Clearing an obstacle after takeoff

[kwmoore]

I got a phone call from someone in Idaho who's considering a Katmai or King Katmai. His most demanding (and frequent) mission would be flying into and out of a 900-1000 foot hard-pan strip with ~100 foot tall trees at either end. This is demanding enough already, but field elevation is 4500 feet! :eek: He wondered whether the King Katmai could be relied upon for that mission. I demurred, telling him that he'd better talk to Todd about that one. Nonetheless, it piqued my curiosity.

So, I tried some obstacle-clearing climbouts to see how long it took to get to 100 feet. Conditions were: sea level (PAO) or ~200 feet (WVI); OAT 75-77 deg F, for a density altitude of 700-900 feet. Wind was about 5 kt at PAO, light and variable at WVI. We were at about 2800 lb TOW with full fuel at PAO, a little lighter at WVI. I applied full power allowing only a little roll (not with the brakes locked) then let 'er rip, rotated at 40 and climbed out at 50-55 with 20 deg flaps. Vx for the 260se is quoted as ~57-58, so I figured with our reduced weight AND the wingtip extensions, 50-55 was pretty close to the Katmai's Vx.

I was off in 350-400 feet or less but I'm guessing it was 1200-1400 feet total to get to 100 feet agl! Perhaps the altimeter was lagging...In any case I figure that my 260 hp plane under these conditions on a paved strip would be a fair model for the 300 hp plane on a dirt strip at 4500 feet, flown by a competent (me) but not expert (Todd) pilot. I don't think I'd be wanting to get out of that Idaho strip on a windless summer day for sure. In any case, to make it work, the pilot would have to pay close attention to the conditions (strip surface and environment), weight, tire inflation, engine condition, etc.; pilot technique would have to be spot-on every time. Very little margin for error with this, and the consequences of imperfection would be severe. :(

To net a slight improvement and more precision next time, I will hold 55 KIAS exactly on climbout, and use my GPSMAP496 and GNS430w altitude readouts (they update 5X/sec) instead of the altimeter. In addition I'll try some landings-over-a-100-foot obstacle, aiming for 45 KIAS and stabilized, at ~120 feet over the pretend obstacle. All-in-all, it should be an interesting set of exercises. Stay tuned...

Unless the outcomes of those tests are significantly different, I think the Idaho gentleman might want to consider an R44 instead of a fixed-wing aircraft, even the King Katmai! ;)

[kwmoore]

...it just ain't gonna work, anyway. Indulge me for a moment...:rolleyes:

Let's say the King Katmai pilot exhibits Todd-like skill on taking off from this 1000 foot strip at 4500 feet elevation and uses but 400 feet, lifting off at an optimal Vx (55 KIAS) and climbing at a sprightly 1000 fpm right from the moment s/he breaks ground. Well done! Not likely to happen, but hey, you're an ace!

55 KIAS is about 62 mph, or ~90 feet/second. But wait, we're at 4500 feet, so even on a pleasantly cool 60 deg F day, 55 KIAS is more like 60 KTAS, or 69 mph (~100 feet/sec). The King Katmai has 600 feet to go to the trees, 6 whole seconds of pre-impact air time! :eek: But, 1000 fpm is 16-17 feet/sec, and the plane will climb but 96-102 feet agl in this 6 seconds. At the very least, the King Katmai owner can expect plenty of chlorophyll stains on the plane all the way up to the cowling inlets, not to mention on the propeller blades! More likely s/he will leave paint stains on the treetops and a trail of charred tree bark and aluminum fragments all the way to the forest floor, then after release from the hospital s/he will have to get in the 1.5-year long queue for another King Katmai to try it again. :D

Even if you're light and can get off the dirt in ~300 feet, you'll still have to accelerate to Vx and then achieve that 1000 fpm or better ROC to have any chance of escaping with only a few twigs caught in the wheelpants. :rolleyes:

How about waiting for a headwind? Well, maybe...you'll get off the ground faster but then you'll be climbing against a downdraft due to the wind spilling over and down from the tops of the treeline at the far end of the strip. No, thanks.

That R44 looks even better for this job! :p

[joejenie]

This guy is crazy! He needs a helicopter for sure and the R44 isn't the right bird either. They don't do very well in DA. It would work as long as he leaves early in the morning!;)

I'm in a hotel in Wichita right now trying to go to sleep. Kind of tough when I'm picking up my baby finally! Now hopefully the weather cooperates tomorrow.

