Anybody got any neat tricks for figuring the height of obstacles that you need to clear on take off? Is it just experience? I'm pretty new but I have no idea how tall 50 feet looks, except that it looks VERY tall without climbing speed.

If you're on the ground, I could see a way to use the width of your hand to measure an angle from a paced off distance, and use the back of a wizz wheel to guess the height, provided you knew how many degrees your hand was wide (with arm outstretched) and how long a pace is.

You can try some bush trig--pace off the distance from the edge of the trees until it looks like your line of sight to the tallest tree top is 45 degrees (use your thumb and forefinger to make a 45 degree angle--thumb level and finger pointing at the top of the obstacle). The trees are about as tall as the distance you've paced off. If you're Yao Ming, add 7.5 ft. No whiz wheel required, but it only works for the objects you can walk from (it's no good for figuring clearance from rising terrain farther off the departure end).

If you really need more precision in your calculations to achieve the required margins than you can achieve using this method, you probably don't want to try taking off, at least until there's a good wind howling at you down the pipe and you've made the a/c as light as possible. Maybe not then either.

CAVU

There is a way to use your outstretched hand and each finger was X number of mils, artillery guy's would use it to adjust fire, but I'll be darned if I can remember the mils each finger was, or how many mils equal a degree. That with the laser rangefinder you use for hunting would get you darned close once you did the math to determine the ratio.
Now I think about it when I was a kid flying model rockets we built a homemade inclinometer out of a stick, piece of string and a plastic protractor. We could then calculate the height the model rocket went to. You could do that if it were worth the trouble. Laser rangefinder would give exact distance and the inclinometer would give you the angle, that's two sides of the triangle

Identify what kind of tree your looking at and get to know how tall they grow.

Compare them with utility poles. Most utility poles on distribution lines are 30, 40 or 50 feet tall. (Transmission lines are much taller.)

After a while you can glance at a tree and approximate it's height.

This two bits may be worth $0.00 since I live in the land of more power lines and wind mills than trees.

Bill

a64pilot wrote:There is a way to use your outstretched hand and each finger was X number of mils, artillery guy's would use it to adjust fire, but I'll be darned if I can remember the mils each finger was, or how many mils equal a degree. That with the laser rangefinder you use for hunting would get you darned close once you did the math to determine the ratio.

That sounds like what I was thinking. I think it's used in astronomy too, don't know how.

So, I guess most of you guys just draw on experience to know that there's enough room to take off and miss the trees.

I guess that if you're familiar with the climb angle, you can just pace the runway length from the point you know you need to be climbing by, which is most of what anybody'd need to know.

Try this,
Standing some 25 yards from the tree, with a pencil or stick held upright in the fully extended hand, first move the thumb up the stick until the exposed length covers, to your eye, the lower two yards or meters of the tree (the height of a man). Now move hand and pencil up in two yard jumps till the top is reached. Multiply the jumps by six and add any odd yards left at the top.

Climb the tree in question, and drop down your handy-dandy hundred foot tape when you reach the top.
OR
Before you land, drag the strip several times... lower each time, until you can estimate the elevation of the tops by comparing them to your altimeter.
OR
If the trees are an issue when you land over them (if you do), they're probably gonna be an issue climbing out over them too.
Words to live by: when in doubt-- DON'T!

Eric

If you need to know the height of a tree in order to determine whether you can clear it on takeoff, you can't. Or maybe you can, but you shouldn't. Nobody can reliably calculate their climb rate to +/- 50 feet at point X, and nobody in their right mind would be happy clearing treetops by a wingspan while climbing out.

I think the whole reason they put the 50 foot tree in the aircraft performance charts is to show the difference between the ground run and the space required for low level approach and departure...a subtle reminder that even though your landing ground run is only 200 feet, you can't land in a 300 foot field with trees on each side.

mr scout wrote:Try this,
Standing some 25 yards from the tree, with a pencil or stick held upright in the fully extended hand, first move the thumb up the stick until the exposed length covers, to your eye, the lower two yards or meters of the tree (the height of a man). Now move hand and pencil up in two yard jumps till the top is reached. Multiply the jumps by six and add any odd yards left at the top.

Hammer wrote:If you need to know the height of a tree in order to determine whether you can clear it on takeoff, you can't. Or maybe you can, but you shouldn't. Nobody can reliably calculate their climb rate to +/- 50 feet at point X, and nobody in their right mind would be happy clearing treetops by a wingspan while climbing out.

