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  #16  
Old 08-23-2007, 11:54 AM
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Default Re: baggage pods

Quote:
Originally Posted by Marc Zeitlin View Post
The speed loss (due to drag increase) from the pods is attributable to the increased wetted area of the pods and pylons, and the accuracy with which the installer aligned them with the local flow field, not to their attachment scheme to the wing.
Well, i'll be sure to email Mike of ar5 fame, his aerodynamic consultant and inform them of their mistake.
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  #17  
Old 08-23-2007, 12:27 PM
Marc Zeitlin Marc Zeitlin is offline
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Default Re: baggage pods

Quote:
Originally Posted by Dust View Post
Well, i'll be sure to email Mike of ar5 fame, his aerodynamic consultant and inform them of their mistake.
The far more likely scenario (judging from the historical records) is that you've misinterpreted some part of the video you watched, and that there is no mistake on it at all. The issue is not them.

One cannot add two pods, 8 feet long each and one foot in diameter, with a wetted area of ~15 sq-ft each, as well as another ~10 sq-ft of wetted area on the pylons, and not have some substantial drag associated with them, no matter how streamlined they may be.

A small BID pad that wraps around the leading edge of a non-laminar flow wing is not going to have the same order of magnitude effect as the pods/pylons themselves. Will there be interference drag between the pylon and wing? Sure. Is it a function of the small wraparound at the leading edge? Not to any substantial extent. I have no doubt that Mr. Arnold and Bruce Carmichael (who is the aerodynamicist to whom you refer, I believe) would agree. See:

http://www.ar-5.com/kitcarm93.html

and:

http://www.ar-5.com/condrag94.html

for Mr. Carmichaels comment on the Ar-5.
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  #18  
Old 08-23-2007, 01:08 PM
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Default Re: baggage pods

Quote:
Originally Posted by Marc Zeitlin View Post
The far more likely scenario (judging from the historical records) is that you've misinterpreted some part of the video you watched, and that there is no mistake on it at all. The issue is not them.
Well, have you watched the video? Please review it as that is where they talk about interference drag in relation to the leading edge attachment.

Nowhere have i said, as you want to imply, that there is not going to be drag from the pods, the point is where is the optimum attach point to create the least amount of drag.

BTW, my history is just fine, as far as changing my mind when new facts or ideas come my way, then, yes I do change my mind when warranted.

Try to imagine that you are gonna make your own pods and attach points and look at it from that stand point. That is where i am comin from. I have a wood lathe and can easily glue up 2x4's and turn 48" long 16" diameter shape and do more turnings for the nose and tail. The body would be turned using "spindle" techniques and the nose and tail using a faceplate. On all the pieces, if for example i find that the top should be flatter than circular, i can turn a cylinder with craft paper in the middle of the glue up and after turning, split the shape in half and insert a foam wedge to flatten the top. That is the only way i could cut a 16" diameter piece as my band saw has only an 11" cut. Course i could make a carriage for it and stop by the local mill and run it through their band saw too. Fun stuff to do after i am flyin.

Because i will probably do that, i would like to take a fresh look at the pod shape and attach system, as I can now install the points needed in the future.

When i study an area i like to walk away with simple rules to remember. in decreasing drag, one of the rules i have learned is move the air gently, that is why wings have LEADING edge fairings to the fuselage as well as trailing edge fairings. That is why it was so easy to remember Arnold's point on the attach point for the landing gear, it fits with that simple rule. Just like a 3,4,5 triangle. Simple rules for complex problems

You got a better rule?

If you are gonna buy pods, then you have no choice, do it as designed and can be purchased. If you are gonna make em and wish to take a fresh look at the idea, then that is what i am doing.

I build for a hobby, before, during and after the plane build, it is what i do to relax.
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  #19  
Old 08-24-2007, 12:37 PM
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Default Re: baggage pods

Well, i was thinkin about this last night and i learned a lesson from eracer. last year i had occasion to closely inspect lear jet hard points . these were on Leer jet 25's and i think 35's. If memory serves me correct they were standard NATO 1000lb 28 inch hard points.

I have attached a pic.

the pod you see attached holds a cable that is 30,000 feet long, i'll repeat that, 30,000 feet long and when i talked to the chief pilot he said they just crank in a little trim and are good to go.

BTW, this info is from a competitors web site, both pics and the info so i am not revealing any confidential info.

