Designing/building a steel cage around the cockpit

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JayKoit

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Hi Everyone,

As a follow-up to my bubble canopy thread, this is something else I've been noodling:

I was looking at the SAM Aircraft a while back and really liked the fact that it was an aluminum design with a steel cage built around the cockpit for crash protection.

It got me thinking: Could I, or has anyone ever, built a steel cage into an existing aluminum aircraft design (Zenith, Vans, Sonex, etc)? I haven't looked really hard at any of these planes' plans, but I was thinking a cage could be built from square 4130 tubing, with the tubing laid out exactly where the aluminum angle/stiffeners were, so they would essentially replace the angle and the skins riveted to that (plus any reinforcing tubes, of course)...or if lots of rivet holes weakens the tubes too much, maybe the tubes get placed in between the original angle/stiffeners, and maybe a some welded flanges serve as attach points for the cage to the airframe/skins? Here's a picture of the SAM cage, since that design has a round fuselage, they used rounded bulkheads which attach to the cage via flanges, and the fuselage skins attach to the bulkheads:
SAM steel cage.jpg

I'm sure it would be a bit of a headache, but I wonder if it's possible. And I know it would cut down on useful load quite a bit, but the RV-9 and the 650 have useful loads between 600-700 lbs so there's some wiggle room there.
Thanks!
 

lr27

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You might want to check up on what the Formula 1 car racing guys do about driver protection. (I'm guessing they care even more about weight than other racers do.) Also you might find out how this is done in cropduster designs.
 

BoKu

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...Could I, or has anyone ever, built a steel cage into an existing aluminum aircraft design (Zenith, Vans, Sonex, etc)?...
I really like that structural model in general. I think it could constitute a substantial safety benefit for certain airplanes. Back when I was trying to sell an ultra Sport class racer powered by the Jack Kane EV-12 motor, I drew the airplane around what was essentially a triangulated SCCA-spec crash cage of 1.75" OD, 0.120" wall tubing. The cage joined all the major subassemblies (engine mount at the front, wing on the bottom, seat and controls inside, etc), and was attached at its aft end to the monocoque carbon fiber aft fuselage. The cage was dressed out in molded panels of carbon/foam sandwich attached with quarter-turn fasteners. No buyers so far...

Two other examples with good records for serviceability and crashworthiness are the AT-6 and the BT-13, both WWII trainers. In each case the steel cage is combined with semi-monocoque aluminum aft fuselage and tail. And of course there's the GlasStar, and probably the Mooney M20 counts as well.

As to whether you could do that for an existing small airplane design, I'd have to say the answer is no. Which is to say, you'd end up changing so much of the airplane that it would essentially become a new design. To make it effective, you'd end up with a fully-triangulated truss that reacts all of the major aerodynamic forces from the wings, tail, and cockpit. Then you'd dress it out in fairing panels of lightweight construction that react only locally applied skin loads. For a Zenith or an RV, you'd probably end up using almost none of the fuselage structure unaltered.

I'm not an engineer, but my sense is that below a certain mass the overall scheme has enough structural redundancy that it starts to reduce safety instead of improve it. My thinking is that the extra mass increases stall speed and kinetic energy, which would outweigh the crash protection it provides. From there on down, you're probably better off ditching the aluminum skins and using stringers and fabric cover over the steel truss ala Kitfox.

--Bob K.
 

lr27

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I'm guessing the cage would be at least 80 lbs? Don't they use carbon fiber cages in some racing classes? It does seem silly to have all that strength and not use it. I wonder how much weight an SCCA style cage would add to a steel tube airplane.
 

ScaleBirdsScott

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The Panther SPA is steel cockpit frame with alum skins riveted on, check that out for a good example. The Titan T51 is similar and is a 2-seater.

But yes retrofitting a design would require a lot of work to do what amounts to a ground-up rework. Unless the design already is based around a, say, aluminum tube frame and you're just going to swap for an equivalency in steel, the whole way it would distribute and handle loads is just very different.
 

lr27

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Would an aluminum cage be much worse than a steel one?
 

BoKu

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I'm guessing the cage would be at least 80 lbs? Don't they use carbon fiber cages in some racing classes? It does seem silly to have all that strength and not use it. I wonder how much weight an SCCA style cage would add to a steel tube airplane.
In the 24 Hours of Lemons racing series I'm familiar with, the crash cage adds about 200 lbs to the car. SCCA differs in the technical details, but probably has about the same mass. For the EV-12 airplane I described above, the cage would have added around 150 lbs over the mass of the equivalent pure carbon fiber fuselage structure. However, having seen the results of two Thunder Mustang crashes at Reno, I think it would be a penalty well worth paying. In addition to offering better pilot protection, it would also have better access for service and repairs, and I think that alone would probably make it worthwhile.