I'd say that's a pretty complete answer! Thanks, guys!

"If you have to ask, you can't afford it."

OMG, That is some of the finniest things I have ever read. B&Y Luscome I bet you wish you would have not asked that question for this bunch. I had tears in my eyes reading this.

CAVU wrote: (use your thumb and forefinger to make a 45 degree angle--thumb level and finger pointing at the top of the obstacle)

--------THAT IS SO FUNNY

CAVU wrote:No whiz wheel required

------ That’s great

a64pilot wrote:There is a way to use your outstretched hand and each finger was X number of mils, artillery guy's would use it to adjust fire

a64pilot wrote:Now I think about it when I was a kid flying model rockets we built a homemade inclinometer out of a stick, piece of string and a plastic protractor.

-------This is great!!! LOL

I can see a couple of passengers on the runway of a backcountry strip waiting for their ride out. They see their pilot at the end of the runway with his arm outstretched and his fingers and thumbs pointing at a few trees at the end of the runway. Or they see their pilot with a stick, piece of string and a plastic protractor. They say to the pilot, “What are you doing?” The pilot says with a straight face, “Measuring the trees!” ROFLMAO

I hope none of you take any offense to this but this really makes me laugh. The sad thing is a lot of pilots, including me, was sitting here understanding what was being said and then saying, “ya that would work.” Thanks guys for a great laugh!!!!!!!

My grandpa has this tiny handheld theodolite with a bubble level and a sight. You pace off a known distance from the base of the tree and shoot the angle. Make your calculations back in the cockpit so as to avoid any further questioning from the likes of pif_sonic.

Pack a good sharp chain saw with you. "When in doubt...Wup it out"

pif,
Sometimes it's fun to figure out how to do something without figuring out why you would want to in the first place. Logically at least some of my flying should be done in a pressurized metal tube with a bunch of other people, but I avoid that whenever possible.

I found this trick in "Water Flying Concepts" by Dale De Remer. (Seems to be a good book for wilderness flying, though I've only read the first chapter. Some good ideas that, with a little adjusting, apply to land planes as well.) Unfortunately, it only works if you can get to the base of the obstacles.

Go to the base of the obstacle, mark it with toilet paper, and pace off a distance perpendicular to the runway, and mark that distance. Go back along the runway to where you can see both marks, and measure this horizontal distance with a thumb or something, and compare it to the height.

And regarding the SHOULD or SHOULDN'T issue, De Remer describes a ratio of run to climb distance to clear an obstacle, (his example C-180 floatplane has 62% of the 50' obstacle clearance distance being water run) which allegedly stays approximately the same or conservative for an aircraft. For this example, he would put a NO-GO flag at 62% of the lake's distance. This ratio can be determined from your handy-dandy POH (which is quite a splendid piece for the Lusk.)

I know the 70% TO airspeed at half of runway rule of thumb, but it doesn't guarantee room to climb, as De Remer's does.

Buy a cheap clinometer ($4) from any hardware store and use triangulation. See
http://www.envirothonpa.org/documents/MeasuringTreeHght.pdf for an example.

Check the top branches for ailerons, wheels, spark plugs and the likes. If you find anything as such, then assume the tree is approximately two tall.

a64pilot wrote:There is a way to use your outstretched hand and each finger was X number of mils, artillery guy's would use it to adjust fire, but I'll be darned if I can remember the mils each finger was, or how many mils equal a degree. That with the laser rangefinder you use for hunting would get you darned close once you did the math to determine the ratio.
Now I think about it when I was a kid flying model rockets we built a homemade inclinometer out of a stick, piece of string and a plastic protractor. We could then calculate the height the model rocket went to. You could do that if it were worth the trouble. Laser rangefinder would give exact distance and the inclinometer would give you the angle, that's two sides of the triangle

Here's the method... A mil is 1 meter at 1,000 meters distance. If you hold your arm straight in front of you, the index finger width is approximately 30 mils, 70mils for two fingers, 100 mils for three fingers, and 125 mils for four. Basically, if you pace off 200 meters from the obstacle, 1 finger width equals about 20 feet of height, 2 fingers equals about 45 ft, 3 fingers equals 65 ft, and 4 fingers equals 82 ft. Those are my quick calculations for 200 meters. Average arm length... average fingers. Slightly nerdy and approximate, but I like it.