C172 Spar

ProudPilot

ProudPilot

Aeronautics Geek
Hi Everyone,

I got a great question, but not a great answer. For the C172S model at least, how is the main spar carried through to the other side? As far as I know, the spar joins the slanted wing spar which carries under the fuselage up to the slanted wing spar on the other side. So, how much compression can the top fuselage take?

My student asked if in a high G maneuver, will the wings bend before the roof starts to compress? I don't have a great answer, wanted to see what you guys know.

Thanks!
 
ProudPilot said:
Hi Everyone,

I got a great question, but not a great answer. For the C172S model at least, how is the main spar carried through to the other side? As far as I know, the spar joins the slanted wing spar which carries under the fuselage up to the slanted wing spar on the other side. So, how much compression can the top fuselage take?

My student asked if in a high G maneuver, will the wings bend before the roof starts to compress? I don't have a great answer, wanted to see what you guys know.

Thanks!
Click to expand...

Is your student a structural engineer or something?

Tell him I have an old M model I could use the insurance cashout on. PM for the key and base.
 
ProudPilot said:
My student asked if in a high G maneuver, will the wings bend before the roof starts to compress? !
Click to expand...

Is your student under the illusion that the wing would bend downward? Why would it do that? The g forces on the airframe are produced by the wing's lift; if anything, they would bend upward. (and the struts are not part of the spar.)
 
tgrayson said:
Is your student under the illusion that the wing would bend downward? Why would it do that? The g forces on the airframe are produced by the wing's lift; if anything, they would bend upward. (and the struts are not part of the spar.)
Click to expand...

? G force(load factor) is the ratio of how much a wing can support in relation to it's weight. How would the wing bend upwards?

Are you talking about the wing doing a clap?
 
Socal321 said:
? G force(load factor) is the ratio of how much a wing can support in relation to it's weight. How would the wing bend upwards?
Click to expand...

Put a ruler on the ground and place the ball of your foot in the middle of it, then pull upwards on each end of the ruler. This is a rough demonstration of what's going on in flight. The wings pull the aircraft upwards and the fuselage resists this pull. The point of stress is where the wing root meets the fuselage. Weight in the wings, such as fuel, does not contribute to this stress, which is why many aircraft have a zero fuel weight which is less than gross weight.

Wing struts are normally in tension, rather than compression.
 
ProudPilot said:
Hi Everyone,

I got a great question, but not a great answer. For the C172S model at least, how is the main spar carried through to the other side? As far as I know, the spar joins the slanted wing spar which carries under the fuselage up to the slanted wing spar on the other side. So, how much compression can the top fuselage take?

My student asked if in a high G maneuver, will the wings bend before the roof starts to compress? I don't have a great answer, wanted to see what you guys know.

Thanks!
Click to expand...


The 172S spar doesn't carry trough. The 177 and 210 spars do.

As far as compression on the roof, well, they put skylights in some models. I'd think you'd have to snap a strut before you could get that. Then, the wing bolts would go and...well, I wouldn't be analyzing all this in my final seconds.

How many Gs? Let's go with +3.0, +3.8, or +4.4 based on weight and configuration.:D
 
ProudPilot said:
My student asked if in a high G maneuver, will the wings bend before the roof starts to compress? I don't have a great answer, wanted to see what you guys know.
Click to expand...

I wouldn't think the roof would compress at all even in extreme (ultimate load factor) g maneuvers. I question the idea that the roof even has a load put on it from high g maneuvers. The aerodynamic stress occurs because of, what I like to call, an aerodynamic increase in aircraft weight.

Basically the aircraft feels heavier (I know it really doesn't feel), so the wings have to support that increased weight through increasing lift. If you have drawn vectors for aircraft in a turn, depicted in any basic flying manual, you see that in a 60 degree turn the wings have to produce twice as much lift.

This stress on the wings in this scenario is identical to throwing weights in the aircraft to double its weight. Sure it would never take off, but if it did the wings would be subjected to far more stress to lift this weight. That stress would act in a manner pushing the wings upward, as the ruler example demonstrates.

Here is a video of how they test that, as you can see they don't pull the wings down-ward to test this:

[YT]pe9PVaFGl3o[/YT]
 
deadstick said:
The 172S spar doesn't carry trough. The 177 and 210 spars do.
Click to expand...
Quite the contrary. As someone who has restored a Cessna and replaced both the fwd and aft spars......The do have spar carry throughs. Look on the top skin over the cabin and you will see two closely spaced rows of rivets, these show where the spars pass through. They (spars) are also where the shoulder harnesses are mounted.

The spars are a U shaped channel of aluminum passing through the top of the cabin.


ProudP: The spar (load) doesn't pass through the strut to the lower fuselage. The strut only helps carry the load. The spar(s) runs from near the tip to the wing root. One bolt holds each spar to the spar carry-through (4 total).
 
29.92inHg said:
That is a sobering thought. The thought of losing my tail in a airplane makes my stomach turn. Not that losing the wings would be better
Click to expand...

Don't pass limit load factor and if you ever do be sure you don't hit ultimate load factor. :) Then nothing will rip off on you.
 
ppragman said:
http://www.youtube.com/watch?v=LPbhQS6IljU&feature=player_embedded
Click to expand...

Training on the "youtube" attachment on any of these: if u put yt between the [] and then /yt between them at the end to set it up to work on here find the youtube link. In this case "watch?v=LPbhQS6IljU&feature=player_embedded" and just after it says watch?= take that and copy it and past it between the [].

In this case ...yt]LPbhQS6IljU&feature=player_embedded[/yt. I excluded the [] completion of the link to demonstrate its use, but it took me a good 20 minutes to figure this out. That way you can post youtube links in your replies as viewable videos instead of just posting links. :)
 
NJA_Capt said:
Quite the contrary. As someone who has restored a Cessna and replaced both the fwd and aft spars......The do have spar carry throughs. Look on the top skin over the cabin and you will see two closely spaced rows of rivets, these show where the spars pass through. They (spars) are also where the shoulder harnesses are mounted.

The spars are a U shaped channel of aluminum passing through the top of the cabin.


ProudP: The spar (load) doesn't pass through the strut to the lower fuselage. The strut only helps carry the load. The spar(s) runs from near the tip to the wing root. One bolt holds each spar to the spar carry-through (4 total).
Click to expand...

How about that? Learn something new every day. This spar, though, doesn't provide the same support as a traditional spar (177/210), right? That's why you have the struts?
 
deadstick said:
This spar, though, doesn't provide the same support as a traditional spar (177/210), right? That's why you have the struts?
Click to expand...
I'm not sure what you would call a "traditional" spar. Each manufacturer does it a little different.

The struts just allow for a lighter structure based on leverage. (Wider distribution of the load.) The Cessna 210 has two larger bolts mounted vertically on the main/fwd (wing) spar instead of the one on the strutted 100 series planes. The overhead cabin structure is quite a bit beefier to accept the full load.

Below is a C177 wing (upside down). You can see the two wing mounts in the center of the wing.

2022001-5_194.jpg



ProudPilot said:
So, how much compression can the top fuselage take?

My student asked if in a high G maneuver, will the wings bend before the roof starts to compress?
Click to expand...

There shouldn't be any compression in the cabin. Most of the damage in high G loads comes outboard of the struts. Not that that is any better.
 
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