How are take-off performance charts developed?

[BackPacker]

I'm involved in the operation of a relatively powerful and noisy STOL aircraft (GA8 Airvan) which is operated mostly from dry, paved and long (>2000m) runways.

In order to conserve fuel, reduce engine wear, be a good neighbour and so on, we're looking into developing reduced-power take-off performance charts. Obviously these reduced-power take-offs will only be used when we have "plenty" runway, otherwise we'll simply go full power and use the POH charts.

The question is "What is plenty?" We're going to gather a lot of data points during our normal operations but realistically, even with 100+ data points we're never going to produce a full take-off performance chart with smooth lines on them. Heck, even test pilots do not perform sufficient landings (to a consistent standard) to make all the data points on the chart look like smooth lines.

So what formulas and methods are used to interpret/interpolate/extrapolate the data points gathered during test flying, and make them into proper graphs? I know the CAA factors for various conditions but these are simple multiplications only and will not yield the complex shapes that you find on a typical performance chart.

Or, alternatively, is there a handy formula that I can apply to the current, full power take-off chart, and that will give me my reduced-power take-off chart?

The GA8 is powered by a Lycoming IO540-K1A5 by the way. Our full-power chart are for full-throttle, 2500 rpm (2700 rpm being available in emergency only), and we're looking at developing charts for a 25/2500 power setting (75% power, which is - not coincidentally - our continued climb power as well).

[Gertrude the Wombat]

The question is "What is plenty?"
Yes indeed.

The POHs I've looked at give performance charts for "short field" take-off but give no clue as to how to work out how long the runway needs to be for you to forget the "short field" technique and revert to "normal" take-off.

[MarkerInbound]

I'd question whether just reducing the throttle will make any difference as a "good neighbor." A fair amount of a recip powered airplane's noise comes from the prop. If you're not reducing prop rpm you won't have much effect on the noise. In fact you may make it worse. In an open space distance has a large effect on percieved sound levels. By using less power the aircraft will be closer to the ground longer and therefore seem noisier to more people.

Fuel savings become a calculus problem. Yes, you don't have the initial high TO fuel flow but you have a longer time at climb power before you reduce to cruise power. In an IO-540 I wonder if you'll see a noticeable difference given all the day to day operational variables?

[dirkdj]

I would advise against partial throttle operations in a piston engine for take-off. At full throttle a 'fuel enrichment valve' opens and makes the mixture extra rich. If you retard the throttle just a bit, this valve closes and the CHTs start upwards. Cruise climb is something else, there you have more airspeed to help with the cooling;

[Mark 1]

Noise doesn't propogate as efficiently over the ground laterally as it does once airborne. So it makes more sense to take off normally and reduce power at a safe height.
Airliners that use 'cutback' for noise abatement typically climb to 7-800' and then reduce to maintain about 4% climb gradient, though that technique isn't used much these days.
Reducing RPM is much more effective than reducing MP for noise reduction, so worth checking the minimum revs allowed for full throttle.

[Pilot DAR]

but give no clue as to how to work out how long the runway needs to be for you to forget the "short field" technique and revert to "normal" take-off.

The length of the runway is really hardly relevant, it's much more the departure path available (obstacles). The ground run portion will be nearly identical in any case.

Using the generic Cessna 100 series pilot handbook of your choice, you'll probably find that the only difference in the techniques between "short" and "normal" is whether you tear the hell out of your prop, and drive your nosewheel hard into the ground, with full power before brake release, or let it get rolling with the nose light as you open the throttle. The use of takeoff flap will be the same, knowing that it is preferable to use more flap than less for safety and reducing wear and tear on the plan.

Other than that, the differences are the climb away speed, which is all obstacle clearance. I do not accept the notion that there should be any real difference in the way you configure and handle the plane for the rolling portion of the takeoff, unless it's a soft field takeoff (for which you will not see performance data).

For one of my projects, (a Caravan) complete new set of performance tables were required. I did many hours of data gathering takeoffs, and after that, a flight test engineer developed the new performance tables, and the bill was $16,700 for that service. I cannot imagine a manufacturer going to any effort to define less good (than maximum) performance for their aircraft.

It's a big job to get that data to a certifiable level. For "company operations" within the certified limits, you can generally do what you want, other than putting the plane into an unsafe (poor performing) phase of flight without out good justification.

To make this easy, I suggest that you establish the (sea level, I presume) power which you would like to use for your neighbourly takeoffs, and convert that into percent power. Then find the altitude in the manufacturer's performance charts which equates to that percent power. Direct your pilots to use that power setting, and refer to the climb performance in the Flight Manual for that artificially high elevation takeoff, then just tell your pilots to go to maximum power if that is required at any point for safety of flight.

You SOP says something like for takeoff distance determination, when conduction takeoffs at XXXX RPM and XX MP, enter the takeoff distance and climb performance charts at X thousand feet higher than the actual runway elevation.

Doing that is simple, and will not create a situation where the pilot is being asked to fly in accordance with data other than that of the aircraft manufacturer.

[Big Pistons Forever]

In single engine aircraft altitude is life if the engine fails. The minuscule savings in fuel and wear you save by a reduced power takeoff in no way makes up for the lower net takeoff flight path, IMO. I fly Vy to 1000 ft AGL on every SEP.


