Rating the Homebuilts - speed/power coeff

Low timers and high timers alike usually break their airplanes against the ground. Try using landing speed / stalling speed.
I've done a similar exercise, although with a different purpose (I was trying to optimize load per time per dollar). Since engine power and top speed have some direct relation to each other, I wouldn't have used them together in the same equation. Try picking performance figures that are as independent as possible (also a contentious issue).



Steven Fahey, CET
"Simplicate, and add more lightness" - Bill Stout

Steven,

Thanks for your reply. I found that the results of my two-seater study gave a wide range of Vmax/(power^0.20) ratings. The "skyrockets," even low powered ones, had ratings ranging 90-100. The more docile planes ranged 40-50. (For comparison, the Warrier was rated 46; the later Bonanza, 66.)

In addition, I found that the low-wing models ranged higher than the high-wing models, 50's-100's vs. 30's-80's. It indicates that the low-wing models are designed racier. This study is aimed at keeping the new homebuilt flyer out of trouble. It goes back to an experience I had some years ago, when I saw a new homebuilder go around the patch with high PIO. He nearly killed himself.

Mech Engr, CFI ret, and EAA Tech Counselor.

High wings and retractable landing gear don't go together well, so your conclusion is not surprising.
In my analysis, the Van's RV-9 and Zenith 601's just couldn't be beat for overal utility per dollar. They beat out lots of cheaper planes because I was also factoring in speed. Using my non-dimensionalized performance numbers, popular certified aircraft cost 2-3x more.
By tweaking my numbers I could skew things more toward utility, and put the high-wings on the top of the list.

It would be much harder to quantify and compare more complex issues like control authority, trimmability, and how easy it is to keep an airplane rigged properly. Even issues like CG travel during flight could be brought into an equation, if you really wanted to get fancy.

For top speed, have you been using strictly V_H (maximum level airspeed, in regulatory parlance)?

Steven Fahey, CET
"Simplicate, and add more lightness" - Bill Stout

Steven,

Now you have my curiosity. What non-dimens group do you use?

My view is that Speed/(Power^0.2) (~Benefit/Cost) has a lot of parameters buried in it. For example, low wing area and high stall speed would be coupled with high Vmax. This would produce a high rating.

I confirm that the RV-9A and the Zodiacs are relatively docile airplanes suitable for low timers, and I have presented this to my EAA chapter. Counter to most kit designers, the RV-9A and the Zodiacs are more docile than their earlier cousins. They seem to be aimed at the low time pilot and sport pilots.

Thanks for your comments.

Guys...

Guys… my (2cents worth)…

I developed the following relationship [A] for an aviation safety study I almost started years ago. It was an attempt to define relationship between size, power, weight, performance and aerodynamic “cleanliness, etc… for propeller driven acft [piston or turbine]. The relationship was an attempt to define a relative factor that would scale up with size, power, spee, altitude, etc…

The “simple” equation boiled down to the following…

Wt/Aw | Vmax(75) |**2
--------- X |----------| = [CF]
Wt/HP(75) | Vmin(sl) |

Which simplifies to the following equation

HP(75) | Vmax(75) |**2
------ X |----------| = [CF]
Aw | Vmin(sl) |


[CF] = relative Complexity Factor

HP(75) = power [horsepower, etc] driving propeller at 7500’ above sea level [ASL] **

This accounts for affects of engine “aspiration” on performance. Obviously, piston engines that have turbo or super charging will develop full power at 7500-Ft ASL… whereas normally aspirated engines are capable of only about 75% max sea-level power. For turbine engines this also accounts for “full” VS “flat” power ratings at Sea level. Also propellers are adjusted for optimum power output at intended flying altitudes and speeds.

Aw = Wing area (no flaps) **

Vmax(75) = True airspeed at “Max power” at 7500’ASL **

Vmin(sl) = Minimum true airspeed attainable at sea level with flaps [“essentially just above stall]. **

** data can be extracted directly from pilots operating handbook.
----------------------------------------------------------

Relative relationships for a few acft I am familiar with [using some estimated numbers]…

Cessna 150: [A] = 3.08

Aw =~120-ft2
HP(75) = 75
Vmax(75) = 100Kt
Vmin(sl) = 40-Kt

Thorp T-18 [normal aspirated]: [A] = 10.17
Aw = 86-Ft2
HP(75) = 135
Vmax(75) = 140-Kt
Vmin(sl) = 55-Kt

Thorp T-18 [turbocharged]: [A] = 18.84
Aw = 86-Ft2
HP(75) = 180
Vmax(75) = 156-Kt [Yellow-line]
Vmin(sl) = 55-Kt

P-51D [Stock, turbo-supercharged]: [A] = 104.40
Aw = 300-Ft2 [estimated]
HP(75) = 1400 [estimated]
Vmax(75) = 350-Kt [estimated]
Vmin(sl) = 74-Kt [estimated]



Regards, Wil Taylor

Wil,

"Complexity" may be the wrong word, but your relationship seems to scale up without too much trouble. Take, for example, the Cessna Caravan: P=630 (@8000'), A=280, Vmax=182 KTAS (@8000'), and Vmin=60 KCAS

This gives me a CF=21.
A King Air 100 got a CF=122.

I would consider the Caravan more "complex" than a Thorp, and a King Air more complex still, but mostly because it brings to mind the number of subsystems on board. For performance beside a turbocharged Thorp, it's a cow. It's far less likely to kill you than a P-51. I like plasgears' term "hot rocket" more. Perhaps the term "demanding" would be suitable.

My attempts to non-dimensionalize aircraft performance weren't so focused on pilot ability (though maybe they should have). I was interested in utility and cost in terms that I could use to directly compare with a car. For example, miles per gallon can describe fuel efficiency of travel by either means, by dividing airspeed (mph) by fuel consumption (gph). Initial costs and overhaul costs relate well, and so does "payload", ie. occupants and baggage. I even went so far as to include "parking" fees for either mode of transportation.

So by using my car as a "standard", I was able to rate the utility of various airplanes. Clearly airplanes are luxury items, because none scored even close to driving.

Steven Fahey, CET
"Simplicate, and add more lightness" - Bill Stout