Flight loads

[HankF]

I did a search of all the forums and got no response.

Is there any documentation in the field of model airplanes similar to (USA) FAR (Federal Aviation Regulations) Part 23 for the design of man carrying aircraft?

What I need are the flight loads one can expect model aircraft to experience during operation in order to design the lightest weight structure to handle such loads without failure.

Hank

[Usta Bee]

That depends on what you plan on doing with the plane, an indoor flyer would be built lighter than a gas powered stunt plane. What type of model are you trying to build ?. Also structural material has a huge effect on overall strength, have you ever seen AeroVironment's Helios in flight ?. It looks like it's gonna snap in half, but since it's built of composite material it can take that kind of abuse. Rutan's Voyager plane was the same way, that's why it broke a wingtip on takeoff. (Flexible Flyer)

[HankF]

Hasn't anybody ever measured flight loads (like putting a g meter in the plane) and published the results?

FAR Part 23 talks about three categories of airplanes: Normal, Utility and Aerobatic. Each of these has its load limits, 3.8g for Normal, 4.4g for Utility and 6g for aerobatic. Loads are considered to be caused by maneuvering, gusts and landing.

Hank

[RyanPSU21]

Is this for a school project or contect or something like that?

You could probably take an estimate of the G's during a turn by taking the mass of the plane, taking a guess at the flight speed, do your sharpest rolled to knife edge 180 degree turn and time how long it takes. Assume your airspeed is constant through the turn and calculate a radius for the turn from the time it takes, then you have enough to get a rough estimate of the G's your pulling through the turn.

[Sparky Paul]

Flight loads are computable, and the structure needed for those loads can be determined.
There's no AMA version of the FARs, other than the AMA safety code.

[Sail 'n Soar]

What's the fastest you expect your plane to fly - you define the condition? Calculate the load at that speed, add a safety factor (1.5 in normal aircraft design) and design and build to that. Alternately, design to a lower speed and load and ALWAYS use your elevator gingerly - no max deflections above design speed. If you see the wings flex more than you fell confortable with, don't pull back that far.

Calculating max speed is another discussion. Programs like MotoCalc will give you a max straight and level speed, but this won't cover the speeds you may encounter in the way you fly, e..g., a shallow to moderately steep dive. To calculate max g's encountered, divide your expected max speed by your stall speed and square the result. For example, if your max expected speed is 60mph and your stall is 20mph, max g's will be 3^2 = 9 g's. If you want to include the 1.5 safety factor, that says to design to 13.5 g's - or restrict your control throws when going fast. In that since, for full scale aircraft they have both a do not exceed speed and a maneuvering speed speed, which is lower than the do not exceed speed and is the max speed where you can apply full control deflections (without potentially permanently bending something!!!)

[HankF]

Flight loads are computable, and the structure needed for those loads can be determined.

Appropriate flight load LIMITS are not computable but are a matter of experience.

Where do you suppose the FAR's came up with 3.8g's ,4.4g's and 6g's respectively for the limits in PART23? The idea is to have a safe structure so that it will not fail at loads lower than the limits. This implies that exceeding the limits in actual flight has a very low probability.

You can design a structure for any load but if you set the limit too high, the structure may not function at all. For example if the limit for an airplane is set too high and the structure to withstand those loads is solid steel, it probably won't fly at all.

Limits have to be reasonable and usually come from experience as I said before.

For model airplanes, safety may not be the criterion for setting limits. Of course we do have to worry about hitting somebody on the ground so there is always some level of safety concern. What should determine the load limit to design against is the question.

Hank

[RyanPSU21]

Out of curiosity. Why does dividing your max speed by stall speed and squaring give you an acceleration? The units would come out dimensionless for that calculation. What is the basis for that estimation?

[Usta Bee]

Eagle Tree Systems has an onboard G-meter with their Seagull wireless telemetry system if you're so inclined.

When was the last time you saw a model company or a scratch builder do destructive testing like the "big boys" like Boeing and others do on full scale aircraft, by bending the wing and measuring the amount of deflection till it breaks ?. I've never heard of it, and yet I've seen several stories on here where people's wings have broken in flight.

[HankF]

When was the last time you saw a model company or a scratch builder do destructive testing like the "big boys" like Boeing and others do on full scale aircraft, by bending the wing and measuring the amount of deflection till it breaks ?

Well, what would be the point? As modelers we don't have any limits like the Boeing's have.

How tight does a loop have to be at any given speed? Is a big loop as good as a small loop? For man carry, you have only to worry about how much force the pilot can stand to make a loop. How much can a spectator on the ground stand?

Still, it would be nice if there was some guidance that says: if you build an airplane strong enough to withstand the standard aerobatic patterns, it need not be any stronger (and thus heaver). We just don't have anything to work to.

Is a bare foam a reasonable substitute for the stick and film wing of a Todd's Attitude or must it be reinforced? Certainly, it will fly bare and maybe well enough too. Who can say? It would be nice to look in a book and see that a 3-D airplane has to be able to withstand 10 g's in order to do the tricks it is supposed to be capable of.

The fact that there are so few inflight failures in the modeling world leads me to believe that our models are overdesigned and, consequently, overweight.

Hank

[Usta Bee]

Deflection testing would give you a good relative idea of the strength differences between various building methods, and strength to weight ratio. For a given wing weight you could compare lets say balsa and monokote versus foam fiberglassed with Minwax Polycrylic. What would happen to ultimate tensil strength of the covering material if you substituted 1/2oz ripstop polyester instead of the monokote ?.

