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Thread: Kit Plane Glide Ratios

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    Registered User rbrochey's Avatar
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    Kit Plane Glide Ratios

    One of the questions I always asked kit manufacturers was what the glide ratio was for their particular aircraft. Personally I like a nice glide ratio... most planes seemed to be 9 or 10 to 1, some higher, some lower with the SD-1 at 12:1. Does anyone else out there find the glide ratio as important as I do?

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    Registered User DarylP's Avatar
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    Re: Kit Plane Glide Ratios

    You bet...but with STOL (which is what I like) aircraft it seems that nobody wants to divulge that. That being said, they are not really designed to glide anyway. Still it would be nice to know. Say for the Zenith CH750 for example, which their site shows nothing. I think you are right as it would be nice to know how far I could glide before I impact mother earth.

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    Registered User djschwartz's Avatar
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    Re: Kit Plane Glide Ratios

    Aircraft optimized for efficient cruising at altitude tend to have the highest L/D. Aircraft designed for STOL operation tend to have relatively low L/D. These are natural outcomes of the design tradeoffs that must be made to best suit each flight regime. For STOL operation minimum sink rate is much more important that L/D. Minimum sink rate translates almost directly to lower stall speed. Overall these are relatively useless numbers for powered aircraft except for the emergency case of engine failure. STOL approaches are usually made with power unless a very steep approach is required for terrain or obstacle clearance.

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    Registered User DarylP's Avatar
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    Re: Kit Plane Glide Ratios

    Quote Originally Posted by djschwartz View Post
    Overall these are relatively useless numbers for powered aircraft except for the emergency case of engine failure.
    Exactly....that's my point. I wish I knew what it was....just in case of that emergency. I mean before I buy the plane.

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    Moderator autoreply's Avatar
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    Re: Kit Plane Glide Ratios

    Quote Originally Posted by djschwartz View Post
    Aircraft optimized for efficient cruising at altitude tend to have the highest L/D. Aircraft designed for STOL operation tend to have relatively low L/D. These are natural outcomes of the design tradeoffs that must be made to best suit each flight regime. For STOL operation minimum sink rate is much more important that L/D. Minimum sink rate translates almost directly to lower stall speed.
    Well, actually L/D is even more important. With a constant L/D, sink rate is proportional to the speed. With a constant wing loading (stall speed), sink rate varies steeper as the L/D does.
    Usually the reason STOL-aircraft have low aspect ratios is because the aircraft is more compact and the wing is lighter. That's more of an issue as minimum sink.

    Penetration is by the way as important as L/D. If your plane glides just fine at 40 kts, but you have a headwind of 20 kts, you have to make two turns, one to go downwind and one to turn into the wind to land. Glide at 60 kts gives you much more range, 80 kts is even better, though it reduces your time to think.
    Especially the European ultralight glide excellently, but have the penetration of a parachute.

    @ DarylP, the CAFE400 site has many of them, a bit of googling will find you the rest.

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    Registered User djschwartz's Avatar
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    Re: Kit Plane Glide Ratios

    Quote Originally Posted by autoreply View Post
    Usually the reason STOL-aircraft have low aspect ratios is because the aircraft is more compact and the wing is lighter. That's more of an issue as minimum sink.
    All the real STOL aircraft I know of have relatively high aspect ratio wings for a power plane. Pilatus Porter, Twin Otter, Beaver, Helio Courrier, even the old beast, the AN-2. Serious STOL pilots often add wing tip extensions to their Cessna 180's for both more area and higher aspect ratio at the same time. The lowest aspect power craft, such as the Grumman Yankee and early Hershey bar winged Cherokees, are generally regarded as poor performers in the STOL arena; though, aspect ration is only one small factor in that difference.

    One of the most critical specs for a STOL plan is obstacle clearance distance. On takeoff that includes distance for acceleration to lift off speed, lift off, acceleration to climb speed, and finally climb. The actual straight ahead stable climb is a relatively small part of the picture. Add to this that STOL is often combined with rough field conditions in the real world and the need to get in the air at a slow speed quickly dominates the equation over how efficiently you get in the air.

    On approach there are similar issues. A typical STOL approach is a steep descent followed by flare and roll-out. Again, minimum energy is good. Flaps are used not only to reduce speed but also to add drag to degrade L/D to get that steeper descent. STOL planes don't use spoilers the way a glider does because of that need to maintain as much lift as possible at the same time they're increasing drag to steepen the approach.
    Last edited by djschwartz; October 19th, 2010 at 08:53 AM.

