Common / Useful Design Metrics for Aircraft Comparison?

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SamP

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When watching Barnaby Wainfan's videos, he brings up the concept of transportation efficiency (useful payload / Gross weight * Lift/Drag) as a metric to compare aircraft. I wonder what other metrics are used to evaluate different designs. Pure Lift to Drag is a common one as well. I've also seen from Torenbeek's Advanced Aircraft Design Mach # * L/D as a metric.

Are there other ones that are typically used?

Thanks!
 

Vigilant1

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I have seen the maximum speed (or cruise speed) to stall speed ratio used, I suppose it is meant to denote the flight speed flexibility of an airplane. A plane with a high ratio (3:1 is good, some planes are 4:1) can land on a relatively short strip and still make relatively good speed enroute. Now--can it take off from the short runway it used to land?
 

addaon

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Yeah, I find max cruise / stall very useful -- especially for understanding design proposals that are "like <X>, but 30 kts faster"; if that comes with a 10 kts increase in stall, it makes the change uninteresting, and basically equivalent to just scaling down the wings to increase wing loading.
 

BJC

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When watching Barnaby Wainfan's videos, he brings up the concept of transportation efficiency (useful payload / Gross weight * Lift/Drag) as a metric to compare aircraft. I wonder what other metrics are used to evaluate different designs. Pure Lift to Drag is a common one as well. I've also seen from Torenbeek's Advanced Aircraft Design Mach # * L/D as a metric.

Are there other ones that are typically used?

Thanks!
You might be interested in the C.A.F.E. Efficiency races. https://cafe.foundation/v2/pdf_cafe_cafe400/81CAFE250.pdf


BJC
 

arj1

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@SamP, what about the ones used by the commercial aircraft?
For example, fuel consumption, i.e.: L/100nm/PAX
 

wsimpso1

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Span loading - gross weight per unit length of span, good indicator of wing efficiency and altitude capability;
Wing loading - gross weight per unit area of wing, good indicator of stall speed and runway needed;
Power to weight ratio - power to gross weight, good indicator of runway acceleration and climb rate;
 

cluttonfred

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I find span loading (gross weight/span), wing loading (gross weight/wing area), and power loading (gross weight/max engine power) to be the most useful. I also find that playing around with design spreadsheets like these from the LAA helps me to better understand the tradeoffs.

 
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llemon

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Gabrelli-Von Karmen;
gvk diagram original.png
Dividing the speed by the G (P/WV) results in a "score" that makes comparing different aircraft easy. The maximum line = 6000
gvk scores.png

Another is Pazmanys efficiency score, which is what led to CAFE;
= (Vmax/Vso)*(cube root (S/P))*(square root (W/S))

CAFE 250, this was used in the first few (2?) years of CAFE races
= Vmax*MPG*(sqrt(payload sans fuel))

CAFE 400 is what superseded 250;
= (Vcr^1.25)*MPG*(payload sans fuel^0.75)

Finally CAFE had the "Triaviathon"
= (28110625*(Vmax*ROC)^2) / ((4100625+Vso^4)x10^9)

Crawford gave this performance metric in his book;
= (Payload * RoC)/(hp * 33000) * (1-(Vmin/Vmax)

Austin Meyer, the developer of X-plane, came up with this metric;
= (square(Vcr)*(max range w/ 45min res)*(payload w/ full fuel) ) / (Vmin * fuel in gal)

There is also this metric I found in a Aeronautical Engineering Review article;
aircraft performance rating aero eng rev 48-5.png

My own preferred is Joules per Joule, ie how much energy is consumed vs the kinetic energy of the payload. For piston engines it'd be
=(kg of fuel consumed * joules per kg) /(0.5*payload sans fuel in kg*velocity in m/s^2)

I like the above because it allows for a fair comparison between electric, diesel and gas. It also accounts for structural efficiency and prop efficiency.
 

arj1

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I’m kind of simple minded, so I apply KISS, and reduce it to a single metric: “How fast does that thing go?”
Surely you count the Fuel Flow in this? Otherwise, the Jet Fighter with afterburner is the best. :)
 

PiperCruisin

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Depends on what is important to you.
I work in rail. We care about energy required per payload (or passengers) distance or [(payload or passenger) x (distance)] / (energy or quantity of fuel consumed). Obviously rail is quite good.

Personally, for aircraft, I care about takeoff distance, rate of climb, payload, cruise speed, range and miles per gallon. I live in a mountainous, high desert area. I'm less concerned about crossing a bunch of boring landscape quickly than making sure I can takeoff from non-paved airstrips and not make a crater in a mountain side.

In the end, the biggest factor is cost. Does it get in the air and can I afford to do that repeatedly and safely.

So how do you choose something? Establish minimum requirements, and then look at efficiency? Go with what looks pretty? Looks are an important criteria to a lot of people. Me, I don't like bug ugly, but I put a premium on function. Maybe do a House of Quality matrix. There is still subjectivity, but helps with decision making.
 
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dog

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a large interior volume to gross weight ratio (iV/gw)™ will it carry more than its own weight
and is the difference between stall and cruise
more than 3x
wheels would be nice for sociability
 

dog

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It always is, if you select a favorable value for x.
I'll add the notion of a plane thats easy to get in and out of and comfy nice to sit in,which is of course enough info to derive a value for X
slow airplanes still go a 120 knots
and my interests are in lifting #1200 at the lowest
speed ,cruising at 3x that,and having some extra
oomph for contingency
 

cluttonfred

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I'll add the notion of a plane thats easy to get in and out of and comfy nice to sit in...

That's probably an aspect of homebuilt aircraft design that gets less attention than it really should. Here is how Eric Clutton tackled it.

 

dog

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You missed my point, but that’s OK.

Vc - Vs = 3x is not the same as Vc > 3Vs

Math jokes can be obtuse. 😉
I got your joke,and chose to be oblivios
my math head space has been entirely taken over with applied geometry in the shop,and money math in the office
now people are admonishing me to be better at
time managment,which is testing my patience,as they are all people who want stuff done on there
shedule(time),and pushing,and I know that if I suggest that any help that they can provide in changing the time constant would be great,I know I will get blank missunderstanding
back to getting stuff built @ 1min/1min
 
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