I have a silly question. If you in fact you did have a guideline that told you "Plane X" should be built to withstand 5 g's, just how would you know if the wing you were building was in fact capable of a 5 g loading ?.

[Sparky Paul]

Why the confronational attitude?
Models ARE overdesigned.. the one's that aren't, collapse.
Don't do it that way next time!
When structural components are known to work, use those.
If the desire to is to build the lightest possible plane which collapses -after- the last flight it needs to do, that can also be done, but few of us have that intention.
We want our planes to survive, so they're overbuilt, relative to what a precision design team would come up with.
I'll go for overbuilt and heavier than the minimum possible all the time!
And the one's I've built that failed, well, I do it better next time.
The "limits" for an SAE lifter are far less than a DS'er. Don't build an SAE plane to DS specs, or vice versa.
If you don't know what works, look at kits, plans in magazines.. watch other guy's stuff fail in the air, and find out why.
Then don't do it that way. :)

[mwraight]

Boeing and other aircraft manufacturers do, in fact, perform this kind of destructive testing. They bend the wings as far as then can until they break so they understand what kind of loads the wing can handle. Fuselages are also stressed by hydraulic rams. Those planes go through full destructive testing - just not in a real world environment. But you can believe that companies like Boeing know EXACTLY how many G's their aircraft can withstand without coming apart.

But I sure don't know of any model company doing this kind of testing. I'm sure that many of them do testing as they are building the aircraft - if your plane weighs 10 pounds and you want it to be able to handle 6G's then you know the wing needs to be able to support 60 pounds. You could do it just like they do those bridge building contests at schools -- put 60 pounds of weight in the center of the wing with the tips resting off the ground. If it breaks, try again! This is simplistic...but will work.

You'll definitely need to know what the all up weight of the plane is going to be and how fast you intend on flying and what types of manuevers you plan on doing. If you're just going to putt around with a light little plane then you don't need much. Look at the Slow Stick. If it did go much faster it would fold those little meat tray wings up!

[HankF]

Deflection testing would give you a good relative idea of the strength differences between various building methods

But what conclusion can you draw if one method fails at a lower load than the other method? Was the load the it failed at sufficient? Maybe, even though it failed at a lower load, it was strong enough. And that's my whole point, there's nothing to tell you except for endless experimenting which some of us don't find particularly entertaining. What is a sufficiently tight loop?

Why the confronational attitude?

I'm sorry, I don't mean to sound that way. Basically, I'm lazy and really don't want to build a lot of samples to "get it right". I'd rather be able to determine that before I start building. To me, computing is easier than building.

As a practical example, I wiped out my "Attitude" and I want to rebuild it. I have foam and a cutter. If I reproduce the wings in bare EPS, will they be strong enough? The only way I have to determine that right now is to actually build the wings and go fly it. But what flight testing should I do, maximum powered dive with maximum deflection pull-up? But is that really necessary for a 3D airplane? If not, what is a reasonable test?

Hank

[mwraight]

Hank -- then I'd suggest you look at previous designs and emulate them. There's really no reason to re-invent the wheel. Use the same size spar, the same number of ribs, etc. and you should be golden. If you don't want the trial and error then use someone elses expertise and have no worries.

[HankF]

Come on, guys. Help me here.

What is a good enough loop, diameter and airspeed?

Hank

[mwraight]

It all depends on what you want to accomplish. What you're asking is like asking someone to help you design a car without saying what you want to do with the car. Do you want a car that rides well, can go off road, gets good fuel economy, has lots of interior space? What is this plane for? What kind of flying are you going to do? What size plane do you want? How fast DO YOU want to fly? No one can tell you what a good airspeed is - that's for you to decide.

If you don't have in mind what you want to accomplish, it's very unlikely anyone is going to be able to help you. They'll tell you their preferences...and that'll be it. Throw us a bone and give us some more info and I'm sure you'll get your answers!

[Purdue Aero Man]

Just design the thing for 1E100000000000 G's and you shouldn't worry too much about failures.

[mwraight]

Hail,Hail to old Purdue...all hail to our old gold and black... Ummm, nice picture. Rather gets the point across - not a big Vintage1 fan? I agree 1E1000000000000 G's would definitely do the job.

It's hard to design anything without knowing what the primary goal is. It rather reminds me of a Dilbert cartoon...with the pointy haired boss giving him something to design and not having a clue what it's supposed to do. You've got to know your objectives before you can effectively design something.

Ever faithful, ever true, as we raise our song anew...

And to those of you non-Boilermakers, there are a few lyrics in here to the Purdue fight song.

[RyanPSU21]

I would be willing to bet money that no kit manufacturer out there is going out and destructively testing their planes statically on the ground. Who even knows how much load a given piece of balsa can actually take? Then you have differences in balsa weight and strength going from there. We all know about what works since plane construction is similar in material and guage for a given size plane. I'm sure none of us are going out and doing FEA models and testing static loading and trying to optimize the plane. There is so much error involved in FEA that you need to have a good feel for estimating loads to much sure it isn't feeding crap back at you.

For the most part our planes are more or less seat of the pants designs. You can always go about the method of trying to produce a plane that is very lightweight by building it, if it works okay, remove some material and fly again. If it still works okay remove some more material. Repeat, when it fails in flight add back the last bit of material you removed and you're there.

Current Boilermaker student by distance ed.

[Usta Bee]

OK, let's say we told you the plane has to be capable of 5 g loads, and be able to do a 6 foot diameter loop at 100mph ;) . Just how in the heck do you plan on taking this information and turning it into a finished wing ? :confused: . Do you have some sort of computer program that tells you how a 5 g wing is built ?. What if we told you you only need to build it to take 4.5 g's, what are you going to do differently ?. :rolleyes:

[Ollie]

Two (photo and DS speed) examples for long threads:
http://www.rcgroups.com/forums/showthread.php?t=191234
http://www.rcgroups.com/forums/showthread.php?t=101242

[Sparky Paul]

Come on, guys. Help me here.

