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Old 04-28-2010, 02:46 AM   #1
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Curious what the glide ratio is for a Pitts S1S. I found info on 747's, cessna's, wingsuits, and even the space shuttle but no Pitts.



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Old 04-28-2010, 03:22 AM   #2
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A wild guess would be similar to the space shuttle but at a slightly slower speed.



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Old 04-28-2010, 03:29 AM   #3
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Take a streamlined crowbar to a third floor balcony and drop it over the edge. This will approximate the glide ratio of a Pitts. I had an engine failure in my Pitts in 1993, just three miles west of Tract C-A 84 Mesa, an abandoned airstrip 25 miles east of Rangley, CO. I was cruising at 4,300 AGL when the crank gear drive pin on the back of the crankshaft sheared. I went from cruise power to no power in a nano-second. I was on the ground in sixty seconds. I held 100 MPH on the ASI till I flared, so that gives a forward travel distance of about 9,000 feet, and the vertical loss of 4,300 feet, so just over two-to-one. Granted, I was heavy, on my way home from an airshow in Aspen, with all my travel gear and airshow stuff with me, external tank, etc. I suggest you go up to a safe altitude, cut the power, and time your descent at a given speed. I'm sure a light Pitts will do much better than my "heavy" did.


Danny

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Old 04-28-2010, 03:33 AM   #4
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" I'm sure a light Pitts will do much better than my
"heavy" did."

You may have done better heaver. Wt. at alt. is energy.


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Old 04-28-2010, 04:31 AM   #5
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I'm told that the lexan floor panel is a good guide to knowing where you'll end up if you lose power :-)

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Old 04-28-2010, 05:25 AM   #6
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Glide distance heavy vs light is the same for a given airspeed its just your ROD thats higher.

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Old 04-28-2010, 11:53 AM   #7
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I carry a red brick labeled "ENGINE FAILURE" in my Pitts. If my motor quits, my plan is to throw the brick over the side, then follow it down.

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Old 04-28-2010, 12:05 PM   #8
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Dunno about the Pitts---but in our Stardusters---if you can see it over the nose---ya ain't gonna get there! ;-)

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Old 04-28-2010, 12:53 PM   #9
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Hi there,


akt227 your brick is a good idea but you should wear a helmet, 'caus you will be on the ground faster than the brick ... ;o)


I tried a simulated engine failure once with my Ultimate 10-200. Its glide ratio should be very similar to that of the Pitts S-1.


2000ft AGL, cut power, keep 100mph IAS, one full 360 and I was on the ground. So where the brick hits the ground you will too (Think about the helmet.).


After that I estimated the glide ratio with a rough (!)calculation to 5-to-1. This is about the same as a modern parachute has.


Concerning the influence of weight on glide ratio: The glide ratio curve shifts to higher ROD and higher speeds with increasing weight. So if you are heavier your best glide speed will be higher than when you are light.


That is the reason why gliders carry (lots) of water ballast in good thermal contitions. Then they can glide faster and make the same distance in a shorter time which means longer distances over the whole day. Thermaling they loose a bit of time (higher ROD) but if the weather is good enough the advantage in glide speed overcompensates this disadvantage.

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Old 04-30-2010, 12:12 AM   #10
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[QUOTE=akt227]I carry a red brick labeled "ENGINE FAILURE" in my Pitts.















Does that brick get a little wicked when you do your snap rolls?

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Old 04-30-2010, 01:52 AM   #11
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Snaps aren't too bad. Rollers are the worst. Sounds like the percussion line on a rap track.

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Old 02-29-2012, 09:37 PM   #12
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Quite a thread, red bricks and all.
Love it,
and I agree and have corrected a few others elsewhere,

Heavier changes nothing except the time it takes to feel the gentle bump
of your tires hitting the ground.
Figure out your glide ratio, from a manual on that plane or somewhere,
and yes, test it. Why not ?
Learn the angle, 5 to 1 is about 11.3 deg.
Double that angle for margins to account for turns and lining up to land.

L/D is always L/D. Change everything else
but until you change L or D your distance over the ground is the same.
Same holds for altitude / air density.

I believe it was Gertrude Stein who once said,
L over D, is L over D is L over D.

