Weight estimations based on modern composite airframes?

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karoliina.t.salminen

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Typical weight estimation data is pretty old today, like the one in Raymer's book.
I think it would be useful to have statistics of modern aircraft airframe weight statistics split by at least major component, or in minimum the total empty weight of the airframe minus engine and minus propeller. Sailplanes are good in that sense, at least it is possible to know what is the empty weight without engine because there is none. However, to come up with empty weight of airframe one has to know the weight of the engine installation on the particular aircraft. For DA40, I can probably ask from our mechanic because we changed engine and prop on it and the crate had some weight printed on it. How about other planes on market, like Cirrus, DA42, Cessna Columbia models, Lancair Evolution etc.? Does anyone have exact numbers?

In conceptual early sizing estimations it seems that empty weight - gross weight ratio is often used. But this ratio always includes the engine install. The important question would be instead: how much would the composite airframe that can carry total load (including engine) will weight for given speed envelope. This would be useful for example estimating electric aircraft where battery weight is going to be the heaviest component of the plane and the motor/motors have in comparison insignificant weight. As a reference, the Lithium battery pack found in Tesla cars weights around 540 kg which is more than a typical gasoline engine + fuel would weight, and is going to be the main driver of the design of electric aircraft. Airframe needs to carry 540 kg batteries + motors + propellers + ESC, baytery load management/balancing system etc. in addition to the payload. The payload becomes lesser portion of the gross weight as most of it is consumed by the enormous battery pack.
Has anyone Excel sheet or similar with statistics of composite airframe or better component weights, compared to gross weight of that airframe?
 

cheapracer

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I wanted some weights recently, easy as going to relevant brand forums and searching for weight and balance sheets. - or just Google airplane weight and balance sheets.

Oh and by the way, I am involved in the start up of a 4 brand electric car Dealership, I will be seeking the various component weights myself, is that useful to you?

This is my current daily driver ...

http://jacen.jac.com.cn/showroom/iev6s.html
 

Topaz

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Karolina, Jan Roskam's Airplane Design Part 1: Preliminary Sizing of Airplanes has much better trend-line empty-weight-fraction data for composite homebuilt airplanes (it has a specific set of data for that niche alone) than Raymer's book. However, even Roskam's data is for fairly older types: Q2, a composite VariViggen, VariEZE, LongEZ, so this data is, with the exception of the Q2, for fully moldless composite construction.

EDIT: I just checked Gudmundsson's General Aviation Aircraft Design and he doesn't really go much into trend-line data, instead using statistical formulae for particular airframe components. He presents both Raymer's formulae and those of Nicolai, but these are generalized formulae that are probably heavily-weighted towards aluminum airplanes.

Yes, there's a dearth of really good specific data for "modern" composite aircraft. It's possible to create your own empty-weight-fraction trendlines from published manufacturer data, but it'll only be as good as the information you can find on the example airplanes.

Moderator Note: Snipped out three completely off-topic posts. Let's stay even remotely in the neighborhood of the original topic, guys and gals.
 

autoreply

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The short answer is that such data only exists in private collections.

The problem goes much deeper.

The traditional "sizing" method assumes weights are fairly proportional, to loads, wing loading, aspect ratio etc. True for most airframes, whether it's an airliner or a fighter. Not true for many homebuilts.

Two examples truly excel there. One is skin weight. We can do all the calculations we want, but skin weight is mostly driven by minimum gauges, handling loads and what's commercially available. Interestingly, for aluminium airframes the same problem exists.

The other is systems weight. Many control systems are several times heavier than on otherwise comparable airframes. Small decisions and choices add up quickly; I have two airframes in my database that have a control system under a third of the weight of what the trend line would suggest.


Carbon fiber is maybe the biggest contributor. Most data that exists is for fully glass fiber airframes. Carbon fiber totally changes the equation. Strength is only a good bit higher. But stiffness is way higher. Buckling loads drive most light airframe layup decisions, so the change from glass to carbon fiber is highly non-linear.

Worse, many well-known composite airframes are not only glass fiber, even for glass fiber they're pretty heavy. The Cirri and the Diamonds are good examples. If you compare the weight of the composite structure of say the DA20 and the MCR01 (both 2-seaters using the Rotax 912S), you're blown out of the water if you discover that the MCR01 has about a third of the DA20's structural weight.

