# Drafting (behind an 18 wheeler) **URL:** \ **Category:** Factual Questions\ **Created:** [January 19, 2010, 3:45am UTC](https://boards.straightdope.com/t/drafting-behind-an-18-wheeler/525520 "2010-01-19T03:45:58Z")\ **Posts on this page:** 14\ **Page:** 1
**Author:** ![dwpenn](https://avatars.discourse-cdn.com/v4/letter/d/fbc32d/32.png) [@dwpenn](https://boards.straightdope.com/u/dwpenn)\ **Post date:** [January 19, 2010, 3:45am UTC](https://boards.straightdope.com/t/drafting-behind-an-18-wheeler/525520/1 "2010-01-19T03:45:58Z")
Concerning the [1932 Helicron](http://members.cox.net/dwpenn/helicron.jpg), a propeller pulled car. Somehow my nephew and I got into the discussion of drafting, and I postulated that the Helicron would actually have to work harder (be less efficient /propeller spin faster) to draft behind an 18-wheeler because there would be less air for the propeller to grab (kind of like a helicopter at high altitude) . For comparison’s sake, less efficient as compared to a rear wheel drive vehicle of the same weight and profile. 🙂 ---
**Author:** ![snailboy](https://avatars.discourse-cdn.com/v4/letter/s/97f17d/32.png) [@snailboy](https://boards.straightdope.com/u/snailboy)\ **Post date:** [January 19, 2010, 3:52am UTC](https://boards.straightdope.com/t/drafting-behind-an-18-wheeler/525520/2 "2010-01-19T03:52:51Z")
There might be thinner air for the propeller to move, but that’s evened out by the fact that there’s less drag on the rest of the car. Then you consider the tail wind. It should actually have an easier time drafting, at least if the low pressure area is as wide as the car. ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [January 19, 2010, 6:46pm UTC](https://boards.straightdope.com/t/drafting-behind-an-18-wheeler/525520/3 "2010-01-19T18:46:08Z")
Does the helicron also provide drive through the wheels, or only the propeller? If it’s only the propeller, I think it’d be less efficient, since its propulsion would drop, but friction with the ground wouldn’t. Consider the limiting case of a vacuum behind the 18 wheeler; there’s no propulsion no matter how fast the propeller goes, but friction from the road will slow it down. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [January 19, 2010, 8:16pm UTC](https://boards.straightdope.com/t/drafting-behind-an-18-wheeler/525520/4 "2010-01-19T20:16:55Z")
Drafting doesn’t work because there’s less air behind the semi (there is, but only to a completely negligible degree), but because the air that’s there is moving. I don’t see a problem for a propeller-driven car. ---
**Author:** ![cornflakes](https://avatars.discourse-cdn.com/v4/letter/c/9d8465/32.png) [@cornflakes](https://boards.straightdope.com/u/cornflakes)\ **Post date:** [January 19, 2010, 8:29pm UTC](https://boards.straightdope.com/t/drafting-behind-an-18-wheeler/525520/5 "2010-01-19T20:29:22Z")
I see the opposite problem. Since the Helicron is more affected by airspeed it may run a risk that the driver would not be able adjust for added acceleration as it gets closer to the trailer. ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [January 19, 2010, 8:38pm UTC](https://boards.straightdope.com/t/drafting-behind-an-18-wheeler/525520/6 "2010-01-19T20:38:56Z")
> [@Chronos](#): > > Drafting doesn’t work because there’s less air behind the semi (there is, but only to a completely negligible degree), but because the air that’s there is moving. This nails it. The prop-driven car would be capable of lower maximum power due to the very slight decrease in pressure, and the decrease in efficiency a propeller suffers when operating in turbulent air. In return, it has what amounts to a significant tailwind. It’s a big net benefit. ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [January 19, 2010, 8:42pm UTC](https://boards.straightdope.com/t/drafting-behind-an-18-wheeler/525520/7 "2010-01-19T20:42:05Z")
> [@cornflakes](#): > > Since the Helicron is more affected by airspeed… Why is that? > [@](#): > > … it may run a risk that the driver would not be able adjust for added acceleration as it gets closer to the trailer. Can’t he modulate throttle and brakes like any other driver? ---
**Author:** ![TimeWinder](https://avatars.discourse-cdn.com/v4/letter/t/bcef8e/32.png) [@TimeWinder](https://boards.straightdope.com/u/TimeWinder)\ **Post date:** [January 19, 2010, 8:48pm UTC](https://boards.straightdope.com/t/drafting-behind-an-18-wheeler/525520/8 "2010-01-19T20:48:34Z")
> [@ZenBeam](#): > > Does the helicron also provide drive through the wheels, or only the propeller? And if it’s only the propeller, can it drive on a treadmill running backwards? ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [January 19, 2010, 10:39pm UTC](https://boards.straightdope.com/t/drafting-behind-an-18-wheeler/525520/9 "2010-01-19T22:39:14Z")
> [@TimeWinder](#): > > And if it’s only the propeller, can it drive on a treadmill running backwards? No. It runs into the truck. ---
