]

Ldel "Watermolecules in air actually also makes air less dense, because it takes up less space than air molecules - thats another long story "

Point taken and understood.

This is the value of this forum because at least a few of us now understand that humidity can spoil your whole whole day if you dont take it into account when you do your weight and balance and density altitude calcs.

Lets fly as safely as we can.

[Low Level]

Morph wrote:

Hot high and Humid is related to carb icing rather than Density Altitude

True for both. Looking at the simplified formula for calculating density altitude, it is directly influenced by Pressure (which is determined by height - the higher you, go the lower the air pressure) and temperature.

The complete formula takes dewpoint for the air also into account, which is determined by humidity - but the effect is smaller.

Something interesting - a friend of mine's friend came to visit him on a game farm, during November, from Durban with a Cessna, wifes, kids, dogs - the whole caboodle. Took of from a 800m strip in Durbs and landed late afternoon on the farm, close to Naboom. Tried to take off with the same load on Sunday mid afternoon, and ran off the end of a 1200m strip without getting airborne. Fortunately no damage. He had to dump some fuel, and left Monday morning at 5:00 - no problems. His calculations for altitude was fine, but he disregarded density altitude, which at 33 C added most propably another 3 000 ft.

Maybe the guys flying the big birds can confirm something else. I timed a loaded Kulula flight. Taking of in Jo'burg (4.6 km runway) - 50 seconds from moment the thrust is applied till take off. Cape town (3.3 km runway) - same plane - 25 seconds.

[FO Gyro]

Maybe the guys flying the big birds can confirm something else. I timed a loaded Kulula flight. Taking of in Jo'burg (4.6 km runway) - 50 seconds from moment the thrust is applied till take off. Cape town (3.3 km runway) - same plane - 25 seconds.

Comparing a take off out of Joburg vs Cape Town on a Boeing 737-800, for the same weight, we would have very similar speeds for the take off (V1, Vr and V2, V1 being decision speed, Vr - Rotate Speed, V2 - Liftoff speed). For each take off, depending upon the temperature, the runway in use, the elevation, our weight, and the QNH, a different set of speeds are worked out. On light aircraft, one tends to use the same speed for take off.

Getting back to the time between brake release and lift off, one needs to understand the difference between IAS (indicated air speed) and TAS (true air speed). If one rotates at around 145 KTS IAS (a typical speed), one is also doing 145 KTS TAS at the coast on a standard day (15 C, QNH 1013.25). On the highveld, when lifting off at 145 KTS IAS, one's TAS is 162 KTS (at 25 degrees C), so the aircraft has to accelerate that little bit more before becoming airborne at the same IAS.

The engines also produce less thrust on the highveld, and so the effect of this, plus having to lift off at a higher TAS, results in a longer time before lifting off.

What is interesting is that we always save our engines, and never take off with full power, unless required to because of weather conditions (eg windshear, or very heavy rain). We take off with various thrust settings (depending on how light we are). If very light, and at the coast we apply a setting that simulates the aircraft's performance as though the outside temperature is 65 degrees C. This means that the engine is hardly working on take off, and is around climb power, and not take off power.

In my MT-03, not sure what the other guys do, but I only use 5 500RPM (max continuous power), even with a passenger, to save the engine. Only now and again do I use 5 800 RPM (full take off power). Some people say you shouldn't nurse one's engine too much, and other's say you shouldn't use full power for every take off, or your engine life might be reduced.