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Safe Gliding Distance Calculation


Palle

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In the book Stick and Rudder (Chapter 15: The Approach - The Glide Line) the concept of an imaginary horizontal "glide line", located at a certain number of degrees below the horizon, f.ex. 10 degrees (degree based on best glide ratio), is introduced.

 

The idea is to be able to quickly tell how far you can safely glide in the event of an engine failure. Any point that appears below the "glide line" can be reach in a glide (subject to wind and other variables off cause), but any point that appears above the "glide line" is out of gliding range.

 

I've not come across anything like this before. It could be a great tool in a stressful situation and should work regardless of altitude.

 

I understand that the "glide line" should be placed at a higher number of degrees below the horizon for a plane with a low glide ratio (sinks fast), and placed at a lower number of degrees below the horizon for a plane with a higher glide ratio (doesn't sink so fast and can glide longer). F.ex. if the glide ratio is 5:1 the number of degrees would be = X, and for a glide ratio of 15:1 the number of degrees would be < X.

 

However, I can't figure out the algorithm used to calculate the number of degree for where the imaginary "glide line" should be placed. F.ex. what number of degrees below the horizon should I place the "glide line" for a plane with a glide ratio of say 14:1 (CTLS)?

 

Anyone wanna take a guess? Or maybe I'm asking about something that is not used anymore. If so, does anyone know if there is another method to figure out if you can glide to that nice flat landing spot you see "out there past the river"?

 

Thanks.

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It would be nice to have a rule-of-thumb on the gliding distance for a CT. My CFI had one for the C152.... if I remember right, it was a spot he picked on the wing strut. It was pretty accurate. This question is one reason I've always liked AnyWhere Map... they have that cone-of-safety thing that shows your glide range. I guess it's patented, so nobody else has it.

BTW, on my last BFR, during the engine out part, I picked a small glider-port that I thought I could make.... wrong (short) by about 1,000 ft.

Tim

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Oh Dear, this makes my brain hurt!!

 

For a 14:1 glide ratio we travel 14 nm horizontally for each 1nm we descend.

 

Using trigonometry we get an angle of about 4 degrees below the horizon.

 

tan-1(1/14) or tan-1(0.714) or 4.086 degrees.

 

Some will find it easier to use a different calculation.

 

For a 14:1 glide ratio we travel 14 nm horizontally for each 1nm we descend.1nm is nearly 6000ft.

 

So if you are 6000ft above level ground you'll travel 14nm. At 3000ft you'lltravel half the distance, 7nm. At 1000ft you'll travel just over 2nm which isan easy number to remember.

 

But BE WARNED, these are THEORETICAL calculations.

 

They don't take into consideration you probably wont fly the aircraft at BestGlide, nor that you will want to be at pattern height on the downwind leg toyour landing area.

 

The instructor's method of using a point on the strut will probably result in amuch steeper angle than this theory, and is likely to greatly increase theprobability of a successful landing.

 

The fact it doesn't hurt the brain as much is an added bonus!

 

John.

 

 

PS.

 

Oh Dear I haven't flown a CTSW since last year. I forgot it’s a super cleanaircraft with no strut for reference. Back to the drawing board!

 

And the real experts will know if glide ratio is the ratio of horizontaldistance to vertical distance, or the ratio of flight path to verticaldistance. I can't remember.

 

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Would we get ourselves into trouble with the CTsw or CTLS if we use the following as a quick, conservative, easy-to-figure guideline? Take your altitude AGL and double it, e.g., 12000' agl gives you approximately 24 nm and 24 minutes glide...

10000' agl - approximately 20 nm and 20 minutes glide

8000' agl - approximately 16 nm and 16 minutes glide

6000' agl - approximately 12 nm and 12 minutes glide

4000' agl - approximately 8 nm and 8 minutes glide

2000' agl - approximately 4 nm and 4 minutes glide

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The best way to find out these numbers is actual field trials. The paperwork doesn't always work because of too many variables. (i.e. the human variable which can be huge, and weather) The 14:1 assumes to many theoretical things taking place all at one time and ideal conditions, but are not real field numbers. Why not have 20+ CT owners go out to these different altitudes and give it a try. Then reduce the real glide numbers by a safety factor and averages and then everyone would have some real time field tested glide numbers.

I did try once to glide from 7500' to 2400' which should have given me theoretically 13 miles, but I didn't even make 10 miles and would have been 1.5 miles short of the field I was trying to glide to. I was only at idle rpm, zero flaps and 65 knots. Too many variables so having some better conservative numbers would be helpful to all.

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About a year ago I went out to find the answer for my CTLS. Climbed to 5k' near an uncontrolled airfield and turned the engine OFF (not just idle) counted 1001,1002,1003 and then trimmed to 72kts. Repeated this procedure 3 times and found my CTLS glide ratio to be 8.5:1

 

So for conservatism I now use 1.5 times AGL as my rule of thumb. Chances are I'd do worse, not better, in a real life situation. Somewhere along each leg I try and remember to calculate that distance and keep it in my head. FWIW I also find myself tuning my 696 to NEAREST Airports page during long unfamiliar legs. Hopefully it'll make my decision making easier should i need to decide on trying for an airport, looking for a spot, or pulling the "little red handle".