**Author:** ![ducati](https://avatars.discourse-cdn.com/v4/letter/d/cc9497/32.png) [@ducati](https://boards.straightdope.com/u/ducati)\ **Post date:** [January 20, 2010, 12:12am UTC](https://boards.straightdope.com/t/drafting-behind-an-18-wheeler/525520/10 "2010-01-20T00:12:59Z")
> [@Chronos](#): > > Drafting doesn’t work because there’s less air behind the semi (there is, but only to a completely negligible degree), but because the air that’s there is moving. I beg to differ. Drafting works because two vehicles are sharing one surface area bringing their cx down. Having (very foolishly) ridden a motorcycle right behind a trailer for miles and miles over many years, I can tell you the air is very still in a roughly motorcycle-length bubble right behind the trailer. This is especially good to know if you get caught in the rain and must get home. Nice and dry and still back there. ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [January 20, 2010, 3:12am UTC](https://boards.straightdope.com/t/drafting-behind-an-18-wheeler/525520/11 "2010-01-20T03:12:14Z")
> [@ducati](#): > > I can tell you the air is very still in a roughly motorcycle-length bubble right behind the trailer. When air seems still to someone rolling along at 60mph, it’s because that air is moving at something close to 60mph. As Chronos said. ---
**Author:** ![cornflakes](https://avatars.discourse-cdn.com/v4/letter/c/9d8465/32.png) [@cornflakes](https://boards.straightdope.com/u/cornflakes)\ **Post date:** [January 20, 2010, 5:00am UTC](https://boards.straightdope.com/t/drafting-behind-an-18-wheeler/525520/12 "2010-01-20T05:00:49Z")
> [@cornflakes](#): > > I see the opposite problem. Since the Helicron is more affected by airspeed it may run a risk that the driver would not be able adjust for added acceleration as it gets closer to the trailer. > [@Xema](#): > > Why is that? > Can’t he modulate throttle and brakes like any other driver? He can, but the propellor is pulling him through the air; he’s not being pushed along the road by the wheels. As he gets closer, the airspeed will increase until it matches the speed of the semi. I’m pretty sure that it will increase drastically in the few feet behind the trailer. Ignoring tire friction, the inertia of the Helicron and probably a lot of other factors, the propellor will pull the car at ground speed when far from the semi since airspeed is roughly equal to groundspeed. Close to the trailer, the prop will want to pull the Helicron at a speed equal to twice groundspeed since the air is traveling as fast as the semi, and the biggest changes will happen very suddenly. ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [January 20, 2010, 2:49pm UTC](https://boards.straightdope.com/t/drafting-behind-an-18-wheeler/525520/13 "2010-01-20T14:49:48Z")
> [@cornflakes](#): > > Ignoring tire friction, the inertia of the Helicron and probably a lot of other factors… I think this is the problem, since one of those other factors is aerodynamic drag, which at highway speeds is the largest force any propulsion system has to overcome. When the Helicron driver lifts his foot off the accelerator pedal, thrust goes to a very low value and drag slows the vehicle (pretty much exactly what happens in a conventional automobile). > [@](#): > > …the propellor will pull the car at ground speed when far from the semi since airspeed is roughly equal to groundspeed. I’m puzzled to understand this. The ground has no speed. In order to provide useful thrust in still air, the Helicron’s prop will need to discharge air at a velocity considerably greater than the vehicle’s speed over the ground. > [@](#): > > Close to the trailer, the prop will want to pull the Helicron at a speed equal to twice groundspeed since the air is traveling as fast as the semi… It would be much more accurate to say that because the semi is moving a lot of air (and because at highway speeds air drag is significant), the power required to drive the Helicron decreases as it approaches the semi - just the same thing that happens with a conventional automobile. Note that, unlike tires on powered wheels, propellers operate with a lot of “slip” with respect to the medium (air) against which they push. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [January 20, 2010, 6:52pm UTC](https://boards.straightdope.com/t/drafting-behind-an-18-wheeler/525520/14 "2010-01-20T18:52:10Z")
If the driver of the Helicron would ordinarily need to floor the gas pedal to go at highway speeds, and he floors the pedal drafting behind the semi, then he’ll go faster than usual and end up crashing into the semi. But the same is also true of an ordinary wheel-driven car. When the driver of a wheel-driven car pulls in behind a semi, he eases up on the gas a bit to maintain the same speed, and a driver of a Helicron would need to do the same thing.