eta: super efficient 3 wheeled car (project thread)

[Christopher Jordan]

Originally Posted by diesel_john

countersTrike

so you make a mold and lay down a sandwich and what about the plug does it need to pressed. I have been looking at airfoil shapes to put around a car size vehicle but wonder because these shapes have been designed for airplanes (high speed) Is yours as narrow as possible or derived from equations. It is hard for me to believe a shape good at 300mph would be good at 60mph.

The trike was done at a recumbent bicycle factory (yes; on airport land!) by some sort of vacuum bagging used also for streamliners (at Battle Mountain I think). I did not do that part of building the trike, but I think the shell was cooked- very high temp- into shape. The material used was right out of an airplane catolog- that dark green with little gold threads is that fiberglass and kevlar unpainted.

Yes it is thin- about 36 inches wide and 96 inches long. Like many velomobiles, streamliners, electrothons; mine was done with no equations.

My trike is a similar bad imitation to the 62mph Vector trike- maybe Versatron- which held records in the early '80s, or sort of like the current 81 mph record holders. No way would I try 300mph, but I hit 40mph winds at 20 with no effect even though if combined, the 60mph wind should blow me right off course.

countersTrike

[Christopher Jordan]

Originally Posted by diesel_john

I like the shape. How efficient are the closed ones? Can you send more pics?

Vector (198-2) WAY too many pics! 3-wheelers: I gotta luv 'em!!

countersTrike

[Fuzzy]

Originally Posted by diesel_john

I have been looking at airfoil shapes to put around a car size vehicle but wonder because these shapes have been designed for airplanes (high speed) Is yours as narrow as possible or derived from equations. It is hard for me to believe a shape good at 300mph would be good at 60mph.

Reynolds Number (Re) is what really matters. Although velocity is a component of Re, it's not the whole story.

[Gone4]

Reynolds number is important in fluid mechanics but it is not the defining equation, for drag modeling, derived using similitude techniques.

If you are familiar with Similitude and Buckingham Pi then this equation is trivial to develop (if you would like me to derive it, I shall):
Drag = (w/w_m)^2([rho]/[rho]_m)(V/V_m)^2 [Drag]_m

What this tells us is that the characteristics of a model scale using those terms - no more and no fewer. So if you know the operating air density, width (component), and velocity of the airplane and your vehicle, you can scale the drag of an airplane (component) using that equation, to find the drag it will apply on your car. A side effect of this shows that shapes and components developed for airplanes DO scale to lower speeds.

I hope this makes sense

[Fuzzy]

Originally Posted by GenKreton

If you are familiar with Similitude and Buckingham Pi then this equation is trivial to develop (if you would like me to derive it, I shall):
Drag = (w/w_m)^2([rho]/[rho]_m)(V/V_m)^2 [Drag]_m

I might have been familiar with it, but forgotten about it in non-use. But looking at it now I would suggest the following changes:
Drag = (S/S_m)*(rho/rho_m)*((V/Vm)^2)*Drag_m

Where S = area (cross-sectional or planform depending on body), rho = density, V = velocity, Drag = resistive force. However this is only true if Cdo == Cdo_m, which is not entirely true. A better example would be:
Drag = (S/S_m)*(rho/rho_m)*((V/Vm)^2)*(Cdo/Cdo_m)*Drag_m

Cdo and Cdo_m would need to be collected either from wind tunnel testing or CFD, and they definitely are a function of Re.

[diesel_john]

A side effect of this shows that shapes and components developed for airplanes DO scale to lower speeds.

I don't understand entirely

[diesel_john]

Fuzzy and Genkreton keep talking. Is the scaling of the airfoil in my last post legit. I just multiplied the coordinates by 35 to make it big. Supposedly the 0024 was designed for lower drag at the expense of lift. Am I trying to make it too simple. My first question must have off the mark.

[Gone4]

Originally Posted by Fuzzy

I might have been familiar with it, but forgotten about it in non-use. But looking at it now I would suggest the following changes:
Drag = (S/S_m)*(rho/rho_m)*((V/Vm)^2)*Drag_m

Where S = area (cross-sectional or planform depending on body), rho = density, V = velocity, Drag = resistive force. However this is only true if Cdo == Cdo_m, which is not entirely true. A better example would be:
Drag = (S/S_m)*(rho/rho_m)*((V/Vm)^2)*(Cdo/Cdo_m)*Drag_m

Cdo and Cdo_m would need to be collected either from wind tunnel testing or CFD, and they definitely are a function of Re.

The equation of similitude I derived does only work under the assumption that ratios in the dimensioning cannot change. If you halve the length, you halve everything including radial dimensions. The reason for this assumption is if length and width, for example, change disproportionately then the coefficient of drag also changes, as you stated, and it introduces complexities. I'm not even sure Cdo/Cdo_m is valid without sitting down with some paper to check it. The world of secondary flow and eddies makes their modeling relationship nonlinear rendering that a looser approximation at best, or wrong at worst. The goal of the drag modeling equation was eliminating the need for full scale testing, and you are correct, if we modify it by changing geometry disproportionately then we do need to make an actual scale model for testing.

[Gone4]

Originally Posted by diesel_john

A side effect of this shows that shapes and components developed for airplanes DO scale to lower speeds.

I don't understand entirely

A quick example would be if you had an airplane airfoil that is 3x larger than you want for your car you do something like the following:

(1/3 scale factor) * (23.77/7.382 average density at sea level divided by 35,000 ft, scaled) * (60/600 grossly estimated traveling speeds) * (drag on airplane airfoil) = drag added to your car

Also if you only scale, preserving geometric relationships, then the coefficient of drag will not change on most of the sizes we would be discussing. Since you are considering scaling down size AND speed it is especially true. As you decrease speed the secondary layer flow decreases - the area where the fluid sheers off a surface, slowing down. This explanation is probably more than necessary I now realize

[Fuzzy]

The moderators might want to split this off into a separate thread in aerodynamics starting at post #43