My airplane is the first customer delivered King Katmai. It should be fun! It is still experimental it looks like for a few months. DAMN FAA!:mad:

[kwmoore]

I know next to nothing about helicopters, but I inferred from the R44 performance specs (http://www.robinsonheli.com/r44iispecs.htm) that this bird could still climb at 1000 fpm at 6000 feet at MGTOW (2500 lb), even though maximum hovering altitude out of ground effect was only 4500 feet at 2500 lb. Even to me that seems a bit contradictory; perhaps the max rate of climb requires some forward speed?

[Todd]

The gentleman that's thinking about using this area as a strip sent me some pictures. Just looking at them my first thought is yes it probably can be done with a King Katmai. I have visited strips worse than this but the safety margin is small and you need to be right on the money every time. One must fly the airplane in a very small "window" of energy and altitude and being able to recognize that you're not in that "window" early is important as a go around late in the game would most certainly ruin your day.

I told him if time permitted I would look the area over during my next trip to Idaho.

Todd

[kwmoore]

To net a slight improvement and more precision next time, I will hold 55 KIAS exactly on climbout, and use my GPSMAP496 and GNS430w altitude readouts (they update 5X/sec) instead of the altimeter.
Finally got out this morning after nearly two weeks of aviation deprivation :o. Work, personal, and weather schedules had conspired in an unfriendly way.

I tried another obstacle-clearance departure from PAO. Wind was calm, temp about 50 degrees, aircraft weight about 2550 lb. The GNS430w and GPSMAP496 altitude readouts were -12 feet MSL on the ground. I did not hold the brakes but advanced the throttle smoothly and quickly to maximum power. Once again I was off in well under 400 feet, then allowed the plane to accelerate to and hold 55 KIAS (20 deg flaps). When the GNS430w altitude readout passed through 100 feet I looked outside and was just passing over the 2nd turnoff, some 1500-1600 feet from the power-up point (airport diagram (http://flightaware.com/resources/airport/KPAO/APD/AIRPORT+DIAGRAM/png)).

Clearly, the "problem" as posed to me over the phone would test or exceed the limits of this plane. OTOH if the strip were longer or the trees shorter than I was led to believe, it might be doable.

Aside from that, 1500 feet to clear a 100-foot obstacle is pretty darned impressive! :cool:

[nworth]

The Private and Commercial standards do not distinguish between short field and obstacle clearance but there's actually a big difference in technique required if you are actually faced with one (but not both at the same time) of these.

The best way to clear an obstacle is to configure for the best balance of good lift and minimized drag (0 or 10 degrees flaps), fly in ground effect (while retracting flaps if you have time) as long as possible to gather speed, then zoom over the obstacle.

The best way to take off from a short/soft field is to configure for maximum lift, and get off the ground ASAP.

These are very different techniques.

Deakin discusses the difference in one of his typically excellent articles at:

http://www.avweb.com/news/pelican/182089-1.html

Of course, with a 260se, you actually have an alternative option for clearing an obstacle: don't bother and, instead, make a turn right after lift off!!!

[kwmoore]

The best way to clear an obstacle is to configure for the best balance of good lift and minimized drag (0 or 10 degrees flaps), fly in ground effect (while retracting flaps if you have time) as long as possible to gather speed, then zoom over the obstacle.

The best way to take off from a short/soft field is to configure for maximum lift, and get off the ground ASAP.

These are very different techniques.

Deakin discusses the difference in one of his typically excellent articles at:

http://www.avweb.com/news/pelican/182089-1.html

Of course, with a 260se, you actually have an alternative option for clearing an obstacle: don't bother and, instead, make a turn right after lift off!!!
Makes sense. I read recently--forget the source--that the flap deflection corresponding to best lift vs. drag is about the same as full aileron deflection. In other words, set the yoke to full right or left, then lower flaps until they are aligned with the downward-deflected aileron. I haven't tried this out yet as my plane is still in for a 100 hr/annual at EQA, but I suspect that it's very similar to what Todd recommends, i.e. 1/3 to 1/2 of the flap trailing edge visible to the pilot through the rear window.