Carbon fiber tubs are common in open-wheel racing like F1 and Indy. That technology might be applicable to aviation, but it would probably come at a pretty substantial cost in dollars per unit kinetic energy.

--Bob K.
 

rv6ejguy

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I used to build sedan roll cages for a living years ago and they work out to around 120-200 pounds depending on the class and number of tubes involved. An airplane doesn't weigh as much as a car and is much narrower so you could use less tubing and smaller OD and wall thicknesses for similar protection. Probably about half the weight as above for a typical 2 seat airplane.

As Bob says though, you need to attach that roll over structure properly to the rest of the airframe and spread out the loads so those parts don't buckle or collapse.

Entirely possible but you'll have a challenge welding all sides of the structure inside the aluminum. Many fatalities are due to injuries sustained from the high vertical component forces and the cage won't help that aspect. The factory Van's roll bar has worked well in numerous roll over accidents if that's your main worry.
 

lr27

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Maybe all small airplanes should have sagging bellies full of foam to take the vertical forces. I imagine that sometimes it would actually be aerodynamically cleaner. Proximity airbags would probably be dangerous all the time except when crashing.
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Shouldn't we be attaching the front and back of the rest of the airplane to the cage?
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Boku:
There must be an awful lot of pit stops in the "24 Hours of Lemons". I was shocked to discover it was a real event.

https://24hoursoflemons.com
 

Kyle Boatright

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Hi Everyone,

As a follow-up to my bubble canopy thread, this is something else I've been noodling:

I was looking at the SAM Aircraft a while back and really liked the fact that it was an aluminum design with a steel cage built around the cockpit for crash protection.
The SAM and several WWII trainers are basically truncated steel tube fuselages with appendages bolted on. The T-6 and BT-13 have aluminum monocoque tailcones and aluminum wings bolted to the steel tube fuselage. The PT-19 is a steel tube fuselages with wooden wings bolted on. A steel truss is a great starting place for an airplane.

But I wouldn't want to start with an airplane and try to install a steel truss cage inside the existing structure.

Better might be to design a steel truss fuselage around (say) a set of RV wings and tail surfaces.
 

BoKu

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...There must be an awful lot of pit stops in the "24 Hours of Lemons". I was shocked to discover it was a real event.

https://24hoursoflemons.com
A team in the hunt for a class win usually runs 2-hour stints, with fuel and driver changes at the end of each. Teams just out for fun or gunning for the IOE will usually run 1-hour or so stints to make sure that everyone gets some seat time before something breaks. I've never been on a team where everybody didn't get at least a little time on track, but it happens pretty regularly. Usually there's only one real 24-hour race per year in the season; most races run around 8 hours on Saturday and 6 hours on Sunday. That gives you time to blow up on Saturday, change an engine or transmission, and still get some track time on Sunday.
 

Toobuilder

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Take a look at many of the Extra series fuselage structure and with a little bit of imagination it could be a dragster or open wheel race car. But, as pointed out earlier, rollover protection is pretty simple to achieve with bubble canopy aircraft without going to the trouble of a "cage". One simply has to be realistic about what you are trying to protect against. Even the most sophisticated NASCAR or F-1 drivers wouldnt fare so well in their sometimes spectacular crashes if they were also dropped from 50 feet.
 

Jay Kempf

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It's not about "adding" a cage to a design but more replacing some of the cockpit surrounding structure with a cage and then transferring the strength of that cage out to the surrounding structures. It seems that the idea of a strong and malleable cage that can crush some but not enough to allow the occupants to get injured badly is a good idea. Not an easy thing to get right. Both aluminum and composites can be designed for the same task. Carbon fiber is probably going to be better at the task but less repairable. The idea is to have it all crumple surrounding the occupants to dissipate energy and decelerate all of the crash velocities slowly over longer periods of time. The more crush, the more deceleration, the higher the survivability.
 

Mad MAC

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In many of these examples the biggest value of steel tube is that it makes for easy access to the cockpit systems. For the typical aluminum semi monocoque fuse structure, 10 lbs more aluminum would make quite an increase in structural robustness.
 

BJC

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I believe the Glastar uses the same approach.
One of the features of the GlaStar and Sportsman cage is that landing gear sockets for conventional, tricycle, or floats configuration are built into the cage, along with lifting tabs, allowing the configuration to easily be changed on a completed aircraft. The sockets are visible here:
100_1130.jpg


BJC
 

Riggerrob

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The DHC-2 Beaver has hidden steel pillars connecting the wings and engine firewall.
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Same as the high winged Glastar and high winged Zenith 701 bush planes. The more windows and doors around a cabin, the more usefull a steel cage becomes.
 
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