Look at the distance that the pod is from the wing, not far at all. It is forward of the wing for a purpose - the pod needs clean air for it's internal wind powered 32 hp turbine. heh heh heh just a little trim is needed.

my pods are going to go on two titanium hard points that i will now install on the wing root rib, next to the srtake.the top of the pod will be about 6" lower than the wing and the hard points will be connected by a fairing and be adjustable so when i am flying i can tuft it to insure the optimum angle is achieved.

Any thoughts?
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  #20  
Old 08-24-2007, 11:15 PM
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Default Re: baggage pods

Side issues first. Not sure the area rule comes into play with the B737, its critical mach is pretty low. The -100 & -200 with their skinny, low bypass JT8D's fitted under the wing. The fatter, high-bypass CFM56 didn't, so Mr. Boeing mounted them on stumpy fairings in front of the wings. Compare the B707 with JT8D with the B737 of the same vintage & you can see the B707 has taller (and heavier) gear and long pylons. Its faster and heavier.




The pylons on the hunter pods look disproportionally long, but I imagine that will minimise the effect of the pod on the wing, so it works for me. Make the pod bigger you will be tempted to put more weight in & your aircraft is already going to be rather heavy. Best of luck with that.

Why titanium, how much strength do you need? Won't the hard point only be as strong as the fg & foam it is embedded in? How will you determine if it is strong enough?

If you move the pylon rearward you increase the arm from the c of g of the pod, so you will need proportionally more strength for the same weight. You will need to trade off the performance benefit of avoiding the leading edge (if any) against the known penalty of adding more lard to your aircraft.

I wish you well with it, but hope you do some pencil work before you pull out your grinder.
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  #21  
Old 08-25-2007, 10:00 AM
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Default Re: baggage pods

B-737

One of the issues they had with putting the large diameter fans on the later 737s was that of Ground Clearance (GC).

To get more GC, they had to mount the engine higher, the only way they could do that, was to move the engine forward.

If you look at some of the ultra high bypass engines on 737s (CFM-56), they even had to modify the lower engine cowling in order to maintain GC.

The redesigned pod vastly improved structural loads while in flight (Tension), but created chalanges in the pod design while on the ground (Shear)

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  #22  
Old 08-27-2007, 03:11 PM
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Default Re: baggage pods

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Originally Posted by Spodman View Post
Why titanium, how much strength do you need? Won't the hard point only be as strong as the fg & foam it is embedded in? How will you determine if it is strong enough?
I picked the wing forward root cause it already has a "rib" in it, easy to add coupla hard points to attach to.

titanium, light, strong and corrosion resistant. Also the cool factor is there, got to have just a wee bit of titanium.

Weight will be a factor, have to fill it with skinny cloths. my gross will be high on take off and fine on landing.

Cloths for a week for 2 do not weigh that much, but take up volume. back seat will have dowg and a bag or two.

the pods will effect gross more than CG, will have to reopen flight testing when I have them made and installed. Tuft em to insure the angle is good and that they are away from the bottom of the wing/strake juncture enough to allow goot air flow

Life is tuft
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  #23  
Old 08-27-2007, 04:50 PM
chasingmars chasingmars is offline
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Default Re: baggage pods

Quote:
Originally Posted by Dust View Post
Simple rules for complex problems
You got a better rule?
I'll bite.

There is a problem with this philosophy. It's not so much the simple rules issue, most of the rules are simple, it's that there are an awful lot of simple rules, all with differing design objectives, and these design objectives often compete, especially in the tradeoff between aerodynamics and structure, where some fairly strong power laws (for example stiffness going to the cube of structural depth) mean that you often will see compromises that require a good deal of teasing out the original simple rules. Remember that engineering is compromise as high art.