As for noise, as was mentioned in an earlier post, the only way to significantly reduce take off noise is to reduce RPM.

[dubbleyew eight]

flap on takeoff shortens the takeoff roll by a few percent but reduces the overall climb performance. it is often better not to use flaps on takeoff. tyre wear doesnt cost that much. if your aircraft has some tested advantage using flaps on takeoff then be guided by the testing....

[john_tullamarine]

Generally, one starts from one of the standard sets of equations for takeoff and landing analysis which can be used to generate the charts.

The purpose of flight test is to check the accuracy of the model and fudge the equations to make them fit the observations.

The time spent on flight test for performance takeoff and landing work depends on the budget and how important it is to get the final kilo out of the bird. Effort can vary from using a still camera in a triangulated calculation with several good takeoffs and landings per configuration up to a very detailed cinetheodolite record and analysis. The latter we used on the Nomad, for instance. With the advent of differential GPS, it is now a bit of a doddle to get the data.

One needs to keep in mind that STOL is undefined for civil operations and that "normal" means iaw the design standards relevant to the aircraft .. in this case the relevant version of FAR23.

Given that the OP is talking about the Australian Airvan

(a) you might give the lads down at Latrobe Valley a call and ask the question directly. I don't know precisely what was done originally for the aircraft (my guess probably by Col Nicholson) but my best guess is that they would have used the earlier equation set referred to below - there would have been little imperative to try and get anything more accurate when that is such an easy way of developing the charts.

(b) historically, for Australian chart work, there are two approaches. The more recent, dating back probably 25 years now, was developed by John Fincher and was intended for light turboprops. Quite elegant but quite involved. The earlier dates back to the early 70s and was developed by Ian Cohn and Ron Saunders. Very simple, surprisingly accurate and still quite acceptable. Both are in CASA publications but probably not available to the public anymore post 1988. Most of us older folks have copies in the library and have used both in anger over the years.

[Skeeve]

Hi John,

I'm very interested in the two australian publications regarding take-off performance (currently working on a certification project for a light aircraft).
Can you cite the two titles and Doc.-No?
And any source where to get them?

Much appreciated.


Tom

[john_tullamarine]

I was hoping no-one would ask that. I have copies tucked away in storage and will have them out in the next few months after a move but I have no idea just where they are at the moment.

Both documents are Australian DCA/.../CASA (Department of Name Changes over the past few decades) internal TM type advisory papers which Industry folks involved with performance work generally used as well.

You might check with either the flight test folk at CASA (I'll send you some contact details by PM) or, very likely, another PPRuNer by the handle of djpil will still have his copies.

[aeromariner]

The easiest way is just to use the "equivalent altitude" method. Work out what power you want to use and work out what difference in altitude that involves between what the manufacturer used and what you want to use. Say the normally aspirated Airvan is based on 30 inches S/L and reducing and 2700RPM =300 Hp and you want to use 2400 RPM at 30 inches (over square - how naughty) and reducing. Then the power you are using is most likely about 265HP, and that would equate to about 27 inches at 2700RPM. 27 inches is a density ratio of about 0.9 which would be 4,000 feet. Hence a normally aspirated Airvan at S/L with 2400 RPM would perform like an airvan at 2700 RPM and 4,000 feet. So for 2400 RPM just add 4,000 feet to your actual airfield altitude (say 3,000) and read the distance for the total (7,000). There are other effects, but are small in comparison. There is an equivalent temperature method (but you usually fall off the chart). Turbocharged aircraft are obviously quite different. A lot of fincher's "AF" stuff made it into text books, but from what I remember TO distance was ratioed by density ratio raised to the -2.7 for normally aspirated and -1.0 for TC abd SC stuff.

[Genghis the Engineer]

At risk of stating the obvious, have you checked the methodology in AC23-8 ?

G

[Miserlou]

Regarding the straight 'performance' part of the question, as Aeromariner writes.

Look into the manuals and go to the high density altitudes and high temperature performance.

At a high density altitude/temperature the engine will not be developing full power. If you use these power settings at sea level you will still be getting better actual performance than the chart says because your at sea level. You're just pretending the engine is at "x" density altitude or OAT.

Big Pistons.
In europe the biggest problem in surviving is earning money and if you make a lot of noise, you will be severely restricted in your operations and hence potentially, earnings. And it is responsible to be good to your neighbours.

Nothing worse than looking at a blue sky and not having any movements left on your quota.

But the noise regulations are objective whereas 'noise' is subjective. So the same engine driving a 4 bladed prop produces a more irritating higher pitch whine which but has a lower rating on its noise certificate than the deeper bruuum of the 3 bladed prop.

I did a lot of meat bombing with PT-6 powered aircraft 3 (and 4) bladed props.

We just reduced the rpm until the tips speed was quiet. Using the manual we were able to calculate the power to be 609 horsepower were the aircraft/engine were certified to 680 hp for take off and 620 max continuous.

The manual only had performance data for the original 550 take off rated engine.

So I suggest using the method described earlier if you want to know the exact horse power.

Just use the highest rpm below where the prop tips scream and as high over square as the manual allows.
It's very considerate flying.

PS. But it was funny setting off car alarms with a low pass at 100% prop rpm though!!!