One book I have ('Cleared for Takeoff', C2002, from Cessna), the
only book I have seen that addresses this issue
uses a rule of thumb;
~ for each 10% below max gross weight,
~ reduce published best glide speed by 5%.

I do not know if this holds across all aircraft profiles and
I do not have a rule of thumb for increase in altitude or
decrease in air density where L/D still applies.

I did have a CFI ask me once what best glide speed is for the C172 as
we prepared for lesson flight on emergency procedures.

I told her the speed in the book, 56 kts.
She then asked, 'OK that is for gross weight, what about our weight' ?
I looked at her puzzled, "you have never told me a range nor is there one in the manual, so ?????"
Cya C








Edited by: IronmanFly78
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Old 02-29-2012, 10:15 PM   #13
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Quote:
Originally Posted by bf92
I held 100 MPH on the ASI till I flared, so that gives a forward travel distance of about 9,000 feet, and the vertical loss of 4,300 feet, so just over two-to-one.









Enough of this "glides likea wet brick" hyperbole.Now we can accurately say, "glides like a wet wingsuit".
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Old 02-29-2012, 10:30 PM   #14
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Glide is greatly influenced by what prop you have and what the prop is doing. Least drag is a fixed pitch,then the two blade Hartzell. The German props have huge amounts of drag. Three blade on S1S, VNE and 200' on 1/4 mile final, close the throttle and land on the 1000' mark. The S2B with Hartzell 2 blade, 120 abeam the numbers, close the throttle and turn at the same time, you will just barely make the runway in calm wind. S1S I almost always closed the throttle abeam the numbers and rarely added power unless the wind was high. The late John Lilberg lost the prop off a S2B. It left cleanly, no damage to cowl or the rest of the airplane. John told me it glided so well he almost overshot a 4000' runway. Its all about the prop, on the 1 or the 2, the prop arc covers a large amount of wing area. Take away the prop and the Pitts glides quite well.

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Old 02-29-2012, 10:39 PM   #15
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The effect of weight is very noticeable in a jet with high useful load. Max landing weight approximately 90 nm from 30k in calm winds. At very light weight closer to 60 nm.

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Old 03-01-2012, 03:54 AM   #16
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I agree with jrs. Weight at altitude is stored energy. If given a tight crossing restriction in a decent it is good if you note approx. wt. while counting on your fingers and toes.

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Old 03-03-2012, 08:27 AM   #17
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Interesting topic.
The glide distance of an aircraft does not vary with weight, assuming best glide speed is maintained. The "time" in the air will will vary, as weight simply affects the speed for best glide. But you'll still hit the same spot.
Weight may be stored energy, but you cannot "unstore" this energy unless you throw it overboard. Nor can you convert this energy to another form unless your engine is running and converts fuel to thrust. It weighs the same at 10,000ft as it does when you hit the ground. The stored energy is still in exactly the same place as it was originally.
Glide angle (or distance) is a simple function of lift divided by drag. To increase the glide angle (or distance), you must either increase your lift, or reduce your drag. There are no other ways around it I'm afraid!

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Old 03-03-2012, 01:37 PM   #18
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Brett, Are you willing to weigh in on this?

My opinion is not scientific but is based on 30+ years of dealing with crossing restrictions.
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Old 03-03-2012, 03:21 PM   #19
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I checked Wikipedia for Lift-to-drag ratio, and Gliding Flight. Interesting. It even shows the glide ratio for a flying squirrel...but not for a Pitts!


Danny

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Old 03-03-2012, 05:53 PM   #20
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It's a lot more fun to speculate about issues like this than to dig out a book and find the real answers. So, since it's a slow morning, he's my guess:

Induced drag depends on weight. If a plane had only induced drag, weight would make no difference to the gliding distance, as long as the best speed was used for the weight.

Parasite drag is independent of weight. It converts kinetic energy into heat. More weight = more potential energy to turn into kinetic energy and then into heat. If a plane had only parasite drag, a heavier plane would glide farther because it would take longer (at the same speed) to convert the greater potential energy into heat. If speed is adjusted for weight, I think the answer comes out the same.