What is a good enough loop, diameter and airspeed?

Hank
.
For an indoor flier in your living room... the height of the ceiling minus 3 inches, at 4 mph.
For an FAI Masters pattern plane, 750 feet, at 67 mph.
I've actually measured a large number of plane's airspeeds..
http://www.angelfire.com/indie/aerostuff/inflights.htm
The manuvering diameters depend not only on airspeed, but c.g./elevator deflection. There's a combination of c.g./elevator which will yield the smallest diameter loop. Increase either a tad and the plane will snap out.
You get to that point by flying, not design.
Another point to consider with a model vs full-scale is we use the airframe for the landing gear. All of it!
And expect the plane to be useable after an arrival.
Go sloping, and you'll find out all you need to know about this.

[Sparky Paul]

OK, let's say we told you the plane has to be capable of 5 g loads, and be able to do a 6 foot diameter loop at 100mph ;) . Just how in the heck do you plan on taking this information and turning it into a finished wing ? :confused: . Do you have some sort of computer program that tells you how a 5 g wing is built ?. What if we told you you only need to build it to take 4.5 g's, what are you going to do differently ?. :rolleyes:
.
Can't be done.. :)
The initial requirements compute to 112 g.
DSers at 225 mph are pulling about 17 to 20 g, flying in fairly small diameter loops, say 200 feet.
And these guys explode doing it!
Howsomever, once a wing is built, and its structural properties are proven for practical limits, these properties can be used to compute alternative structures whose physical attributes are known.

[10cent]

The problem with being able to "calculate" your way out of iterative model design is that you are working with non-linear materials. I feel your pain about saving time and money with a little brain work but there just isn't any easy way to calculate how a foam wing will behave under loading. Adding to the problem is that foam is not a homogenous material so any inconsistancies (ie. air pockets etc.) will create failure zones.

The long and the short of it is without expensive finite element analysis software you can't get a reliable exstimate of a foam wing's failure point...even with it it would be a challenge. Since you can't really approximate the failure point is there any reason to try to estimate your design loads?

[Ollie]

At 250 MPH, 263 foot diameter lap, lift coefficient 0.2, lap time 2.25 seconds and, G=32, the angle of bank is arccosine 1/32 = 88.21 degrees.

[HankF]

I suppose one way to come up with limit loads would be to have expert fliers in each category fly their planes with g meters installed. That's something the AMA could do since it sponsers so many different contests.

With that data, a designer would have some idea how strong his design for that type of flying needed to be.

BTW check out this poll: http://rcgroups.com/forums/showthread.php?t=284331

Hank

[raptor22]

Just to give an relative idea, and this is no "guide" as that is not possible, rc planes pull HUGE G's, since we can build a strong structure easier than big planes and don't have to worry about the pilot blacking out.

If you want a failsafe system, you need to use the CL and wing area to calculate max lift produced at your design airspeed. This can be used to calculate the G's you can be able to pull or the loads on the spars. Then you need to use wind shear, etc to add in even more strength (big issues in ds and 3d) plus the safety factor of 1.5 or so.

Gotta agree with everyone else; calculating these things for a basis is fine, but practical experience w=should be drawn upon as well.

--Alex

[HankF]

at your design airspeed

Where do I get this number? Can I choose infinity or is something less acceptable and to who?

Hank

[Sparky Paul]

Use infinity.
An inablity to take off at all due to infinitely high gross weight is a magnificent safety feature.
No fly, no crash.

[RyanPSU21]

When you find a way to make something fly at infinite speed let us know. There would be a great market for it. Since parasitic drag increases with something like the square of velocity it is going to have to be one slick craft. Since limit rules with reducing drag will probably put you at a near 0 surface area you will probably then have a stall speed somewhere one very small increment below infinite speed which would make it approximately infinite as well. Going from there you would probably need to make the thing have near infinite power to overcome the near infinite drag meaning you need to be nearly infinitely massless.

Even if you had FEA to use to do this. FEA works best where you build a prototype based on your FEA guess to test and then improve your FEA model. Then use your FEA to indicate how to optimize. I use FEA at work daily for destructive automotive testing and design. The success of the intial design is more based upon the engineers knowlege and intuition and knowing then it is on how well the FEA code works. Blind calculations without a decent understanding of the underlying physics involved and a good understanding of what you are trying to achieve are useless.

What is this plane for or what is it supposed to do? That might help us to give you any kind of useful suggestion.

[Purdue Aero Man]

I think the problem here is that HankF wants to design something but doesn't have a grasp of where to start. Not saying it's his fault, he just doesn't know where to start. The first thing you should do is decide what you want your airplane to do (trainer, sailplane, aerobatic, etc). Then make a guestimate on the airplane's weight by using its approximate size (look at other airplanes that are similar in layout and mission to yours). Figure out how fast you want your airplane to fly, then size your wing and pick your airfoil based on this. For the moment assume changes in Re are negligible. You're delving into a topic that can't be explained on a forum, rather required years of reading or a higher level education in the stuff, and even then, you're still gonna be ignorant to a lot of things with this stuff, I know I am.

It's not so much a hatred for Vintage1, just a disgust for the many poorly spelled, confrontational posts that lack "ly" on the adverbs.

[Sparky Paul]

...