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    Registered User djschwartz's Avatar
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    Re: Kit Plane Glide Ratios

    Quote Originally Posted by DarylP View Post
    Exactly....that's my point. I wish I knew what it was....just in case of that emergency. I mean before I buy the plane.
    In an emergency there are three characteristics to be concerned about. One is glide ratio, which is L/D, and which tells you how far you can go from a given altitude without power. The others are minimum sink rate and stall speed, which tell you how hard you're going to hit once you've run out of gliding distance. Of these, the latter have much more to contribute to engine out survivability. Unless you're cruising at fairly high altitudes, the difference in glide distance between an L/D of 9 and one of 12 isn't going to make a lot of difference in the choices you have available. Assuming you can find any kind of field to land in you'll have to use the last few hundred feet or so of whatever altitude you have to maneuver to land. If you start out at only a couple of thousand feet AGL that doesn't leave a lot of gliding distance either way. The lower you're arrival energy the broader your choices of "acceptable" landing sites. And if there's no good field within reach, then glide distance doesn't matter at all.

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    Moderator autoreply's Avatar
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    Re: Kit Plane Glide Ratios

    Quote Originally Posted by djschwartz View Post
    All the real STOL aircraft I know of have relatively high aspect ratio wings for a power plane. Pilatus Porter, Twin Otter, Beaver, Helio Courrier, even the old beast, the AN-2. Serious STOL pilots often add wing tip extensions to their Cessna 180's for both more area and higher aspect ratio at the same time. The lowest aspect power craft, such as the Grumman Yankee and early Hershey bar winged Cherokees, are generally regarded as poor performers in the STOL arena; though, aspect ration is only one small factor in that difference.
    I checked it before posting and thinking of our typical STOL and fast glass aircraft I came with:
    Lancair IVP/Legacy: 13
    Glasair III short/long tips: 6.7 /7.1
    Questair Venture: 10.5
    Cirrus SR22: 10.3
    Super Cub 6.9
    C 172: 7.3
    Zenith CH 801: 3.6

    So it looks like the typical stol aircraft doesn't care as much about aspect ratio as faster aircraft do.

    Ow, and wikipedia has a great list of STOL-aircraft and their performance:
    http://en.wikipedia.org/wiki/List_of_STOL_aircraft

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    Registered User djschwartz's Avatar
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    Re: Kit Plane Glide Ratios

    Dehavilland Beaver: 9.2
    Twin Otter: 10.1
    Caribou: 10.0
    Pilatus Porter: 8.8
    Quest Kodiak: 8.4
    Britten Norman Islander: 7.4
    8GCBC Scout: 7.3
    Westland Lysander: 9.6
    Short Skyvan: 11.1
    PAC 750 XL: 6.6
    Dornier Do 27: 7.4
    Antonov An-2: 8.3 (approx., each wing)
    Kitfox: 7.8
    Sportsman 2+2: 9.1

    All at least average or higher aspect ratios for powered aircraft. The Zenith 801 seems to be an unusual stand-out with its exceptionally low aspect ratio. Time will tell whether this works out well or not.

    And yes, you'll see the highest aspect ratios in aircraft designed for efficient high altitude cruising. These include the Lancairs, Questairs, Bonanza's, Piper Malibu, etc. For example: the Boeing 787-8 has an AR of 11.2

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    Re: Kit Plane Glide Ratios

    It is low span loading that one should be comparing not aspect ratio(AR).

    The key to to STOL aircraft is low min power to stay afloat so that they have a lot of excess power to attain a high climb rate. Min power to fly is proportional to spanloading^1.5. Hence STOL aitplanes need low span loading:

    super cub 42lbs/ft
    801 70lbs/ft
    Lancair 86lbs/ft
    Cirrus SR22 88lbs/ft

    The AR of STOL will be low because in order to exploit the low min power they have to fly relatively slowely and so will have more wing area for a given span. The slow speed of min sink will also give a high climb angle for a given climb rate.

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    Moderator autoreply's Avatar
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    Re: Kit Plane Glide Ratios

    Quote Originally Posted by ragflyer View Post
    It is low span loading that one should be comparing not aspect ratio(AR).

