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Airplane Design Q&A


mikegarrison
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13 minutes ago, JoeSchmuckatelli said:

I realized as soon as you asked the question that I had goofed; I've actually seen the 'interior insert' modules

Was it for a USN C-40A? That's what I did some noise testing on. (There was a drain valve for the cargo door that was causing a hissing noise during takeoffs, and I needed to get some data on it to justify fixing it. I used our Aachen Head to collect the data, so I could replay a full binaural recording of the noise. When the program managers were able to hear the noise themselves, they agreed that it was a problem and ok'd the fix.)

https://www.head-acoustics.com/products/artificial-head-binaural-recording

In this picture you see some service members loading cargo into a C-40. But if you look at the interior you can see that it has overhead bins and regular passenger cabin sidewalls (with insulation and noise treatment like normal). (They also appear to have some sort of protective sidewall cover that is protecting the passenger sidewalls from being damaged during cargo handling.)

US_Navy_100126-N-0705K-003_Naval_Air_Cre

You can also see the stowed airstairs under the forward passenger door.

Edited by mikegarrison
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The thing I saw was different.  It was kind of like those big cargo inserts (name?) that can slide into / out of the fuselage, except it was set up like the interior of a passenger compartment.  I'd never seen anything like it before or since, but I was like 'this is head-slappingly obvious... why don't more planes do this?'

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  • 2 months later...

I'm just posting here because I worked for more than six years to develop the ICAO CO2 airplane certification rule. The rule is now in place in Europe. (It is still working its way through the legal system in the US.)

EASA just recently announced that they have certified their first airplane under the rule, the Airbus A330-900. This is very exciting for me to see.

My work on this rule is quite possibly the most significant thing I have done as an engineer.

https://www.easa.europa.eu/newsroom-and-events/press-releases/easa-completes-first-co2-emissions-certification-airbus-a330-900

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On 9/26/2020 at 4:44 AM, mikegarrison said:

Some airplanes, like those designed for aircraft carriers or the new 777X, have folding wingtips. This is so they get the benefits of that lovely extra wingspan when flying, but can still fit into smaller spaces on the ground. However, that adds weight, cost, complexity, failure modes, etc.

I know I'm extremely late to this conversation but I still can't quite wrap my head around the whole 777X falling into ICAO Code E (FAA Class V) aircraft category. This particular brochure just reads crazy to me.

Spoiler

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I do know that the ICAO Codes classification might well have been plucked out of someone's back - it was written when there weren't anything in Code E or Code F airplanes in the 60s - but it still feels pretty something to see 777X fitting in there. I wonder if there're going to be any followers in this regard after 777X... maybe Code D or Code C...

Edited by YNM
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  • 3 weeks later...

As a starting point for an R/C glider project, I am attempting to estimate the wing loading (W/S) using an equation given in Dan Raymer's Aircraft Design: A Conceptual Approach, where wing loading is expressed a follows:

W/S = [-(-G) +/- {(-G)2 - (4 CDO / pi A e)}](1/2)*[(q pi A e)/2]

(An equation generator would be a nice addition to the editor)

Where G is the ratio of the vertical speed and forward speed of the glider, CDO is the zero-lift drag coefficient, q is the dynamic pressure, A is the aspect ratio, and e is the Oswald Factor.

Assuming a glide ratio or 20 to 1, and an airspeed of 10 m/s, the rate of decent will be approximately  0.5 m/s, thus G will be -0.04995. CDO and e are assumed to be 0.015 and 0.8 respectively. Earlier on in the text, Raymer identifies an aspect ratio for sailplanes/ gliders to be about 4.464. And lastly, q was determined to be 60 N/m2.

Now. The trouble I am having is that when evaluated, the difference within the radicand is a negative value. Meaning, one can not obtain a real solution with this equation. A complex one is obtainable, but is that appropriate? Is there a different method for estimating wing loading other than that presented by Raymer.

