Helicopter Performance....

[zaplead]

If I understand correctly, pilots flying reciprocal powered helicopters use manifold pressure to determine the power availible and the type of take off (e.g. towering or creeping cushion) that they can perform.
On a gas turbine helicopter I understand that N1 indicates the power level being provided, but I have never seen any reference to how the amout of power reserve relates to making landings or take off's..........can anyone help?

[Randy_g]

N1 is one of the power indications used with a turbine. However it isn't necessarily the best indicator of how much power is being used. Since it measures the compressor speed, it will change with density altitude. The higher you go in temp, and/or altitude the higher your N1 will be for a set power setting.

The one that is used most often is the torque gauge. It measures oil pressure at a set of gears usually in the main rotor transmission, and is a real indication of how much power is being transmitted through the transmission. The guage is either marked in %, or in p.s.i. You will have a maximum (red line) and that is all the power that you have available (turbine temp, and N1 allowing of course). So if the a/c requires 75% to hover, and you have a maximum of 100% torque allowable, you have 25% in which to use for departing. Depending on the a/c, at least 15% torque is required to vertical out of a confined area.

Hope this helps clear it up. :)

Cheers

http://randyspics.tripod.ca/gifs/naughty.gif Randy_G

http://randyspics.tripod.ca/gifs/bear_eating_picnic_md_clr.gif

[BlenderPilot]

Here at higher altitudes (8,445 FT MSL, MMTO) power is measured different ways, on the 212 for example on some days you will reach the N1 limit before you reach Torque or ITT (temp.) limits, then others ITT will be your limit and torque will almost never be a limitation becasue it will never be reached before the other limits, consequently your measure of power available changes.

Just this morning I took off with 2 pax and 70 Galons on a 206B3 and my limitation was torque, I could go to 100% TQ and I didn´t worry about TOT (temp.), then the same flight at noon with 10 degrees more and I couldn´t reach 80% TQ before reaching the TOT (temp.) redline.

All of this is basically due to the high elevation, if you take both helicopters to sea level your basic power available indication will certainly be TORQUE.

[ppng]

Randy G: 'The one that is used most often is the torque gauge. It measures oil pressure at a set of gears usually in the main rotor transmission . . . '

Randy, we should all be careful when suggesting anything like 'this is the way it is done'. For example, the Super-Puma series of helicopters uses a 'hall-effect' probe to measure torque (Nick Lappos may explain, if you ask) which has nothing at all to do with oil pressure and everything to do with twisting moments.

Anyone wonder why Eurocopter don't have a Nick Lappos equivalent?

Anyone wonder about the future of ECF?

Anyone wonder if the two questions are related?

[Shawn Coyle]

The problem with rules of thumb is that the turbine engine is dependent more on temperature for power available than on density altitude (the explanation is too long for this forum, check out my book - the Art and Science of Flying Helicopters - shameless plug).
The example above proves the point - a 10C change in temperature made the engine temperature limited instead of transmission torque limited.
Rules of thumb I've seen worked for low density altitude conditions, and were related to the margin between maximum N1 or torque or TOT and the existing N1, torque and TOT in a three foot hover - these are good for the pre-takeoff condition, but of little help if you have to land at higher altitudes.
The favored way to look at the pre-land conditions is to go VY (typically 60 knots) and look at the margin between existing power and maximum (and you may have to pull to the maximum to find out which parameter it is) and then make your decision on the margin.
Sorry no hard and fast numbers here as they will differ from machine type to type.

[zaplead]

Thanks for the great info guys, it has makes life a little clearer.
Have had considerable difficulty finding any info on this subject in reference books, thanks again & take care..........Zap:)

[John Bicker]

zaplead,

I think it's called the flight manual. Leave the flight testing to the flight testers. Where I operate if your are going to land above 4500' amsl you must comply with the following: Establish where you are going to land and the height - map or otherwise. Confirm OGE hover performance from the flight manual with the current met conditions. Confirm power assurance from the flight manual. You arrive at the landing point with ample power. Subsequent landings may be conducted at the same point using IGE hover performance. Pedantic - yes, the law - yes. Seems to work though. At least this method will let you gain experience with some safety over "suck it and see".

[Aladdinsane]

Zaplead

As an example a rule of thumb I use in a 206 is check what power (torque) it uses to maintain level flight at 80KIAS about 500' above your intended landing site. Add 20% (torque) to that figure to give you a IGE hover. To climb back out of the landing site add another 10% and if you have picked up some frieght or Pax add 1% for every 15Kg.