It's not so much a hatred for Vintage1, just a disgust for the many poorly spelled, confrontational posts that lack "ly" on the adverbs.
.
Gratuitously insulting a respected contributor to the depth of knowledge on this forum in a thread where he hasn't posted at all is "blindsiding" at best, and chicken**** at worst.

[gkamysz]

I don't understand what you are trying to accomplish Hank. Models experience -10 to -20g stresses on landings the way many pilots land. Sure you could build an airframe based on the fact that will only see +/-5g in the air, but do you want to repair the model every other landing?

From my reading in aviation design all the requirements are based on the mission. This means you need to decide what the aircraft needs to do, then design it. Once you know what the requirements are you can do the math.

But realistically no, there is no set guideline for designing models. The cost for such research is tremendous. Most designers go by experience and trial and error. Those that can calculate structural designs to a tee are already degreed aerospace engineers for the most part.

Because there is no payload for models and very little in the way of systems and such, there is a huge advantage in models that virtually all of the structure is used to support the airframe and the loads imposed on it. You don't have to worry about payload falling through the cargo floor in your scale transport because the floor doesn't need to do anyhting except hold it's shape. So all models are very overbuilt compared to the loads experienced. As you go up in size there is less and less margin.

FAR has minimums. You'll see that current production aerobatic aircraft are rated to +/- 10g. The field now demands this kind of performance so the manufacturers came up with a design to do the job.

My opinion.

Hank, which aircraft deisgn books have you read? Most give a basic understanding of loads, and structures. many describe typical wing designs and what can be done to optimise them. Many also point out what the limits really are and why certain things can't or haven't been done. I suggest the book below.

http://www.aiaa.org/content.cfm?pageid=360&id=529

Greg

[Sail 'n Soar]

Out of curiosity. Why does dividing your max speed by stall speed and squaring give you an acceleration? The units would come out dimensionless for that calculation. What is the basis for that estimation?

First, in case there was some confusion, I was referring to the stall speed shile flying straight and level. Approaching stall the CL is at CL max. Flying straight and level, the plane is flying at 1 g, that is, the lift is just equal to the weight. Ignoring any change in CL max with increased speed/Reynolds number, the highest lift you can creat at any speed will be based on CL max and that speed squared. Thus, dividing the square of the max speed by the square of the stall speed will give the maximum load the wing will generate measured in g's.

Question, what if you limit mechanically the elevator throw? In that case the minimum flying speed will be higher. Use the flying speed at that trim limit in place of the stall speed, square the ratio of the two speeds and you will get a lower g limit to design to.

[vintage1]

One thing no one has mentoned is turbulence. I have had a wing simply snap on me in slow climbout when the model hit a bump.

Its probably true to say that apart from wings, landing lodas far exceed flying loads, so a fuselage that stays together on landing will meet all flying requirements - the excepitin being long sldnder sialplane fuselages.

Stiffness is also and issue.

The calculation of wing bending moment is not too hard, its probably the same as an evenly loaded centrally suported beam in reverse, as lift is dstributed over the entire wing area. but teh weight is mainly at the center.

Stressing for 2-3g is probably enough for light wind flying and modest aerobatics. Full size aerobatic planes can pull up to 13-15g, and are stressed to 20g or more.

Best stiffness is acheved with full depth I beam or even stressed skin D box type structures. Normally its necessary, but seldom done, to increase stiffness and strength towards te wing roots.

Asfar as teh fusealge goes, stiffness to prevent flutter is needed in the tail area, but herwise there is very little load except in landing.

I am gradually evolving structures that will connect the motor, pack, servos and receiver to the landing gear at high sterngth, and ot too much stiffness to reduce shock loads: This seems to give best results on rough field landings.

[Ollie]

Study Dr. Drela's design for structural specs.
http://www.charlesriverrc.org/articles/allegrolite2m/markdrela_allegrolite2m.htm
and info plus structural detail.
http://groups.yahoo.com/group/Allegro-Lite/?yguid=108420033

[HankF]

The impression I get is that model design is a chick-n-egg process. Maneuvering capability depends on the model's strength to weight ratio and the model's strength to weight ratio depends on what maneuvers are desired.

Upward and onward without limit forever.



Do you have some sort of computer program that tells you how a 5 g wing is built ?

Yes, I have and use Algor, but it goes more like this: propose a design and test it with Algor then refine it until it is capable of 5 g's.

Hank

[markdrela]

The impression I get is that model design is a chick-n-egg process.
Yep.


Maneuvering capability depends on the model's strength to weight ratio and the model's strength to weight ratio depends on what maneuvers are desired.
Yep.

For a aerobatic airplane, I would want to make sure that as long as it's flying below a certain "maneuver speed" Vm, it doesn't break no matter what the control input is. This Vm choice is arbitrary, but I would pick it to be well above the max speed expected during normal flying. Then you KNOW the airplane cannot break up during normal flying (assuming the structure was correctly designed to this Vm). You can still fly faster than Vm, but then you have to be careful with control inputs.

Anyway, the chosen Vm defines a maximum lift which the airplane must withstand, using the usual lift formula:
L_max = 0.5 rho Vm^2 S CL_max
and the corresponding g-level is
g_level = L_max / weight
I would assume CL_max = 1.2 for plain symmetric airfoils, and 1.6 for airfoils with elevator-driven camber flaps. This L_max or g_level can then be used to size the wing spar and other primary structure.

You can also compute L_max for the tail surfaces, and size their structure like the wing. The fuse must be designed for the tail loads and the g-loads on the heavy items like the battery and motor.

Using FEA for this is very time consuming and massive overkill IMO. Simple spar sizing formulas are far more effective.

