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  #11  
Old 11-18-2011, 05:40 PM
humptybump humptybump is offline
 
Join Date: Feb 2011
Location: USA
Posts: 3,179
Default Margins around the Envelope

Quote:
Originally Posted by flyboy1963 View Post
VNLK / LUA

12% uphill gradient landing, downhill takeoff ( obviously)
Elevation: 9100 ft (2774 m)
Runway: 1500 ft (457 m)
how do you calculate this?
Your scenario is outside my curent personal flight envelope.

The way I approach "the envelope" is with a very big safety net. My personal minimums start with big margins. From there I find a place to "try out" the edges of my personal envelope.

If my plane is capable but I am not, then I look for a talented and experienced flight instructor. I've found some great (and thankfully patient) instructors. If I feel capable and the plane is capable then I find a similar scenario with large margins. In your example, I would look for sloped runways much longer than 1500' and mark for 1500'. Same for altitude. Then practice practice practice.

Larry Bird use to say, "I don't have natural talent so I have to work twice as hard.

I have a 1900' strip now. The RV-8 was new to me. I didn't make it into my strip. I used a 4000' runway for a couple of days. Then I made my field. Wind is no different. I recently started expanding my personal envelope on wind velocity and cross winds. I go back to big runways and work it where I have safety margins.

If you want it, work on using less runway at the big fields. You'll get betting with practice. Soon, hose shorter fields won't seem as short. On your next biennial, expand your envelope.

It's all good.
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  #12  
Old 11-18-2011, 06:13 PM
Vac Vac is offline
 
Join Date: Jul 2009
Location: Niceville, Florida
Posts: 434
Default

One technique is to compute takeoff roll based on wing loading (see AC90-89A), essentially a "most conservative" estimate and then adjust for density altitude effects and slope. A "best case" answer could be estimated by using test data from your airplane, if it's available, and then adjusting that. Britain's CAA has a good primer with some handy rules of thumb (Safety Sense Leaflet #7, Aircraft Performance) and the standard Koch Chart will provide estimates for altitude effects and correct for non-standard temperature. From a risk-management standpoint, it's best to compute "worst case."

RV-4 Estimate, 160 Horsepower Engine

Assumes 73% horsepower available at 9100' MSL under standard conditions

Standard temp at 9100 MSL: 26 degrees F

Worst Case Estimate, 1500 lbs gross takeoff weight, wing loading 13.6 lbs/foot: 850' required to accelerate and smoothly lift off at MSL (Per AC90-89A Minimum Recommended Runway Length Chart)

Density Altitude Adjustment:

Method 1, 10% correction factor per 1000' increase in DA: +774 feet
Method 2, Koch chart correction factor 180%: +680 feet

Net range for roll 1530-1624 feet corrected for density altitude affect (at standard temperature, no wind)

Slope Adjustment:

5% reduction per 2 degrees of slope: Net reduction 30%

Net range adjusted for slope: 1137-1043'

Surface adjustment: Dry grass (up to 8") on firm soil: Net increase 20%

Net range adjusted for altitude, slope, runway surface: 1252-1364 feet

So I'd guesstimate 1400' to get airborne. Not much margin for error, and no acellerate stop option, i.e., I'm committed to crash beyond the runway once I commit to takeoff.

Climb Performance Estimate:

Estimate rate of climb 992 FPM @ 102 MPH CAS (from bootstrap derived performance data corrected for density altitude affect). Koch chart estimates 70-75% reduction in rate of climb from standard, sea level conditions, so that works out to approximately 500 FPM, banding estimated climb performance between 500 FPM and 990 FPM.

Assuming worst case 500 FPM climb rate, climb gradient would be 345' per NM, so 3.4 degree climb angle required to clear obstacles.

Assuming best case 990 FPM climb rate, climb gradient would be 682' per nautical mile, or 6.8 degrees.

Therefore, I'd estimate climb gradient 3-6 degrees when looking at departure terrain.

If I had to adjust for non-standard temperature, I'd increase the distance 10% for each 10 degree C (18 degree F) increase. Headwind would increase performance, but I wouldn't factor it in (i.e., most conservative estimate), any tail wind would increase the estimate by 20% for each 5 knots of tailwind component.

Bottom line, I'd wait for my wingman to go first, and if he makes it, then I'd roll...

Cheers,

Vac
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Mike Vaccaro
RV-4 2112
Niceville, Florida

Last edited by Vac : 11-21-2011 at 06:43 AM.
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  #13  
Old 11-18-2011, 06:37 PM
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Flyfalcons Flyfalcons is offline
 
Join Date: Jul 2009
Location: Bonney Lake, WA
Posts: 295
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12% downhill is like falling off a cliff. 1500 feet would be no problem. Even a 2% slope is quite noticeable. I used to fly a 206 into one of the San Juan Island airports that was on a steep slope and the usable portion was probably around 1500 feet. I always had to add power to get to the end of the runway after landing, and never had a problem taking off.
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