Tips and tricks for building a LIGHT aircraft

HomeBuiltAirplanes.com

Help Support HomeBuiltAirplanes.com:

autoreply

Well-Known Member
Joined
Jul 7, 2009
Messages
10,751
Location
Rotterdam, Netherlands
Anyone know the weight difference between aluminum and standard fiberglass per square ft? Say on a cowling etc?
None. A panel of similar dimensions (width, thickness, height) will have close to the same weight, with vacuum bagged glass being a tad above 2 tonne/m3 and alu being about 2.7 tonne/kg (but the alu is also a bit stiffer allowing for slightly tiner panels). Panel stiffness drives required thickness. GFRP and alu are close there together.

Carbon is way lighter, at maybe 1300 kg/m3 and also way stiffer. The biggie however is that with a composite cowl (or any shape) you can easily add a local stiffener and optimize thickness to what's needed. Just forget about that with any sheet metal unless you have deep pockets.

Spending too much to save a tiny bit of weight could actually arrest the whole project. One has to weigh (pun intended) the value of some stuff. If it's going to cost you $3000 more, say, to cover with Oratex instead of Poly-Fiber in order to save ten pounds, I wouldn't do it unless I had really deep pockets. You'll just be a lot longer building the airplane, and as the build time lengthens, the likelihood of finishing it diminishes.

It's hard enough just matching the designer's target weight. Most plans-built homebuilts end up too heavy simply because builders weren't careful to copy exactly the designer's plans. They add a few instruments or a heavier tailwheel or a bigger engine or a few more coats of paint to get that deep gloss. Or they decide that those 1/4" x 1/4" rib capstrips are too fragile and make them 5/16" square instead. Or make them 1/4" thick by 3/4" wide so they can glue the fabric to the ribs to save stitching. Or they use Home Depot wood that has cockeyed grain that splits when you bend it so they have to make it bigger or start patching it or reinforcing it. Or they use oak just to be sure it will be strong enough. Or a metal prop instead of the specified wooden prop because they got a deal on Ebay. One has to watch for the "savings" in time or money that most often end up costing way more than one intended, and resist the temptation to modify or upgrade or spiff up. Weight that creeps in also has a way of making an airplane's CG end up outside the specs so that more weight is necessary, in the form of ballast, to bring it back where it belongs.
That, a thousand times over. Such extreme weight-saving measured are just a waste of money.

"Low weight" is not a metric that one should optimize blindly. The virtue of lowering weight is a viscious circle of lower weight, lower engine power, smaller engine required, less fuel required etc. The Dyn'Aero MCR4S is a good example. Outperforms a C172 with a lower stall, higher cruise, smaller engine (100 hp Rotax) by going for a very low weight, thus lowering cost.

If you're committed to building a Cassutt, build it reasonably, not ridiculously light. Because if you look at the money/effort expenses of such extreme lightening, you'd be better off by designing and building a composite wing, a composite (monocoque) fuselage, or just a from-scratch composite plane. sub 400 lbs empty for a starterless, non-electric O200 powered ship then doesn't sound impossible. But then, you could also put on a VW-derivative, cash out the money saved on the engine, have the same performance as a "real" cassutt and have a lot of money left over for fuel...
 

Midniteoyl

Well-Known Member
Joined
Sep 3, 2003
Messages
2,406
Location
Indiana
None. A panel of similar dimensions (width, thickness, height) will have close to the same weight, with vacuum bagged glass being a tad above 2 tonne/m3 and alu being about 2.7 tonne/kg (but the alu is also a bit stiffer allowing for slightly tiner panels). Panel stiffness drives required thickness. GFRP and alu are close there together.

Carbon is way lighter, at maybe 1300 kg/m3 and also way stiffer. The biggie however is that with a composite cowl (or any shape) you can easily add a local stiffener and optimize thickness to what's needed. Just forget about that with any sheet metal unless you have deep pockets.
Didn't Orion use to always say that a properly designed (for the materials used) and built aircraft, of the same design, will always end up weighing very close to the same whether wood, metal or composite...?
 

bmcj

Well-Known Member
HBA Supporter
Joined
Apr 10, 2007
Messages
13,392
Location
Fresno, California
One potential area for weight savings is in the instrumentation. There are very light and flat alternatives in instruments and avionics compared to the old standard that used to be the only thing available.