[HankF]

This Vm choice is arbitrary

This is the root of my problem. Or like I said it how tight does a loop have to be. It turns out that tighter is better (for a given speed) in terms of "Wow" factor. That is what flying contests are made of. This year's winner will lose next year to the equally talented contestant who has stronger/lighter equipment next year because he will be able to put more "WoW" into his performance.

It seems that there is always a flier who can reach the limits of the model airplane since he doesn't fly in it. If he did, he'd be the limit rather than the machine. Is there a machine that is always ahead of the pilot?

Hank

[RyanPSU21]

A really large fast loop is infinitely more impressive to watch then one that is really small diameter. It's interesting to see a fun fly ship do very tight loops but no where near the wow factor of a big pattern ship doing a very large very fast loop.

With most sport planes out there you could go into a dive, build up a lot of speed and pull full elevator and do as tight a loop as the plane is capable of doing at that speed. Not sure where getting ahead of the pilot has anything to do with it. We can already use expo and dual rates to help make a plane that is overly sensitive on the controls a bit easier to fly.

You can probably make a plane that will fly faster and be more sensitve to the controls then you can keep up with. I know personally I have a very hard time with speed 400 size planes, especially small ducted fans. Give me a plane going the same speed that is much larger and I wouldn't have a problem with it. Make that plane go very very fast and give it a really fast roll rate and I'd have a problem with it. Each pilot has their own limit as to what they can control. For some people it is a trainer. For others it is a ducted fan going a 100mph.

If your trying to design a pattern plane to wow people at contests. A good place to start would be existing designs and see how you can improve upon it. What about current pattern planes would help a pilot score higher. You still haven't really said what the purpose of this plane you are trying to design is or why you want to make it.

[mwraight]

I'll agree with RyanPSU21, a big graceful loop looks a lot better than a tight 'n tiny loop. Ultimately the bigger loop looks more realistic and is much more impressive.

I'm not sure why anyone would want a plane that's stays ahead of the pilot. Those types of planes generally aren't any fun. You should always be ahead of the plane...otherwise you're just going to have to spend what could be good modeling money on new underwear. I want the plane to follow my lead and command - not decide what it wants to do and then let me react. That's a good way to lose a plane - and you stated that you don't want to be building a whole lot - I opt for the plane following your orders rather than the other way around.

As for the WOW factor...its not always the lighter, faster plane. Pilot skill has something to do with it...as well as putting together a program that looks good. I don't care what you can pack into a show -- if its just a jumble of crud and doesn't look like anything then what's the point? Quality over quantity still has its merits. And it's not always about winning - but enjoying what you're doing! That's a win in itself!

[Ollie]

It seems that there is always a flier who can reach the limits of the model airplane since he doesn't fly in it. If he did, he'd be the limit rather than the machine. Is ?

Hank

If there is a machine that is always ahead of the pilot, find a better pilot or more practise, gyro, etc. If the machine can limit a pilot, improve the designer's design, add flaps, manufactured quality, etc. Radio, sunglases limit, etc.?

[RyanPSU21]

There's always going to be planes that are ahead of the pilot. There will always be planes with short life spans as well because the pilot can't keep up. That story about all the WWII warbirds people have built as their first airplane and immediately crashed. Those planes were all ahead of the pilot. Practice and you will be able to work up to it. I'm sure somewhere out waiting to happen is some turbine powered plane capable of going who knows how many 100mph that no one is going to be able to keep up with regardless of what electronic's gyro's etc.. you can put in there.

A large graceful loop takes much more pilot skill then doing a really tight loop by just cranking the elevator to full travel and holding it. It looks much more impressive as well. Anyone can just pull the stick as hard as it will go. That takes no pilot skill at all.

[vintage1]

Oh yes. I agree SO much. Ive got planes that have to be flown all the way throuh maouvers to make them look effortless, and others where I simply back off the throttle going over the top of the loop and ease back gently for fear of snapping the wings.

And I've got planes that are too fast for my eyeseight and reflexes. Every time I fly them its a butterfly stomached knee wobbling nightmnare till I get them under control, and yes, pulling a 100ft loop in a 30" plane is very warbird like.

High speed snaps are I think the worst manouver for G stresses. Apart from crashes.

I have no idea what one needs to stress a plane to - but 20g ought to be reasonably safe.

[Sparky Paul]

Here's one that passed two stress tests..
the first when it crashed, the second just to do it..
required some bouncing to break the wing.

[HankF]

a big graceful loop looks a lot better than a tight 'n tiny loop. Ultimately the bigger loop looks more realistic and is much more impressive.

Ah, yes! But how do you quantify quality so you can design to it?

A common saying in "quality" programs in the corporation is: I may not be able to define quality, but I'll know it when I see it." Subsequently, the program fails.

Hank

[Ollie]

You begin by desiding what type of flight characteristics you want the plane to have. Then you try to quantify and prioritize those characteristics. This data then becomes a standard against which you can judge the virtue of the many design decisions to resolve conflicting objectives that are involved. You can start with the characteriatics of the model that the equipment came from. Calculate its performance and deside how you want to modify it for "improvement." The process goes something like this: analyze, modify, reanalyze, judge results and consider new modifications. With sufficient iterations, the process should converge on your objectives.

Nature evolves its designs by a slow process of natural selection. You can evolve your designs so very much faster by insiteful variations.

Equations analyze. Design decisions synthesize.

You can't bring any judgement in desiding to quantify and prioritize what type of flight characteristics?