We've recently seen that pop-up on HBA with a discussion on wing weight. (https://www.homebuiltairplanes.com/forums/showthread.php?t=27115) For a glass airframe the tradtional "sizing" method works reasonably well and lightest wing weight for a conventional composite wing will be an aspect ratio of 5-6. For a carbon fiber spar, the optimum is suddenly over twice as high; since most of the wing and spar weight is already at the minimum gauge and thus weight, even if we up the AR quite a bit.

Personally, I design more by "equivalent" designs: what do comparable designs get for weight of this or that component. This gives a good realistic number, but it takes a lot of hard data from existing designs.

Why I won't simply put all that data out in public? That's how I make a living ;)
 

karoliina.t.salminen

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Yes. I know the MCR01, I have flown one. But the reason is simple. The MCR is a mini-plane with full carbon structure whereas the DA20 has carbon only in spar. The Diamond has also (according to my mechanic) not so good fairings (from weight perspective): they have wing eg joint filled with lot of micro and that is sanded smooth instead of the mold having this shape included and this adds extra weight as there is a lot of solid filler in the joints. They do not have the 5-axis CNC there apparently that eg Pipistrel has and they seem to be shortcutting some details in quite homebuilderish kind of way. Which is somewhat surprising. But this is how state of the art in airplane manufacturing seems to be, quite low tech that even homebuilders are able to match.

However the data described above by everyone is coming with systems and engine weights and I was looking for the bare composite shell weight fraction to MTOW.
 

karoliina.t.salminen

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Basically for the electric plane, it would make sense to exclude weight of everything except airframe. Landing gear, control surfaces, canopy and and flaps belong to the airframe, so these shold not be excluded, but engine and all of its systems should be excluded to estimate how much there would be allowance for batteries.
 

TFF

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Estimation wise, just subtract TCDS airframe weight from TCDS engine weight. It would be within 100 lbs of right. 100 lbs is optimization space. The difference of a 900 lb battery and 1000lb one is very small until you really start flying one. Even then testing will be a while before capacity would be used to max.
 

proppastie

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TCDS=Type certification data sheet, and same thoughts I had, All aircraft need seats and the various other weights.
 

ThadBeier

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The plane that most impresses me as a light composite plane is Facebook's drone Aquila. Now, I recognize that there's nobody on that plane -- but it has a wingspan greater than a Boeing 737, but weighs less than 1,000 lbs (400 kg.) And, of course, it's designed to fly from 60,000 to 90,000 ft for three months. An astonishingly light airplane.

https://www.facebook.com/notes/mark-zuckerberg/the-technology-behind-aquila/10153916136506634/
 

Norman

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The plane that most impresses me as a light composite plane is Facebook's drone Aquila. Now, I recognize that there's nobody on that plane -- but it has a wingspan greater than a Boeing 737, but weighs less than 1,000 lbs (400 kg.) And, of course, it's designed to fly from 60,000 to 90,000 ft for three months. An astonishingly light airplane.

https://www.facebook.com/notes/mark-zuckerberg/the-technology-behind-aquila/10153916136506634/
Yeah, you can do a lot by getting rid of the pink parts.
 

wsimpso1

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I agree, sharing data is good where we can get it. I also understand the professional's requirement to keep hard won private data private...

I will share my bird and compare it to Thurston's plot in Design for Flying. Thurston's 2000 pound gross weight was what I grabbed for comparison to my 2150# 6g airplane:

Engine + Constant Speed Prop 425/21% 430/20%
Flapped Wing 280/14% 220/10%
Fuselage + Cowl 190/9.5% 176/8.2%
Fixed Stuff 180/9.0% 143/6.7%
Landing Gear 100/5.0% 108/5.0%
Tail Planes 46/2.3% 50/2.3%

Where do I see differences from the traditional? I saved weight in the wing, mostly because I have higher wing loading and thus less wing area than on traditional birds...Other help is that modern avionics is translating to less weight. I suspect that Thurston's info is for a 4.2g airplane in metal which means design to 6.3g, while my bird is designed for designed to 12g loads, but in most of the structures, that will have only minor effects. Everything else in my carefully designed fiberglass bird lines up pretty well with traditional numbers, and was not really a surprise. In fiberglass, we can not actually expect to save much over metal except in places where we can tailor things - spars and cockpit security zone mostly. Now if the bird was graphite cloth, there would have been more taken out of the skins everywhere, but a lot of other stuff would have been the same weight anyway...

I hope that this helps...

Billski
 
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