The best part is that the instruments tend to be close to the CG, so not much shift in balance.
 

autoreply

Well-Known Member
Joined
Jul 7, 2009
Messages
10,751
Location
Rotterdam, Netherlands
Didn't Orion use to always say that a properly designed (for the materials used) and built aircraft, of the same design, will always end up weighing very close to the same whether wood, metal or composite...?
I don't recall him saying that and would actually be surprised if he thought so, given the many conversations we've had.

I do recall him often saying that by referring to "black aluminium", where you simply copy an existing alu airframe (or wood for that matter) and end up with pretty much the same weight in composites if not heavier. His Warren truss wing designs were a favorite showcase to highlight that composites really required a different structure to grossly outperform other materials. Same for his monocoque Pitts' wings.
 

Pops

Well-Known Member
Log Member
Joined
Jan 1, 2013
Messages
8,162
Location
USA.
One potential area for weight savings is in the instrumentation. There are very light and flat alternatives in instruments and avionics compared to the old standard that used to be the only thing available.

The best part is that the instruments tend to be close to the CG, so not much shift in balance.
Bob Borrows ( designer of the Bearhawk) eats and breaths lightness. Also allergic to electric :) His Bearhawk LSA had a steam gauge airspeed, altimeter. For engine, a Grand Rapids Tech EIS. The instrument panel almost looks empty. He powers the EIS with some D size NI-Cads in a holder behind the panel.
Dan
 

Midniteoyl

Well-Known Member
Joined
Sep 3, 2003
Messages
2,406
Location
Indiana
I don't recall him saying that and would actually be surprised if he thought so, given the many conversations we've had.

I do recall him often saying that by referring to "black aluminium", where you simply copy an existing alu airframe (or wood for that matter) and end up with pretty much the same weight in composites if not heavier. His Warren truss wing designs were a favorite showcase to highlight that composites really required a different structure to grossly outperform other materials. Same for his monocoque Pitts' wings.
Of course I cant find it, but it was something along the lines that since you need to design to 2x instead of 1.5x for metal, that most planes end up the same weight in the end.

I do remember where he said simply copying a metal plane in glass would get you a heavier plane, but also that properly redesigning say, a RV-8, in glass would still give you the about the same weight in the end.
 

Topaz

Super Moderator
Staff member
Log Member
Joined
Jul 29, 2005
Messages
14,110
Location
Orange County, California
Of course I cant find it, but it was something along the lines that since you need to design to 2x instead of 1.5x for metal, that most planes end up the same weight in the end. ...
My recollection is that he was talking roughly along those lines, saying that the differences, after all was said and done, wouldn't be as large as people were claiming. I don't think that he claimed they would be the same weight, but again, rather that the differences wouldn't be as large as some of the composite proponents were claiming. FOS was a large part in that.

He had a simply huge aversion to "black aluminium" techniques such as Autoreply is describing, and as is practiced largely by the various aerospace primes even to this day. He thought it was the worst of all possible worlds.
 

autoreply

Well-Known Member
Joined
Jul 7, 2009
Messages
10,751
Location
Rotterdam, Netherlands
Of course I cant find it, but it was something along the lines that since you need to design to 2x instead of 1.5x for metal, that most planes end up the same weight in the end.

I do remember where he said simply copying a metal plane in glass would get you a heavier plane, but also that properly redesigning say, a RV-8, in glass would still give you the about the same weight in the end.
For sure, why one would nowadays still build in glass (for major structure) is beyond me, the price difference between CFRP and glass is tiny if you look at the total cost (molds, hours, engine).

Comparing CFRP to glass, you can save almost half of the structural weight. On most otherwise optimized light aircraft, structural weight is only a quarter to a third of empty weight however, with empty weight being say 60% of gross. That means going to CFRP increases your payload with 10-20 percentpoint of gross, or 25-50% over the original. That's why that vicious circle is so important, lower mass, smaller engine, less fuel required, even lighter structure etc. Without it, you're just lightening up a useful but tiny bit.
 