[RyanPSU21]

If you go out and you can do the most perfect loop that your current plane will allow you to do. You just can't improve it any further by pilot skills. Then think about characteristic of the plane will help you do better. There is no perfect design or perfect plane for everyone or everything or we'd only have one plane flying at every field across the country. You should look at current pattern plane designs and see why they have evolved to what they are and understand why. Then you may have some idea of what to do to improve it. Your not likely to be successful just starting from scratch without doing a lot of research first.

[vintage1]

Just a few random bits of information to throw into the pot.

(i) Deflection is in no way an indication of final strength to failure. Think of a rubber band, which will deflect massively before snapping.

(iii) G forces. Someone mentioned that an RC plane can do 'hundreds of G'. I thought it might be instructive to consider the impact of a plane doing say 60mph and coming to a halt in say 12 inches, which is roughly what happened to one of my planes.

using X=1/2V^2/a, and solving for a we get 3872 fps/s or with 1g about 32 fps/s a mere 120g.

(I hope this isn't too confrontational here)

The plane did of course break...but the avionics including the pack survived.

Now consider pulling g's coming out of a dive....where acceleration is V^2/r where r is the radius of the pull out Or turn. I'll be a bit more brutal and say the plane is doing 120mph, and we expect to turn it in what - 30 ft radius? So its vertical at the top of your house ( if you have chimneys as big as mine) and pulls out in time to just miss the ground.

My. That's only 32g. Give or take. Assuming the wing doesn't stall.

At 60mph that 30ft radius loop or pull out is a mere 8g.

So no, we only get a little over 100g in a full lawndart experience.

(iii) Lets take a look at how many G its possible to pull.

Now my maths gives out on stall speed computations, but Motocalc is reasonably fair at estimating stall speed from wing loading, so I took a model I fly a bit - 30" span, 19oz AUW and increased its weight to 190oz. Equivalent to a 10g loading and guess what...that wing stalls at 60mph, which is just about its flat out estimated speed.

So, taking Motocalc on trust, I simply can't pull more than 10g in that plane even from flat out level flight, without it stalling and reducing the G loading.

Ok out of a dive at 80mph, I might manage 12g or perhaps a bit more.

This is consistent with some figures given over the tannoy at an air display that I attended where a small aerobatic plane was held to be stressed to 23g positive, and 17g negative and the pilot was taking it to +12 and -9g.

The basic fact is that unless you have HUGE velocity, the plane will simply stall before it can generate more. My little plane is operating at 165W/lb. Not very efficient watts - its only a can motor - but still pretty powerful, and it seems I can't push it much past 12g.

Even if you manage through gross abuse of the elevator to turn the plane broadside on to the direction of travel in a fully stalled state, I cant see it e.g. stopping from 120mph in 4ft, or whatever is needed to generate 100g.

(Interesting aside: That is the sort of speeds and distances a head on auto racing accident generates with a car going into a tyre wall. With modern protection, the drivers can often walk away from that. If well strapped in, its estimated that bones start to break and internal traumas happen up around 200g or more).

My tentative conclusion is that the wildest of planes needs only be stressed to a bit over 20g. The major point of structural failure is usually the wing roots, and its easy enough as the pictures posted earlier show, to test for that.

Flexing at that load is not a huge issue. But a very flexible structure will sffer from flutter and break up under far less G loading.

The other thing to be wary of - as I have found to my cost - is of heavy items like packs and motors tearing themsleves out of the structure during high g manouvers. The point loading is very high especially if you have e.g. outrunner motors mounted as cantilevers. Its worthwhile sticking the equivalent of 20 motors worth of weight on the motor casing to make sure it doesn't fall out. and ditto the pack, especially when inverted. DAMHIKT :D

One final point, its an unfair test to load a wing with weight at the center, and support it at the tips. Ideally it should be evenly supported by something elastic along the whole span, but the bending moment at the center will be the same if its just supported halfway from span to center. Or is it .7071? Not sure. Too long ago. Anyway if the wing structure will take 20 times the estimated all up weight of the plane when supported about halfway along each wing, its probably good enough for all except the most hair rasing DSS type machines.

YOU can calculate if you like, but as far as I am concerned, thats down to knocking up a simple box girder structure, - spars only really - and testing it to failure, and adding structure till its good enough. An I beam with vertical grain shear webbing, or a D box stressed skin structure, is about as good as it gets for built up structures. With stressed skin over foam, watch out for the foam buckling: It may be worth inserting vertical grain shear webbing into the foam before skinning. Or carbon fibre strips for choice.

[Sail 'n Soar]

Now my maths gives out on stall speed computations, but Motocalc is reasonably fair at estimating stall speed from wing loading, so I took a model I fly a bit - 30" span, 19oz AUW and increased its weight to 190oz. Equivalent to a 10g loading and guess what...that wing stalls at 60mph, which is just about its flat out estimated speed.

OK, for those who followed my previous post relative to calculating g loads based on stall and flight speeds, here goes. (For those who wish to argue, please ignore this post.)

If MotoCalc gives the stall speed of 60 mph for Vantage1's ship ballasted to 190oz AUW, then the stall speed at the 19 oz AUW would be 60/sqrt(190/19), or 19 mph (true, Vantage1?) Conversely, if the max speed is 120mph in a dive, then the max g's in the dive could be calculated by either 10 g's*(120mph/60mph)^2 = 40 g's, or 1g*(120mph/18.97mph)^2 = 40 g's. The most g's that model can hit at 120 if it stalls at 19 straight and level is ~40 g's, not 100's!!!!! At 120 the Reynolds number will just over a factor of 10 higher. Assuming a 10% increase in CLmax due to the higher Reynolds number and you still get a max g's at 120 of ~44.