 

My formula goes something like: (k' AGL x 1.5 = nm)

 

2k' AGL = 3.5nm

4k' AGL = 6.0nm

6k' AGL = 9.0nm

 

No guaranty expressed or implied, your mileage may vary, yada, yada.

 

Oh, and I try never to cross water at an altitude lower than that allows me to glide to shore from the point of no return using this formula.

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Hi Bill,

 

I would think those are pretty close to good real numbers because the 14:1 is theoretical and assumes everything including the pilot is perfect.

 

 

 

According to the NEW Flight Design CTLS POH AOI Pilot operating handbook Airplane Operators manual:

 

"Document Title Document No. Revision Date Page

Pilot’s Operating Handbook CTLS-LSA AF 0430 0017 00 12-Nov-11 5-17

Approval Ref.: Approved on the Basis of Manufacturer Self Declaration

5.2.4. Gliding Characteristics

The following chart shows the distances the aircraft can glide, dependent

upon altitude, assuming smooth air, no wind and no vertical air currents.

▲Warning: Thermal activity can stretch or shorten these

distances. Turbulence always leads to a reduction in

gliding distance. One should never expect favorable

conditions when estimating a possible gliding

distance!

 

Glide angle of the CTLS-LSA can be assumed in practice to be 8.5 to 1

with flaps 0°, and 7.9 to 1 with flaps -6°. With flaps further extended this

atio gets worse. One effect of moderately set flaps is to reduce the

minimum sink, but the speed at which the minimum sink is observed

educes faster. This results in a reduced possible gliding distance. Speeds

or best glide at flight mass can be assumed as follows:

 

Mass 400 kg

(880 lb)

Mass 500 kg

(1100 lb)

Mass 600 kg

(1320 lb)

Flaps -6°

60 kt

(111 km/h) IAS

67 kt

(124 km/h) IAS

73 kt

(136 km/h) IAS

Flaps 0°

58 kt

(107 km/h) IAS

65 kt

(120 km/h) IAS

71 kt

(132 km/h) IAS

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When I was in flight training we assumed a 7 or 8:1 glide ratio for a C172. A lot of the forums use a similar number. I find it hard to believe that the CT is almost the same per the POH. I'm not going to shut the engine down, but I will have to test my CTSW.

 

I've been over the center of Lake Michigan at 7000 ft, pulled the power and never had to apply it until I got into the pattern at KLDM with a 10 kt tail wind (about 18 nm). The 16 number may not be real, but I don't think it's 8 and change like the C172.

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Here was a quick test I did and posted awhile back with the engine off in our CTLS. The book I think is conservative which is a good way to go and a good way to plan on the safe side.

 

 

Ok, here is the first data point I captured today.

 

First the setup:

 

2009 CTLS, with myself and 1/2 tank of fuel, and a few pounds of stuff in the foot storage. That puts me about 1100lbs.

 

Starting pressure altitude 9000ft, temp 11c, density altitude 10571ft. Ending pressure altitude 7000ft, temp 16c, density altitude 8688ft.

 

Winds aloft light and variable, turbulence none.

 

Test 1:

Flaps 0 and maintaining the book glide speed from the CTSW of 63kias. Engine OFF, prop stopped.

 

At this setting I managed to glide 1.9nm every 1000ft of altitude lost so that calculates out to a glide ratio of 11.5 to 1. Average rate of descent was 645fpm.

 

Test 2:

Flaps -6 and maintaining the book glide speed from the CTLS of 78kias. Engine OFF, prop stopped.

 

At this setting I managed to glide 1.8nm every 1000ft lost giving a 10.9 to 1 ratio. The average rate of descent was 835fpm.

 

 

This is just one quick test, as I get time I will try to do some more longer glides at some lower altitudes and post those results.

On a side note, my engine on idle rpm at 63kias was about 2200 and that reduced the descent rate to about 600fpm so even at idle the engine is making a little thrust. With the engine off and at 78kias the prop/engine will flip over a blade occasionally. Not near enough to start but it does rotate once in awhile. Oh yeah, it gets pretty quiet ithe cockpit with the engine off!

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The best way to find out these numbers is actual field trials. The paperwork doesn't always work because of too many variables. (i.e. the human variable which can be huge, and weather) The 14:1 assumes to many theoretical things taking place all at one time and ideal conditions, but are not real field numbers. Why not have 20+ CT owners go out to these different altitudes and give it a try. Then reduce the real glide numbers by a safety factor and averages and then everyone would have some real time field tested glide numbers.

