Optimal speed

[dan-stevens]

I have not seen one but its an interesting question.

In ICE vehicles, they have transmissions which make them more efficient at close to highway speeds, in their top gear at engine efficiency RPM’s, usually designed for best efficiency at 50-60MPH.

Electric vehicles are different, they have to overcome a certain amount of rolling resistance friction, electric motor efficiencies but after that, propulsion becomes less efficient as speed increases. For best efficiency though, you have to think about hotel loads (lights, HVAC, charging phones, entertainment system, whatever).

With ICE vehicles, they tend to have a highway MPG rating higher than a city MPG rating. Electric vehicles are the reverse of that, with higher city MPG and lower highway MPG.

All that said, I’ve noticed in almost every electric car I’ve driven, best efficiency is usually around 30-40MPH, depending somewhat on how much work the HVAC has to work. Probably lower, around 20MPH, if you shut everything else off and just move the car.

Note: This is a very unscientific perspective, based on my limited understanding of batteries, electric motors and my personal experience.

[NetworkRick]

True, but still aerodynamics dictate most efficient speed. In aeronautics it’s called velocity stall, the laminar flux will rip of over a given speed creating turbulence.

In fact this is true also on cars where drag increases exponentially over certain speeds as seen in wind tunnel studies.

The inefficiencies you mentioned while being by no means negligible could become trivial compared to drag (turbulence) at a certain velocity. So I’m curious 🤓

[ROMAD]

I’d love to see actual wind tunnel tests of a full size Aptera.

[dan-stevens]

As I understand it, planes are generally calculated for optimal speed based on friction drag which makes sense, you need lift.

Cars are calculated based on wake drag, which is mostly the turbulence created by moving the car.

Since you don’t need lift for a car, the most efficient aerodynamic speed of a car is basically as slow as you can drive it. Other things will make this not the most efficient overall speed, but for aerodynamics, it seems that simple.

And yes, planes have significantly different rules because you want them to get off the ground. Formula 1 cars have their own set of rules as they have significant both friction and wake drag.

That said, its been a LONG time since I studied fluid dynamics. I’m open to being told I’m missing something obvious.

[NetworkRick]

True for sure.

Let me reformulate then: I’m curious at what speed an Aptera will develope significant rear turbulence 😄

I’d also love to see a video dedicated to aerodynamics of an Aptera. Not that I think they’ll reveal their secrets for now, but still…

Maybe I’ll need to bring mine into a wind tunnel when I get it 😂

[dan-stevens]

I wonder if they would post some of their virtualization software renderings?

They have shown glimpses of them in various videos, would be interesting to get a better walk through of them.

That said, my experience with multiple EV’s is between 50 and 70mph, you can see a marked drop in efficiency, then above 70, it really drops like a rock.

This part of class I remember though, aero drag is the square of the speed, so it makes sense that as you go faster, the tail turbulence is going to increase causing less efficiency. Just the point you really notice it can vary a bit I guess.

[gary-aulfinger]

A long time ago, I put good old equations for aerodynamic drag, energy to move mass, etc into a spreadsheet and plotted range vs. speed for a number of cars, like Chevy Bolt and Aptera. The optimal range I got for the Aptera is at about 31 mph, compared to 28 mph for the Bolt. Once aerodynamic drag kicks in, range goes down. It’s more a question of how quickly range goes down at higher speeds.

In my model, the Bolt’s 65 kWh battery goes from 359 mi at 28 mph to only 181 miles at 75 mph. The Aptera 40 kWh modeled 443 mi at 31 mph and 296 miles at 75 mph. It’s just a model, though from my Bolt experience, the numbers are plausible. Still, the rate of decline with speed is seriously improved by Aptera’s efficiency (roughly 67% of optimal range retained with Cd 0.13 of Aptera vs. 50% with the Cd of 0.308 Bolt).

• This reply was modified 1 year, 6 months ago by  Gary Aulfinger.
• This reply was modified 1 year, 6 months ago by  Gary Aulfinger. Reason: Minor typos

[dan-stevens]

Interesting…

Wish I understood how to do this kind of math!

I know when the Nissan Leaf first came out, a lot of owners wanted to join the 100 mile club (100 miles on a single charge, car was rated for 80’ish). They would literally drive for 3+ hours at 30-40 MPH, which sort of mirrors your statement of optimal speed.

I always just assumed it was balancing a known, this works speed, with the patience of the driver, mixed a bit with bladder control and a touch of you couldn’t run the HVAC system to make it work.

I know from my SparkEV, Bolt, Tesla that driving at 30-40 MPH gets me FAR more range than 70+ but always assumed driving slower would be better except I didn’t really have patience to drive slower for any real distance.

[Jeff]

What’s CX?

[NetworkRick]

Aerodynamic penetration coefficient.
Calculated as CX = DA / ½* ρ* V2 *A
where DA is the air resistance divided by half of air density multiplied by velocity squared multiplied by the area exposed.

The lower the CX the better

[SteveW]

Your question is one of many similar ones on multiple forums caused by the lack of real-world information from Aptera. I hope they give us some numbers with delta. I think the gamma is 3 wheel drive and we should really have numbers with 2 wheel drive so gamma could be out. The delta will have the newer motors so we could have to wait for that. They need to do a loop test at 50,60,70 mph giving watt hours per mile at each speed. Nothing else matters to anyone who will take it out on the highway. The EPA test doesn’t matter. Wind tunnel testing is a waste of time and money to get another 0.1%. A simple loop test gives real world numbers and answers the questions people are asking when trying to make buying decisions related to battery size. It would take an hour or two to do. Drive a given speed, look at the screen, note the wh/mile, go the other direction, repeat, average the two. Done. They could be really responsive and get these numbers RIGHT NOW with gamma and say that it will be better with new motors and 2WD, or as soon as they have a delta. The other extreme is they could furnish only the EPA number whenever they deliver the first Paradigm and leave it up to owners to do the tests.

[NetworkRick]

Agree 90%, the remaining 10% is for the 3 wheel drive I want to know about as I’ll get this one 😉

Don’t care for EPA numbers, totally useless in real world 😅

[paul-kirchner]

Not sure that Gamma is three wheel drive.

[kerbe2705]

@Paul Kirchner The Gamma prototype is the 3-motor variant, still using the “old” Elaphe motors. Delta should have the new, optimized-for-Aptera motors.

[omar-buitron]

i am hoping for 70 mph as the optimal speed for the Aptera, because of the low coefficient of drag

it would be GREAT if it was 80+ mph for the highways speed here in Texas but that might be to much to ask for even from the great design they came up with. i do know i can not wait to receive my order of tri-motor w/1,000 range to see/feel how the car handle on imperfect city’s roads and high speed driving at night when the highways are empty 😈

[NetworkRick]

For me 60 would be fine, 70 seems pretty high but might be possible. 80 would be a wonder, I know no petrol car with max. efficiency at 80 mi/h

[jonah-jorgenson]

Physics would say optimal speed much lower than 70mph. Aero makes it more efficient at that speed but not most efficient. But we can hope I guess. 😉

• This reply was modified 1 year, 6 months ago by  Jonah Jorgenson. Reason: correct spelling

[NetworkRick]

As long as I can use highway speeds around 62-68 mph (100-110 km/h) without her chewing up battery like a hog I’m happy for daily commutes, leisure drives are a different thing altogether 🙂

[Qiang]

If “optimal” speed is defined as the speed at which the range is maximized, then the “optimal” speed is where the power output = power input from solar. The range is unbounded at that speed. 😀

I vaguely remember seeing a post showing that with 700w solar input, the breakeven speed is somewhere around 15 mile/hour.