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Thread: 24 FPS Gust and Thermal

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    Registered User proppastie's Avatar
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    24 FPS Gust and Thermal

    I see by another thread (http://www.homebuiltairplanes.com/fo...hlight=thermal) that a 3000 ft/min thermal is not unusual. Simple question, if 24 FPS gust is the design spec. of a high performance glider....24x60=1440 ft/min. So if I hit one of these thermals at Vg (design gliding speed) will my tail/wings fall off, or is there more to those numbers than I currently understand.

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    Re: 24 FPS Gust and Thermal

    That 3,000 fpm thermal is surrounded by rising air. The core may be 3,000 fpm, but you must first transition the slower rising air which starts you climbing. As you transition those surrounding layers, you shouldn't cross any boundaries that are marked with a 3000 fpm (or even 1440 fpm) difference across that boundary.

    Sorry, poorly worded, but you get the idea.

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    Registered User Matt G.'s Avatar
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    Re: 24 FPS Gust and Thermal

    A 3000fpm thermal in NJ would be highly unusual, from what I have read about soaring conditions in the northeast.

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    Moderator Topaz's Avatar
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    Re: 24 FPS Gust and Thermal

    First off, BMCJ's post is the key point - on entering a thermal, there is never a "sharp edged" gust where a glider transitions "instantly" from 0 fpm rising air to 3200 fpm rising air. The width of the transition depends on the radius of the thermal, for the most part.

    But where are you getting 24 fps as a gust spec for sailplanes? EASA CS-22, which is the basis for both the EU and USA glider certification standards, specifies gust loading requirements in 22.341. The specification is not a flat gust value, but rather a surprisingly complex formula based on ten variables describing the sailplane and the gust itself. Gust velocities at various points in the flight envelope are specified in the associated Figure 2. The maximum gust velocity specified is 15 m/s, which works out to just over 49 fps. So 24 fps is woefully inadequate, at least in terms of EASA CS-22.

    Gliders also have g-loading limit specifications in the applicable certification rules. EASA CS-22 specifies "Utility" and "Aerobatic" minimum limit loadings (there is no "Normal" category), where the lowest allowable limit g-loadings are +5.33, -2.65. Gliders are tough, because we deliberately fly in what the rest of the pilot population calls "turbulance". The g-loading limit specification must also be met as well as the gust loading loading.

    You can find EASA CS-22 here: http://www.easa.europa.eu/certificat...red-sailplanes

    The latest version at the time I'm writing this is "Amendment 2".
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    Registered User proppastie's Avatar
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    Re: 24 FPS Gust and Thermal

    Quote Originally Posted by Topaz View Post
    But where are you getting 24 fps as a gust spec for sailplanes?
    Faa Glider Criteria Page 5

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    Re: 24 FPS Gust and Thermal

    Quote Originally Posted by proppastie View Post
    Faa Glider Criteria Page 5
    I have a copy. While that book is an interesting read, it's terribly, woefully out of date. I'm not saying it's useless - there's a lot of good knowledge in there, especially if you're designing a traditional wood sailplane or similar - but I wouldn't use it for design criteria. My edition is dated 1962, and we've learned a lot in the intervening decades.

    IMHO, download CS-22 from that link I provided, and go with that for design criteria. It's not only much more up-to-date, it's much more comprehensive, too.
    "If you have built castles in the air, your work need not be lost; that is where they should be. Now put the foundations under them." - Henry David Thoreau

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    Re: 24 FPS Gust and Thermal

    What's strong enough must be a subject of controversy given the range of designs out there. Carbon Dragon diagram shows 20 fps, till today did not give it a though

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    Re: 24 FPS Gust and Thermal

    No controversy. The Carbon Dragon was designed for very smooth, weak-thermal days, and takes advantage of that to reduce weight. Also, it was designed without any thought at all towards certification. It's just "some guy's" ultralight sailplane design. Maupin was extremely talented, and may well have set up his own set of design criteria tailored to his intended mission for the aircraft. But the 1980's was also a time where there was a lot of "eyeball engineering" going on, which ultimately doomed the majority of the ultralight industry. So he may not have done so, either.

    So there's really no controversy today. The CS-22 standards are what the industry and government have agreed are satisfactory standards for general use sailplanes used in "average" conditions, either Utility or Aerobatic categories. The Carbon Dragon and the less-sophisticated "airchair" designs may, or may not, be designed to those standards. CS-22 may, indeed, be "overkill" for something intended only to be used on very light, smooth days. Or, it may not. It's a decision only you can make for your interpretation of the design.