In this case, I suspect some of the ideas that you want the attach points to be under the wing a distance back of the leading edge comes from seeing airline aircraft attach points like this. Part of this comes from the idea that swept wings should have a vortilon or fence to modify the vortex flow field about the span, which optimizes the flow pattern about the wing to produce more desirable stall characteristics while allowing a better overall maximum lift coefficient (and therefore, smaller lighter wings for a given mandated max stall speed). Essentially, the idea is that a swept wind can't fully exploit the lift capabilities of the outboard wing panels and still ensure root-first stall so a vortilon is put on, or a leading edge fence is incorporated into an attachment structure. These need to come forward of the wing to be effective at high AoA, but if they did so from the leading edge, would interfere with the spanwise flow at cruise, so they attach on the underside. Why doesn't this all translate to the cozy, why can't we just figure out our simple rules from looking at pictures of airliners? Well, the cozy is a canard, and as such, gets it's safe stall characteristics from the forward flying surface, not so much from the twist and optimization of a swept main wing (although it's an open and interesting question whether a well designed vortilon would modify the Cl max distribution enough to increase main wing stall margin, I suspect it might, but that's just a hunch without data to back it up)... moreover, the effect on spanwise flow at cruise is much more important at high mach numbers, and so it makes much more difference to keep the vortilons or fence structures off the leading edge (or, more correctly, behind/below the stagnation point in climb and cruise attitudes) for an airliner than it does for us, who are (or should be) more concerned about the structural advantages of a leading edge attachment - remember, weight eventually becomes drag, and better structure is less weight. Even so, these are only a couple of the myriad considerations, so things like ground clearance dominate on some designs (and ground clearance becomes landing gear weight becomes drag). Airliners are very highly optimized. Cozys are not. But that doesn't mean that we should make Cozys like they make airliners.
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Last edited by chasingmars : 08-27-2007 at 05:15 PM.
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  #24  
Old 08-28-2007, 02:07 AM
Lynn Erickson Lynn Erickson is offline
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Default Re: baggage pods

Quote:
Originally Posted by Dust View Post
I picked the wing forward root cause it already has a "rib" in it, easy to add coupla hard points to attach to.
that might a bit to far inboard most of the pods are mounted about 2 feet outboard of the strake/wing joint. that may be a bit to close to the main wheels in the event of a hard landing. there have been eze's that have a rub mark on the bottom of the strake from the wheel pants hitting them.
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  #25  
Old 08-28-2007, 09:03 AM
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Default Re: baggage pods

Quote:
that might a bit to far inboard most of the pods are mounted about 2 feet outboard of the strake/wing joint. that may be a bit to close to the main wheels in the event of a hard landing. there have been eze's that have a rub mark on the bottom of the strake from the wheel pants hitting them.
Can you elaborate Lynn? Why would moving the pods further inboard have negative effects on a hard landing? I'm thinking the further inboard the better. It would be safer. Obviously the further outward on the wing the more difficulty with balance, handling, etc. We talk about scraped lower winglets, I'd hate to tag one of those pods?
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  #26  
Old 08-28-2007, 10:28 AM
FlyingRiki FlyingRiki is offline
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Default Re: baggage pods

Quote:
Originally Posted by Lynn Erickson View Post
rub mark on the bottom of the strake from the wheel pants hitting them.
Now THAT'S a hard landing.....
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  #27  
Old 08-28-2007, 10:32 AM
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Default Re: baggage pods

chasingmars - no, the thought did knot come from looking at airliner, came from the video from mike arnold on how intersection drag is higher if the attach points are at the leading edge.

lynn - thanks, will now have to look at that and perhaps/probably rethimk
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  #28  
Old 08-28-2007, 10:51 AM
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Default Re: baggage pods

I used the military hard points to look at the basic spacing from the wing down to the pod or bombs or missiles that are generally carried under wing hard points.

From a review of this it appears that the spacing from wing to pod can be much reduced from the baggage pods that can be purchased for our planes.
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  #29  
Old 08-28-2007, 11:35 AM
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Default Re: baggage pods

lynn - just a tinkin, as the current design of the baggage pods has such a long attach arm, would not this hard a landing rip em off too?
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  #30  
Old 08-28-2007, 01:17 PM
Lynn Erickson Lynn Erickson is offline
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Default Re: baggage pods

Quote:
Originally Posted by neverquit View Post
Can you elaborate Lynn? Why would moving the pods further inboard have negative effects on a hard landing? I'm thinking the further inboard the better. It would be safer. Obviously the further outward on the wing the more difficulty with balance, handling, etc. We talk about scraped lower winglets, I'd hate to tag one of those pods?
We have seen case of marks on the paint of the wheel pants and a matching mark on the bottom of the wing. we have seen tire marks on the bottom of the wing from a hard landing. the pods need to be outboard far enough to clear the wheel during full deflection of the gear leg. we have seen canards that have landed off field and ripped out the gear and the pods supported the aircraft and the belly never touched the ground.
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Last edited by Lynn Erickson : 08-28-2007 at 01:25 PM.
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