Add in the vector sum of maximum gusts to that 120 and it will raise that number a little, but not all that much. Add in a 20 mph gust from the nose for max V impact and the ~40 becomes ~55. The thing about gusts from any other direction is that they will increase the instantaneous angle of attack - but you won't generate any more lift than you can at CLmax.

Choose your speed - calculate your g's.

[Ollie]

http://www.sloperacing.com/results/ds-speeds.htm
Old:
http://www.shredair.com/album/dsfest5.html
http://www.rc-soar.com/tech/craig.htm

[Sail 'n Soar]

http://www.sloperacing.com/results/ds-speeds.htm
Old:
http://www.shredair.com/album/dsfest5.html
http://www.rc-soar.com/tech/craig.htm

OK, so the dynamic soaring folks hit really high speeds. What are their wing loadings and/or stall speeds? Choose your speeds;calculate your g's

[Sparky Paul]

The g's on a DSer can be computed by the radar speeds, and the time for an evolution..
We did this here sometime ago, and got about 17 g at 200 mph or so.
The DS evolution is a quite small diameter manuver BTW.
The USN requires a 20g vertical load capability for a carrier plane, with 40g! fore-and-aft!
Landing is worst case for these planes.

[HankF]

Excellent, now we seem to be getting somewhere, this thread has become very interesting:

My tentative conclusion is that the wildest of planes needs only be stressed to a bit over 20g.

Now I suppose, it would be useful to define categories of operation, something like: Training, Intermediate aerobatics, Pattern aerobatics, DS & SS, Hotlining, Soaring, etc and put a reasonable limit on each in terms of g's.

Hank

[Ollie]

http://www.rcgroups.com/forums/showthread.php?t=101242
#2
"Let me introduce myself. I am a retired engineer. I am 72 years old and beyond any capability of flying DS at high speeds. However, I am fascinated by DS and would like to participate vicariously through the design process, which I love.

The basic equation derived by Wurts and Drela states that the ultimate DS speed is some constant times the lift to drag ratio of the plane. The constant expresses the ratio of energy gained to energy lost in a circuit of the course. The greater the wind shear the greater the energy gained. The bigger the circle the more energy is lost. The trade is between the ability of the pilot to control the plane in a small circle versus the ability of the plane to maintain its shape and stay in one piece under high G forces."

#11
"The rationale for distributing the mass along the span similar to the lift distribution, is that it reduces the spanwise bending load to zero in the ideal case.

The problem is that nose weight is required to balance the aft mass of the wing, the tail boom(s) and tail. The solution is to compromize with multiple nose weights, fuselages and tails spread along the wing.
The reduction in bending load is proportional to the square of the number of load increments. I arbitrarily chose four increments which result in a 16 times reduction in spanwise bending moments compared to a single central fuselage with nose weight and tail.

The down side is that the spanwise mass distribution greatly increases the moment of inertia in roll and reduces the roll rate. I do not think that it is a serious problem for an all out DS machine which doesn’t need a high roll rate.

I used the NACA 63-210 maximum lift coefficient of 1.6 to calculate the maximum lift force that the wing could generate at 250 MPH. This is assuming that the wing were suddenly forced to high lift by a gust or inadvertant control input before it could slow down. The lift force cames to 3,000 pounds. Assuming that half the mass is concentrated in four nose weights, fuselages and tails, each fuselage assembly would impose an inertial force of 3000/4=750 pounds. Dividing the wing into eight spanwise segments of equal lift and eight moment producing bending loads acting on 7.5 inch moment arms, the peak bending moments are only 5,625 pound-inches. The minimum airfoil thickness is 0.9 inches and the spar depth could be as much as 0.8 inches, yielding a compression load in the upper spar cap of 7,030 pounds. Using a compression strength of 275,000 PSI yields a spar cap crossection of 0.026 square inches. A precured unidirectional carbon spar cap of 0.06 x 0.5 inches would be more than adequate. Twelve pound per cubic foot end grain balsa shearwebs the full width would easily take the maximum shear load of 375 pounds.

Because the normal bending loads will be associated with a five times smaller coefficient of lift, the actual bending with 200 foot diameter circles at 250 MPH will not be of much consequence."

#16
"Craig wrote:

'I understand that it is not something that needs to be completely calculated. It just need to be taken into account for max air speed and aero load calculations.'

I think I took into account the maximum lift that the wing could produce at 250 MPH when I did the spar design. The maximum lift was 3000 pounds at a lift coefficient of 1.6 and an airspeed of 250 MPH. There is no way the wing could produce more lift than that under those conditions unless flaps came into play.

The advantage of the spanloader configuration is that the mass of the wing and the mass of the four fuselages, nose weights and tails, also, to a large extent, float on the lift. This, in turn, reduces the spar strength and stiffness requirements by a factor of 16 (compared to a conventional configuration) even under the worst possible conditions. The incredibly small spar is for the worst possible case. The spar cap crossection requirement is reduced from .416 square inches to 0.026 square inches by the spanloader configuration!

The spanloader configuration also reduces the torsional load requirements on the wing and the bending load requirements on the tail booms by dividing and distributing the loads. It combines the structural advantages of a small model with the aerodynamic advantages of a large model.

To my way of thinking, all these advantages of the spanloader configuration overcome its disadvantages of high roll and yaw moments of inertia which, hopefully, can be delt with."

31#
"I think it's time to update the design and discuss structures and safety factors.