    The key to to STOL aircraft is low min power to stay afloat so that they have a lot of excess power to attain a high climb rate. Min power to fly is proportional to spanloading^1.5. Hence STOL aitplanes need low span loading:
    Span loading is a pretty useless number, unless you know the power and weight as well. Even then, it's a pretty useless number since it's not dimensionless, or can be scaled up/down to compare it to different aircraft.

    It also doesn't take into account the majority of the aircraft's drag at climb (it only uses induced drag). Aspect ratio, wing loading and power density are much more useful numbers to compare different aircraft, or to design for.

    Just look at your numbers. The super cub has half the span loading of the Lancair. No surprise since the Lancair is about 3 times as heavy. In fact, if you compare it to a STOL-craft of similar weight (the 801) which is otherwise vastly different, you also see similar numbers for span loading.
    In fact, this is true for 99.999% of GA. All have a span that's close to the other values (25-35 ft) and weights vary between 900 lbs (Kitfox) and 2 tonnes. Span loading is really nothing more than the weight of the aircraft, at least in real-world comparisons.

    This is like looking at the power per cubic inch of a racing car. Interesting, but pretty useless if you don't know the cars weight, or displacement and you're interested in the acceleration...

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    Registered User rbrochey's Avatar
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    Re: Kit Plane Glide Ratios

    What great feedback on this! Thanks all, and djschwartz, what is your avatar plane? I haven't seen one of those before... very nice design. My flight instructor once told me the 172 we were flying in had a ratio of about 9:1, then he pointed to a clearing in the mountain pass and as he cut the power, said, "what do think, will we make it?"

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    Registered User DarylP's Avatar
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    Thumbs up Re: Kit Plane Glide Ratios

    Quote Originally Posted by rbrochey View Post
    What great feedback on this! Thanks all, and djschwartz, what is your avatar plane? I haven't seen one of those before... very nice design. My flight instructor once told me the 172 we were flying in had a ratio of about 9:1, then he pointed to a clearing in the mountain pass and as he cut the power, said, "what do think, will we make it?"
    I agree...great information from everyone! When your flight instructor said that, it reminds me of when I was flying ultralights. The guy that instructed me always said, "Always keep it in your head, that at any given moment if you had a engine out, where you can land for the plane you are in?" That I guess is my point in my posts. In the quicksilver we were flying, he had a basic rule. I am not sure if I got this completely right, but he would hold his arm out at a particular angle to the ground and say that if you draw an imaginary circle on the ground at the point you loose power, it should represent an approximate area that you should be able to land in. The higher you are...the bigger the circle. He was trying to impress upon me that I should always be prepared. (There are gauges that will calculate this...right?) He was about a thousand feet over the runway, and then shut off the engine. He said that the glide distance, in this case, was irrelevant as he knew he would land on the runway. It was my first time up and I was a little shaken when he cut the power, but he just spiraled to the ground and touched down with no problem at all. Very cool.

    I know the Zenith CH750 is a rock. And as you have all pointed out, it is made for a single purpose, that being taking off and landing in a small area. I guess that you just buy and build it with that in mind and then in training practice those dead-stick landings. That real experience should give me a feel what the plane can do. Of course I will not go out to buy a STOL plane based on glide ratio, but rather its capabilities as a STOL aircraft.

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    Registered User djschwartz's Avatar
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    Re: Kit Plane Glide Ratios

    Quote Originally Posted by rbrochey View Post
    What great feedback on this! Thanks all, and djschwartz, what is your avatar plane? I haven't seen one of those before... very nice design.
    You've probably seen lots of its descendants. It's a Stephen's Acro, the progenitor of almost all of the current hot rod aerobatic monoplanes. I've owned this one since 1981. It was designed in the late 60's by Clayton Stephens for Margaret Ritchie who wanted a visibly different alternative to the Pitts Special that had already come to dominate aerobatic flying here in the US. Leo Loudenslager was the first to make a real name for himself flying the Stephens. He also kept modifying his and after installing a symmetrical airfoil wing he began calling it a "Laser". The Laser/Z-200 is still available as a kit. The Zivko Edge is a derivative of the Stephens as is the Extra series. There have been many others along the way.

    Oh, and FYI, power off at 80 MPH on final it has the glide ratio of a brick! And it loves to snap roll at the slightest invitation so although it has a phenomenal climb rate and angle, it is most definitely NOT a STOL plane!

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    Registered User Kristoffon's Avatar
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    Re: Kit Plane Glide Ratios

    For comparison does anybody have any idea what's the glide ratio for an helicopter in auto-rotation?

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