I did try once to glide from 7500' to 2400' which should have given me theoretically 13 miles, but I didn't even make 10 miles and would have been 1.5 miles short of the field I was trying to glide to. I was only at idle rpm, zero flaps and 65 knots. Too many variables so having some better conservative numbers would be helpful to all.

 

Any field trial data with a stopped prop?

 

Ernie

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My prop was also stopped. I averaged 8.5:1 with 3 tries. My best was 9:1. I had a glider CFI in the right seat and full fuel.

I thought there was less drag with prop stopped than windmilling. What happened to the advertised 14:1 ratio?

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

;1

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Hi Ernie,

 

Most aircraft glide ratios are theoretical and are based on many things being close to perfect. (i.e. weather, temps, altitudes, weight, pilot performance, ect...) So our 14:1 glide ratio while compared to other aircraft is quite good and much better than most it isn't field realistic. Now think about an aircraft that publishes an 8.5:1 ratio. It has to be less than that too.

We still have a very good glide ratio and it could be just what saves your bacon some day.

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Eric: Interesting trials. If the air density is 74% of sea level when you are at 10,000 feet DA, would your glide ratio have been 35% better, or near 14/1 at sea level? And how did you account for the wind? WF

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Eric: Interesting trials. If the air density is 74% of sea level when you are at 10,000 feet DA, would your glide ratio have been 35% better, or near 14/1 at sea level? And how did you account for the wind? WF

 

Less density means less lift and more ground speed resulting in the same glide ratio.

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Less density means less lift and more ground speed resulting in the same glide ratio.

 

Everything else being equal, the aircraft weight does not change the glide-ratio, but as weight increases, the best glide-speed increases.

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  • 3 weeks later...

my experiments from this weekend:

 

25C at 3500 feet, 0 flaps, with the power pulled I was getting 450-500 fpm decent rate at 63-64 kt indicated. The Dynon said this was 67kt true. Using 500 fpm, that's 6780 ft/m along the hypotenuse, a 4.2 degree glide angle. For every 1000 ft down, I would move 13,520 feet forward, ergo, a 13.5:1 glide ratio, which I think is pretty close.

 

Next I tried it at 68 indicated. At his point the ball really moved to the left as prop drag came into play. Surprisingly, I needed 700 fpm to keep the speed at 68 indicated. This represents (81 kt true) 7180 ft along the flight path for every 700 ft down, a 5.6 degree slope. For every 700 feet down, I would travel 7145 ft horizontally, a 10.2:1 glide ratio.

 

I was surprised to see this drop so quickly with only a few knots of extra airspeed. From now on I'll trim for 450-500 descent rate, not a specific airspeed.

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Afternoon All: Ah turning the CTLS into a glider as in seeing where the earth meets the CT glide path. I regularly fly 10-15 nm off shore when fliying from Pensacola to Louisiana (to see the the grandchildren or to Miami to sponge off relatives) and I have played around with glide distances since we got the CT in 2008. I don't think my results are too different than the ones in this forum. In about 15 or 20 descents from 8500 or 7500 with 0 flaps I simply pitched to 500'/min descent and it works out to about 72 to 64 kts depending on the winds. around 3000' I begin to slow the rate and maneuver to be in position to make 82J (Home), my experience is that if I am within 9 or10 nm of home (or an emergency site) I should make it just about every time. But having said that, it will take a bit of luck because as you start lining up on the R/WY sorting out a glide slope that will get you to the end of the r/wy; meaning a straight in. However, making a base turn presents a set of problems as distances and altitude for the base -- I find (probably stating the obvious) that trying to be a bit high and letting it settle in is best--of course at home I use the grass r/wy.

 

Between yesterday and today we have had over 15 inches of rain and the forecast is for more, so it will be a bit before I fly again. Admittedly, putzing around with glide distances is a hit or miss proposition as I have been doing it, so I will begin a bit more rigorous a program to see what is what. Right now if I were to say what I would do with a engine failure at 7500/8500': Zero degrees flaps; pitch for 70 Kts and 500' rate of descent, be within 7-8 nm of a suitable field or 3-4 mn from an off field site, fly straignt in, or make a 400' base turn within 1 nm . I would plan to be a bit high so I can slip if necessary--I think this is important as you can lose altitude, but you can.t gain it and I combine this with keeping the airspeed as fast as possible, meaning a bit of a margin above stall. So the discussion comes down to Distance, Altitude, and Airspeed.

 

See ya, Ken Nolde (just ticked over 500 hours in our CT)

 

 

 

 

 

 

 

 

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The gps396 map page can display data fields, turn that mode on.Then select glide ratio as one box (I use bottom left)and Glide to Target as bottom right. then punch up nearest airport, direct to , to set the target.Helps you see if you will make it

CTSW with 66k and prop idle, I get about 11:1 as glide ratio . sorry I wasn;t being test pilot recordingDA that spring day. 63k speed, 0 flaps. I didn't try 'quiet mode', aka engine stopped.

So we all can get that glide ratio with a bit of garmin help, but cross check altimeter and clock.

 

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