    This is one of the things that make the "simple" reengineering of an existing design in another material more difficult, in some ways, than designing a new airplane from scratch. You're working without knowing what the original designer's criteria and intentions might have been. The strength of the original design's parts can often be reverse-engineered from a good set of plans and specifications for the aircraft, but that's pretty daunting in and of itself, and you still don't know what the original designer's assumptions and decisions were about how he thought the loads developed and were reacted by the structure. Starting from scratch, you know what those assumptions and decisions were, because you're the one that made them in the first place.
    "If you have built castles in the air, your work need not be lost; that is where they should be. Now put the foundations under them." - Henry David Thoreau

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    Re: 24 FPS Gust and Thermal

    As those that know me here ... I am not afraid to admit my ignorance,,,,its a hobby, not a contest or business. ....So down to cases: here is the VN diagram for the carbon dragon along with the C-22 standard. I notice the 7.5 m/sec spec. which is the same as 24 ft/sec, but not sure if we are talking the same spec. Yes it says 20 ft/sec. but certainly not 100 % off if it is the same spec. Also shown VN FAA Glider Criteria.


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    Re: 24 FPS Gust and Thermal

    Note that the 7.5 m/s gust velocity is for Vmax, and the 15 m/s gust velocity is for "VB", which (I haven't looked and don't remember) is probably the same as our "VA", or maneuvering airspeed. I'm sure it's defined in there somewhere.

    Proppastie, your willingness to learn and to declare that you want to learn is really refreshing here on HBA. You're not alone, but most of the people (especially new members) seem to take the attitude that they're right, and anyone saying otherwise is just insulting them. You're the kind of member that gets the most out of this place. Doing exactly that is why I decided to do my DS54 project "in public": in hopes that better heads than mine will correct my mistakes as I make them! Glad you're here, and I enjoy reading about your project! Since structures is an area where I have a lot to learn myself, I've particularly been interested in your recent review of structural concepts, after your recent test. I don't comment because I don't feel qualified to do so.
    "If you have built castles in the air, your work need not be lost; that is where they should be. Now put the foundations under them." - Henry David Thoreau

    Design Project: Conceptual Design of an "Inexpensive" Single-Seat Motorglider
    Discussion Thread for the Project: Discussion: Conceptual Design of an "Inexpensive" Single-Seat Motorglider

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    Registered User John.Roo's Avatar
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    Re: 24 FPS Gust and Thermal

    Hello!
    Very good source of informations is this book...
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    http://www.cumulus-soaring.com/books...laneDesign.htm
    Best regards!
    Martin

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    Registered User proppastie's Avatar
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    Re: 24 FPS Gust and Thermal

    Design of a glider is beyond me, I can do design, make drawings, been doing drawings for 42 years. I am trying to learn aircraft stress. My focus is to maintain focus and limit the scope of the design in order to not be overwhelmed. Total aircraft design is a huge subject of immense complexity. Aircraft stress is a huge subject of immense complexity. It certainly is necessary for this project for me to understand some aerodynamics and aircraft loading, and these subjects are not easy either. I think as an example just the simple VN diagram is not so simple, connect the dots draw lines......but what does that really mean in terms of loading and stress,......no excuse at this end, I had not realized this thread would go in this direction. Is the FAA glider Criteria/Carbon dragon spec. that far off from CS-22 utility spec?, if my glider is properly stressed designed to the FAA document is it safe at the speeds it is designed to fly at? I use the FAA document because of the easy to use charts in it.

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    Moderator Topaz's Avatar
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    Re: 24 FPS Gust and Thermal

    Quote Originally Posted by proppastie View Post
    Design of a glider is beyond me,
    No it's not, at least not aerodynamically. It's easier than the structural design work you've already undertaken, and that would be no different for an original design. I'm not trying to get you to change your project to an original design, but the aero part really is pretty straightforward, especially for a very light unpowered design like this.

    Quote Originally Posted by proppastie View Post
    ... Is the FAA glider Criteria/Carbon dragon spec. that far off from CS-22 utility spec?, if my glider is properly stressed designed to the FAA document is it safe at the speeds it is designed to fly at? I use the FAA document because of the easy to use charts in it.
    Not surprisingly, that's not a simple question. My personal answer, within my avowed lack of skill with structures, is that I would use CS-22 as a baseline, rather than the FAA Glider Handbook. The latter is geared towards the sailplanes of its day - my edition is dated 1962 - and that means, largely, plywood structures with plywood skins. CS-22 is a more-general set of certification criteria, and benefits from over 50 years of additional experience in the design and operation of sailplanes.

    Is CS-22 "that" different than the Carbon Dragon's design criteria? Well, what were those criteria? You have to remember that the Carbon Dragon was designed towards the end of the heyday of the ultralights. Many, if not most, ultralight and hang-glider designs of the period were not designed to any "standardized" criteria at all. The designer just chose a "g" loading and went from there. Gusts were often not taken into account, nor flutter, nor ground loads... It was really the Wild West of aircraft design. Which is why a lot of the designs failed, and a lot of people died. Now, all that said, the Carbon Dragon is, I'm told, quite a nice bit of engineering and weight optimization, which latter is one of the things that makes it such a complex build. So far as I've ever heard, the airplane has a good reputation for integrity and flight characteristics, so it seems Jim Maupin did a good job when he designed the airplane.

    The big question is, to what set of criteria did he do the job? If he set up his own, then going with CS-22 or the FAA Glider Handbook may be overkill and your all-aluminum version may end up overweight. Or, if he felt the airplane would get "bounced around" a lot because of its very low wing-loading, he may have opted for criteria in excess of those standards, and they may not be enough.