The advantage of this method is it automatically takes into account the altitude you are operating at.

[Arm out the window]

Aladdin, that sounds like an extremely handy rule.
Did you figure it out for yourself from performance graphs, or get it from somewhere else?

I've always found those sorts of things to be great for keeping on top of what's going on performance-wise, something that you can work out quickly in your head when the pressure's on.
Having said that, I guess the performance manual is the one they'll quote in the accident enquiry!

[Nick Lappos]

Some discussion on power instruments:

The only true power instrument for a turbine is the torque, which measures the twisting forced the engine is applying to the transmission. As the rotor bites into the air, the twist must increase, so the torque rises. Torque times rpm is horsepower.

Several torque sensprs exist, the old fashioned kind uses the oil pressure inside the engine to push the helical output gear back into place (helical gears try to walk along their axle in exact proportion to the torque they are transmitting, so the push-back on them is a reliable way to calculate torque.

Most modern engines have a torque tube, where the inside is connected from engine to transmission, and subject to full torque, which slightly twists the shaft. An outside cover is attached at one end, the other end floats above the shaft, and does not transmit torque, so it doesn't twist at all. This outside cover has some bumps on it, as does the regular shaft. When at zero torque, these bumps all align evenly. When the inner shaft winds up with a bunch of torque, its set of bumps shifts over and gets closer to those of the outside cover, so the once even spacing is now a series of unevenly spaced pairs of bumps. We place a magnetic pickup just above the bumps, which sends out an electric pulse every time a bump passes by. As torque rises the pulse pattern shows the shift of the bumps, which is proportional to the torque. The instrument system converts this pulse shift into a reading on the gage.

For most helos, takeoff power is easy to estimate, using the ratio of the power needed to hover. If you can hove at two feet with x torque, then you can make a running hover takeoff (a really flat one) with just 1% torque above that. If you need to make a towering takeoff, you need OGE power plus about 2% more. OGE power is about 1.15 times IGE power, so about 1.17 times IGE power will let you make a vertical takeoff. For example, if you indicate 60% torque to hover IGE, then it will take 71% torque to make the towering takeoff (60X1.17=70.2)

Alladinsane (cool username!) has a fine rule for those times when you can't land and hover first. The only warning is that the slightest descent rate or deceleration while checking the 80 knot power will make the estimate very liberal, and the true performance disappointing, so do the check carefully. I have made similar rules of thumb for Black Hawk and S-76, but usually fell back to a quick reference chart I distilled from the flight manual, based on the operating area and season. A simple table with a small band of altitude (say zero to 5000 feet in 1000 foot intervals) and a small band of temperatures (say 30 degrees C in 5 degree intervals), altitude along the side, temp along the top. Inside each square, put the exact weight your machine can carry at that condition for IGE and OGE, based on the charts in the flight manual. Determine the actual empty weight, plus you and your crew, plus equipment, and subtract all that from the flight manual chart IGE and OGE hover weight. As you fly up to the landing site, note the alt and temp, look up the weight you can carry on your handy chart, subtract your fuel, and bingo, you will be within a few percent of what you can pick up.

A comment on engine temp and N1 as power gages. They do produce information proportional to power, and are very useful, but they are not necessarily very porportional, especially N1. This means that a few tenths of a percent N1 can change performance all out of proportion to the indication. Torque is very linear, and easier to interpret. Both temperature and N1 are limit values that must be respected, and both can be the power limit on a gioven circumstance, of course.

[SASless]

Nick,

Nice idea on the chart construction.....years ago in the hills of Iran and other high hot places where we had to operate from sea level to 14,000 feet msl.....I oft times would use my V toss or Vbroc airspeed and establish a 500 fpm rate of climb in still air....and note the power required. Seems hover power (IGE) usually produced about that same performance in most aircraft I have flown thus if I had the power to climb at 500 fpm at the same altitude as the landing site in still air.....things seem to work fine. The big trick was to maintain the correct airspeed, rate of climb.....and be in still air.

The Alouette III and Lama used a pitch meter to calculate such things and made it very easy......it used OAT, Altitude, aircraft weight as variables to determine the pitch required/available for the data entered......very much nicer setup than any US made aircraft I have flown.