StarJar

Well-Known Member
Joined
Sep 6, 2011
Messages
1,723
Location
El Centro, California, USA
Preface:I waited 'till the end, because sometimes I'm very modest, and also it seems my posts often derail threads, which is as stressful to me, as it is the person who started the thread.
And I probably wouldn't even post this, because of the fights that often insue, but I have had too much time on my hands because I've been waiting a couple days for a package to arrive....And also because Scrapper was pretty cool on my thread, and gave some, IMO, excellent insights and advice.
And it's no big deal, but once I designed and built a plane with a VW engine. (The same one where I discovered the $1 prop balancer.)
Main Point:Well anyway, for the cowling, instead of using many plies of glass, I used only 2 plies, with a 1/16 layer of micro sandwiched in between. It was very strong and much lighter than other cowlings that I had picked up to see how heavy they were. I would say it only weighed about 65% of what a similar sized RV cowling weighed.
OK.Thanks.
 
Last edited:

mustangman

New Member
Joined
Mar 25, 2017
Messages
2
Location
Davidson, Sk Canada
I cut lighting holes in all of the door sills in my Bearhawk with hole saws. Weighed all the pieces cut out, 8 lbs that I never have to carry around. Local friend of mine makes and sales Carbon fiber doors for the Bearhawk, while not cheap, you save from a 4.5 lb door down to a 18 oz door. Almost a 14 lb total saving.

Dan
Hi

Could I get contact info for the builder of the carbon fiber Bearhawk doors please?

Thanks
 

lr27

Well-Known Member
Joined
Nov 3, 2007
Messages
3,822
Weigh some different tires before deciding which to use?

I wonder if a disk brake meant for a tandem bike might be lighter than any airplane gear, but adequate for a light plane like this one? You don't have to go down long hills with your airplane, after all.
 

Little Scrapper

Well-Known Member
HBA Supporter
Log Member
Joined
Jan 3, 2014
Messages
5,711
Location
Wisconsin
Weigh some different tires before deciding which to use?

I wonder if a disk brake meant for a tandem bike might be lighter than any airplane gear, but adequate for a light plane like this one? You don't have to go down long hills with your airplane, after all.
It's might, but it's too risky for me. Not the safety risk, but "time" risk. I don't want to do any re-engineering if it's available off the self and ready to go.

I'm struggling to get this done as it is. With a marriage, 3 kids and a business my free time is in short supply.

Generally speaking the builders who change things almost never finish the airplane. So I'm looking for building minimal using standard airplane construction protocol.
 

pictsidhe

Well-Known Member
Joined
Jul 15, 2014
Messages
7,526
Location
North Carolina
Correx. Light, stiff, cheap (or free if you harvest signs from the roadside some Novembers) Should make a great instrument panel and other stuff. 3sqft of political grade just under 1/2lb.
 

BJC

Well-Known Member
HBA Supporter
Joined
Oct 7, 2013
Messages
11,249
Location
97FL, Florida, USA
Scrapper:

A couple more thoughts about building a lightweight Cassutt.

Keep the varnish (or epoxy) used to seal the wood on the inside of the wing to a minimum. Just be certain that you use enough to protect the wood.

Route out the back side of the leading edge between the ribs.

Do not let an auto body shop painter paint it; it will have three times as much paint as is needed.


BJC
 

Dan Thomas

Well-Known Member
Joined
Sep 17, 2008
Messages
5,343
Do not let an auto body shop painter paint it; it will have three times as much paint as is needed.
Which brings to mind another risk: "Needed" versus "wanted" are often a long ways apart. Many overweight airplanes have fancy interiors, IFR panels for local puddle-jumping, and so on. The desire to impress is vastly underestimated in most of us.
 

Little Scrapper

Well-Known Member
HBA Supporter
Log Member
Joined
Jan 3, 2014
Messages
5,711
Location
Wisconsin
Scrapper:

A couple more thoughts about building a lightweight Cassutt.

Keep the varnish (or epoxy) used to seal the wood on the inside of the wing to a minimum. Just be certain that you use enough to protect the wood.

Route out the back side of the leading edge between the ribs.

Do not let an auto body shop painter paint it; it will have three times as much paint as is needed.


BJC
I was once told that a sanded rib that's varnished has 1/2 the varnish as a finished rib that wasn't sanded. I can't verify if that's true but it certainly seems to.make sense.

In fact, I think it was Leonard Millholland the designer of the Legal Eagle that told me that?
 
Top