Edited by Exploro
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On 6/12/2021 at 5:47 PM, Exploro said:

As a starting point for an R/C glider project, I am attempting to estimate the wing loading (W/S) using an equation given in Dan Raymer's Aircraft Design: A Conceptual Approach, where wing loading is expressed a follows:

W/S = [-(-G) +/- {(-G)2 - (4 CDO / pi A e)}](1/2)*[(q pi A e)/2]

(An equation generator would be a nice addition to the editor)

Where G is the ratio of the vertical speed and forward speed of the glider, CDO is the zero-lift drag coefficient, q is the dynamic pressure, A is the aspect ratio, and e is the Oswald Factor.

Assuming a glide ratio or 20 to 1, and an airspeed of 10 m/s, the rate of decent will be approximately  0.5 m/s, thus G will be -0.04995. CDO and e are assumed to be 0.015 and 0.8 respectively. Earlier on in the text, Raymer identifies an aspect ratio for sailplanes/ gliders to be about 4.464. And lastly, q was determined to be 60 N/m2.

Now. The trouble I am having is that when evaluated, the difference within the radicand is a negative value. Meaning, one can not obtain a real solution with this equation. A complex one is obtainable, but is that appropriate? Is there a different method for estimating wing loading other than that presented by Raymer.

Hmm. I took a week-long class from Raymer one time. (Got paid for it, too!) Actually went out to dinner with him one evening. But we didn't cover gliders.

So wing loading is just lift/wing area. I'm curious why you need to know it. Wing loading affects stall characteristics (and therefore maneuverability). Also affects things like runway length needed and climb rate.

CD = CD0 + CDi

A simple assumption is that CDi = CL^2 / (pi * e * AR)

q = dynamic pressure

L=W= CL * q * S

L/D also = glide ratio

So you see all these terms here. It looks like he just solved them all to get L/S (ie. W/S). 

========

What kind of glider has an aspect ratio of 4? That's a pretty poor aspect ratio for a glider. And no way are you going to get 20:1 L/D with an aspect ratio of 4.

I think the problem here is not the equation. It's the numbers you are punching into it. You are kind of saying, "what's the wing loading required for this brick to glide like a performance sailplane?" and you are getting some kind of negative mass answer, because a brick can't glide like a performance sailplane.

Edited by mikegarrison
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Greetings Mike. Thank you for fielding my question. The reason for estimating the wing loading is to come up with preliminary sizing of the wings. In the edition of the book I am using, the wing's reference area is calculated with wing loading as a variable. Identifying the reference area and knowing aspect ratio will give an estimate on wing span.

The value of aspect ratio I used was actually referenced from Table 4.1, where the equivalent aspect ratio for sailplanes was listed as 4.464 at best L/D. However, I defer to your feedback and will try a different aspect ratio.

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46 minutes ago, Exploro said:

Greetings Mike. Thank you for fielding my question. The reason for estimating the wing loading is to come up with preliminary sizing of the wings. In the edition of the book I am using, the wing's reference area is calculated with wing loading as a variable. Identifying the reference area and knowing aspect ratio will give an estimate on wing span.

The value of aspect ratio I used was actually referenced from Table 4.1, where the equivalent aspect ratio for sailplanes was listed as 4.464 at best L/D. However, I defer to your feedback and will try a different aspect ratio.

Well, I've never designed a sailplane. But ...

In my copy of that book (it was given to me as part of the class), in table 4.1 he says sailplane aspect ratio should be 0.19*(L/D)^1.3

You want an L/D of 20 (taken from your original post that says the glide ratio is 20 -- L/D and max glide ratio are the same). So that would be an aspect ratio of 9.3.

A quick Google search indicates that for R/C model sailplanes, it's apparently hard to make them with an aspect ratio of much higher than 8. If I plug 8 in, I get an L/D of about 17.7.

That makes your "G" be about -0.056. If I plug those new numbers into the original equation, I get a positive value under the square root sign.

Edited by mikegarrison
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I concur. A quick excel spreadsheet done up after reading your post got positive values at AR of 9.75 and greater sticking with the value of G and L/D of 20. A positive value in the radican is positively good news. This is splendid. Thank you Mikegarrison.

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