NACA 66-206, Cdo=0.0035 at Cl=0.2 for Re=3x10^6 (new airfoil)
Span 120 inches
Root chord 16 inches
Tip chord 8 inches
Four vertical tails of 36 square inches each
Four horizontal tails of 36 square inches each
Tail moment arm 60 inches
Wing weight 76 ounces
Radio weight 25 ounces
Total fuselages weight 20 ounces
Total V&H tail weights 16 ounces
Nose weights total 80 ounces (20 ounces per nose)
Gross weight 14.5 pounds
Wing loading 23.2 ounces per square foot
Maximum possible lift coefficient 1.0 at stall
Stall speed 24 MPH (not a problem launching into a 20 or 30 MPH wind)
The circle diameter at 250 MPH and Cl=0.2 is 263 feet and the circuit time is 2.25 seconds making it easier to fly than the first design.
The L/D is a respectable 33 even at the lower lift coefficient.
At 250 MPH and maximum lift coefficient, G=310 and at normal lift coefficient G=32.
The wing spar is designed for the maximum possible lift at 250 MPH."


#1 thru #108 was very long but, was an example of the process for design.

[vintage1]

Excellent, now we seem to be getting somewhere, this thread has become very interesting:



Now I suppose, it would be useful to define categories of operation, something like: Training, Intermediate aerobatics, Pattern aerobatics, DS & SS, Hotlining, Soaring, etc and put a reasonable limit on each in terms of g's.

Hank


Well I will have a stab at that.

Trainer? I have one and the wings flex alarmingly just suporting its own weight. Some have snapped them on this patrticular model. I would say its probably stressed to no more than 3g.

So 5g or thereabouts for a vintage style model would not be far off.

I'd say most warbird scale types or faster models need a bit more than that with 10g being a sensible target to aim for - and full pattern style models - the CAP's Ultmates and Pitts style stuff going up to around 20g or more.

Not sure on 3D models - the speeds are much lower. Probably similar.

Its the ultra fast models - DSS and racers - that need more, and Sail'n'Soars figures seem accurate. Its easier to achieve high structural integrity on a racer - the smaller wings have less stress at the center. Maybe 40g is realistic here, but its out side my area of competence.

The strongest ships would seem to have to be long slender winged fast soarers. But we knew that anyway :D

Interesting that carrier planes experience 40g on the arrestors/catapults.

Intuitively that feels like what a rough field landing does on my planes too.

20mph to a dead stop in a foot is 26g. if the deceleration is uniform.

[Sparky Paul]

Interesting that carrier planes experience 40g on the arrestors/catapults.

Intuitively that feels like what a rough field landing does on my planes too.

20mph to a dead stop in a foot is 26g. if the deceleration is uniform.
.
It isnt' that they get to those levels, it's that they -could- get to those levels, and experience has taught the Navy that a spec to that high a load will usually result in a plane with no damage during a normal landing.
Looking at the data we developed on the ES-3A when doing car/quals back at Pax in '91, the Lockheed stress guy who was working on the C-141 was amazed at the loads the ES-3A saw on a catapault launch itself, being higher than what would have collapsed a C-141. We had some strain gauges on the bottom fuselage beam, and watching the load build up there with the plane restrained by the hold-back fitting opposing the pull of the catapault shuttle, and the structural "twang" when the hold-back released was quite interesting.
The engine thrust would bend the fuselage! And then all of a sudden the load would change to the "yank" of the shuttler, and the plane would be flung into the air.
Landing loads were much higher, as there is NO attempt to flare when landing on a carrier. Several of the tests involved descending with a steep glide path, and some distance off center for the impact point.
Watching LtC Urich do this time after time, with the other 3 guys in the airplane along for the ride.. the "clang" when the thing hit the ground was impressive!
Awesome stuff to see happening 20 feet away! :)

[vintage1]

Well..my experience does not stretch that far...but I have seen racing cars stop dead in under 10 feet from huge velocities...and the drivers come out grinning sheepishly.

although broken ankles are not unusual either.

[Sparky Paul]

The guys on minesweepers frequently get their thigh bones shoved up into their abdomens if a mine goes off under the boat.
Injuries are related to circumstance, and the full anti-submarine harness and head restraint in a racecar have more than proven their worth.
My brother used to race sprint cars, with all the tail-over-teakettle bouncing those go thru, and sometimes ger seriously stretched in an upset tumble, but no lasting damage.
I wouldn't drive 3 feet without my seat belt latched.

[RyanPSU21]

In car crash test going 35mph into a rigid wall the car and people in it can easily exceed 50G's deceleration without having very high injury numbers. That is going from 35mph to complete stop in about 2 feet.

[HankF]

The advantage of the spanloader configuration is that the mass of the wing and the mass of the four fuselages, nose weights and tails, also, to a large extent, float on the lift. This, in turn, reduces the spar strength and stiffness requirements by a factor of 16 (compared to a conventional configuration) even under the worst possible conditions.

The disadvantage is when you have to carry a useful load, witness the Helios. When they put a point load on it, it came apart. There's an interesting article on it's breakup in a recent Avation Week magazine.

people in it can easily exceed 50G's deceleration

As I recall, Colonel Stapp withstood 46g's on the rocket sled at Alamogordo, NM. I also understand his glasses afterward looked like Coke bottle bottoms.

Hank

[Ollie]

HankF,
#52
"Ade,

You can build the spar as strong as you see fit. It won't do a lot of good to over build the spar but it won't be much of a penalty for attaining 250 MPH. There will just be a few ounces of over built spar just going along for the ride."

The several expert pilot's help for judging design checks. My judgment is less than perfect. You can learn for the process, not from a special design.

HankF,

"There's an interesting article on it's breakup in a recent Avation Week magazine."

This is not about the spanloader configuration type but, not for a design requirement to put a point load on it .