    It's very hard to say. While I fly sailplanes, I haven't yet had the opportunity to fly a micro-lift sailplane or an "airchair", and so I don't know what appropriate design criteria might be. My guess is that something like the Carbon Dragon is intended mostly for weak-thermal days, with very little wind. Not really intended much for ridge soaring, unless it's a gentle day. I can't imagine soaring an airplane with such a low wing loading on a really booming or windy day. I think I'd get beaten to a pulp inside the cockpit. With that in mind, if I were attempting what you are doing, I'd probably take CS-22 as a starting point, and then very conservatively adjust some criteria downward, so that the airplane can be made rather lighter. But choosing good and safe new values should be done with a firm grasp of the actual conditions where the airplane will be flown. For example, the CS-22 positive limit g-loading is +5.3g for the Utility category. For an airplane designed solely for "weak" days, that might be overkill, and something like 4g might be more appropriate. But I'd be looking for some way to verify that number before I rely upon it. Is 4g really enough? If you can't find an answer, stick with CS-22, even at a weight penalty. Of course, I'm notoriously conservative in this regard.

    So again, this all comes down to the fact that you don't know Jim Maupin's design criteria for the structure you're redesigning. And, most unfortunately, you never will since he's no longer with us. (Take the following with a very large grain of salt, and hope that someone with more structural design experience either corrects or affirms my thinking.) My best advice is to try reverse-engineering some of the parts (in the original carbon) to see what loads they were designed to withstand at failure. There will be scatter in the results, but after several parts from various portions of the airframe, that scatter should develop to be clustered around a single ultimate g-loading limit. From there, you'll have to try and deduce the margin of safety and therefore limit loading. Holding that information, you can then redesign the structure in aluminum, like you want.

    You have to figure out Maupin's original loads values for each part of the structure, first by reverse-engineering the parts and then cross-checking back the other way from the aerodynamics. It's going to be a lot of hard work. In my personal opinion, it's more work and harder than simply starting a new design from scratch but that's also an individual judgement call. I'm really looking forward to seeing your project continue to move forward. It's going to be great to see it fly one day!
    Last edited by Topaz; January 12th, 2017 at 01:52 PM.
    "If you have built castles in the air, your work need not be lost; that is where they should be. Now put the foundations under them." - Henry David Thoreau

    Design Project: Conceptual Design of an "Inexpensive" Single-Seat Motorglider
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    Re: 24 FPS Gust and Thermal

    As usual I have miss-stated what I wanted to say. Sorry for the confusion....Your answer has pointed out twice to me that mistake, ....words are so difficult. Comparing the FAA Glider Criteria High performance design spec. to CS-22 utility.... are they similar, close, is the FAA spec a safe spec, is the CS-22 utility spec. safe? My goal is to design to a recognized spec. and take a weight penalty if I have to. As you say I do not know what the original design spec. was and for all your stated reasons it is not an area of investigation. Original design studies I did on an aluminum structure showed me I think I can get reasonably close to the original weight using 5.33 G limit loads. My horizontal tail was very close the vertical tail off by less than 2 lb. The Boom is heavier, but I think the Pod will be way lighter. The wings look to be heavier. My hope is the weight savings in fuselage will make up for the engine, but even so that probably will only be 15 lb all up.

    You made it seem that there is a real problem with the FAA spec, and a very quick scan of CS-22 utility made me question that premise. Yes we have learn much since 1962. The spec.s for the CS-22 Aerobatic gliders are much higher than anything in FAA Criteria. Those European Gliders you fly are built tough, and are works of art.

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    Moderator Topaz's Avatar
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    Re: 24 FPS Gust and Thermal

    Okay, I see what you're saying. Let me ask a question, though: What is "safe"?

    The FAA Glider Handbook was state of the art in th 1960's. That's over 50 years ago. At the time, clearly, they considered it a good set of design criteria for a sailplane. Now it's 2017, and the FAA Glider Handbook criteria have been dropped in favor of CS-22. Myself, I would consider that a result of the last 50 years of development in sailplane design. Now, in 2017, the CS-22 criteria are considered a good set of design criteria for a sailplane. There's no problem with the FAA Glider book per se. But CS-22 has 50+ more years of experience behind it. To me, that counts for something.

    Are either of them "safe"? "Safe" is a word that gets bandied about a lot these days, but it is a word that has no absolute meaning, especially in an engineering sense. You can say that something is more or less "safe" than something otherwise comparable, but you can't ever say something is "safe" in an absolute sense.

    I, personally, would say that CS-22 is likely to be "safer" than the FAA Glider Handbook in a general sense. Whether or not one or the other is more applicable to your particular situation, I cannot say. Only you can determine that. Me, I would go with CS-22, simply because there's 50 more years experience and research behind it. That's all I can tell you. I'm no authority. I'm just another "guy on the internet," so take my words with that level of credibility.
    "If you have built castles in the air, your work need not be lost; that is where they should be. Now put the foundations under them." - Henry David Thoreau

    Design Project: Conceptual Design of an "Inexpensive" Single-Seat Motorglider
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