At best glide speeds, light planes likely have a lot more induced than parasite drag, so the same distance argument is mostly correct. But, maybe, big jets glide fast enough that parasite drag is more important than induced drag, so they do glide farther when heavier.

Maybe.


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Old 03-03-2012, 10:07 PM   #21
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Best smelling 'spaining I've seen is that L/D is an angle of attack rather than a speed. So at the higher wts the speed of our average decent is closer to best lift over drag angle of attack (speed) than at lighter wts. If you put a crossing restriction 30,000 feet below current alt. in a flight management system at a light wt then put in one at a heavy wt. it will want you to start down 20 to 30 miles sooner at the heavy wt. We seldom have the luxury of letting the magic box pick the airspeed so this is almost always at an assigned airspeed. This issue is complicated in a jet by the fact that we are not actually gliding. There is residual thrust and that is a larger proportion of thrust to wt at the lighter wts. I paraphrased this so I may have messed up the translation but I think I got the gist of it right.

Sail planes carry water ballast to increase the best lift over drag speed so they can get through areas of down draft in less time.

I think I need to rest now.....


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Old 03-04-2012, 12:21 AM   #22
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All things are not equal when talking about jet aircraft descents.
They are conducted at idle thrust. This is not "gliding" as such. There is another force added into the equation which I suspect might be the culprit (though I admit I haven't thought about it too much).
The effect of idle thrust at very light weight is going to be quite different from the effect of idle thrust at very heavy weight. You can see this on a heavy jet in day-to-day operations. For example, at light weights it'll start moving easily after the breaks are released, without touching the thrust levers. At heavy weights it needs more (sometimes quite a bit more) than idle thrust to get going.
If you switch the engines off, you're back to simple lift versus drag, and equal glide distance at all weights (assuming best glide speed), like any other plane. I don't recommend anyone go out and experiment with this however.

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Old 03-04-2012, 01:49 AM   #23
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<
For example, at light weights it'll start moving easily after the breaks are released, without touching the thrust levers. At heavy weights it needs more (sometimes quite a bit more) than idle thrust to get going.

For the above example - Inertia. Newton's First Law...it goes something like, 'the velocity of a body will always be constant unless an outside force interferes'. In the above case, the parked plane is being interfered with by tires, ashphalt and who knows, crappy wheel bearings on one wheel.
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Old 03-04-2012, 07:42 AM   #24
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Quote:
Originally Posted by darylat8750
This issue is complicated in a jet by the fact that we are not actually gliding. There is residual thrust and that is a larger proportion of thrust to wt at the lighter wts.

But, doesn't this go the wrong way? Residual thrust is independent of aircraft weight, but as a proportion of total thrust (residual thrust plus the component of gravity aligned along the descending path) it is more significant at lighter weights. So, shouldn't it make lighter jets glide farther?

Now that you airline pilots have explained better what you were getting at, there doesn't seem to be much mystery left. I thought you were saying that at the best glide speed for the weight, a heavier jet will glide farther. As has been pointed out many times, that is contrary to all the standard teaching for light planes, which says that at the best glide speed (which is faster at heavier weights) the plane glides the same distance. So, I wondered whether the explanation might be related to the much higher glide speeds you use, which causes the standard teaching to break down.

But, you're actually describing flying at a fixed speed that's above the best glide speed and choosing a descent angle that will let you do it at idle thrust. This requires the second part of the standard teaching about gliding, which is that the farther you are from your best glide speed, the shorter the distance you will travel. And that leads directly into your explanation, darylat8750 . . .
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Old 03-06-2012, 10:37 AM   #25
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That was going to be my next point immediately after my last post!
Aside from residual thrust issues, a jet descent is also not at "best glide" (or min lift/drag) speed, but a constant speed. At some point and some weight it might be close to best glide speed, but the flipside is that it will diverge from that speed too.
I'm an airline pilot. I have never, ever thought that a jet glides further at heavy weight. If you maintain best glide speed (in a straight line), it will still hit the same spot at whatever weight you happen to be at. This is why we have a vague idea of the glide ratio (2:1, 3:1, whatever). And of course in real life this is simply a reasonable approximation of how far you'll go.
In a nutshell: it doesn't matter how much it weighs. The glide distance is determined by aerodynamics and nothing else.



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