# Does the wifi uplink antenna affect aerodynamics or efficiency?

The wifi uplink antenna on airliners seems like a quite aerodynamically inefficient hump, just sticking up into the airflow above the main cabin. If it added even 0.1% more drag, I don't see how it would be financially viable.

(Photo by me, released to public domain)

How much does that big antenna drag? Why not either smooth it out forward and aft or build it into the body of the plane?

• Not directly the point of your question, but it's not a WiFi antenna, it's a radome protecting the satcom antenna/RF amplifier used to transfer the traffic between the WiFi access points located within the cabin and Internet, via a satellite. The satcom antenna actually transfers both wireless and wired traffic on board, and voice as well as data. – mins Nov 17 '16 at 23:18
• Well, yes, if you want to be technical. – RoboKaren Nov 18 '16 at 1:16
• It seems an important detail. The antenna looks mechanically adjustable for tilt and electronically adjustable for pan. From the image the answer to your question is that locating the adjustable satcom aerial it into the body of the plane requires an alteration of the struts because of its size. That means also to update the airworthiness certificate. The WiFi access points antennas are much smaller and not mechanically adjustable don't have this problem.. – mins Nov 18 '16 at 6:21
• @RoboKaren Since there are mostly technical people reading here, why don't you replace "wifi antenna" with something like "wifi service uplink antenna" or "satellite antenna for internet uplink" ? – bogl Nov 18 '16 at 12:42
• One factor you didn't consider is that the entire airline might not be financially viable if it doesn't offer WiFi to business travelers on key routes, even if it increases drag. If you're negotiating a corporate contract for hundreds of seats for busy employees who frequently travel between offices, not having internet access is going to be a big disadvantage. As an example, United had WiFi on its business travel-heavy SFO/LAX-NYC transcon routes years and years before it started rolling it out to the rest of the fleet. – Zach Lipton Nov 18 '16 at 22:50

Any kind of antenna like that is creating an increase in the flat plate area of the aircraft and thus increasing the parasite drag on the airplane. Fortunately newer antenna designs, such as the KU/KA band satellite internet systems can be arranged in a manner which greatly reduces the frontal area of the antenna.

The ideal solution, which will inevitably come about on future airliners as these systems increase in popularity with the travelling public, is the creation of an integral compartment on the dorsal side of the fuselage aft of the rear pressure bulkhead and fitted with a dielectric which is flush with the aerodynamic envelope, thereby eliminating parasite and interference drag as well as affecting airflow over the vertical fin. However for earlier aircraft or those currently in service, this is not going to be possible for several reasons.

First, an installation like that would require major modifications to aircraft structure and pressure vessel of the fuselage and cost big, big bucks in order to get an STC or approval from the airline's engineering department. Current designs are arranged to as they are to be minimally invasive; all that's required for a typical installation is the attachment of a series of brackets and longerons to the cabin frames which can then be attached to brackets outside of the aerodynamic envelope with a minimum series of fasteners penetrating the fuselage. This arrangement is preferred by the airlines as their engineering departments and the FAA or other regulatory authority can sign off on the installs without requiring the OEM's involvement.

Secondly, many jets are not owned by the airlines themselves but are leased from financial brokerage firms and any kind of system which minimizes alterations to the airframe is preferable to major - and irreversible - alterations to the aircraft.

As for the economics of these systems, even if you did have an increase in fuel burn by 0.1% you could easily compensate for the loss if installed on a large enough aircraft. Let's assume on a 747-400 flight from LAX to Narita Intl in Tokyo, you will typically burn about 290,000 lbs or about 44,000 gal of JET-A. A 0.1% increase results in a mere 44 gal difference in fuel consumption at about $220.00 if you price the fuel at 5 USD/gal. For a long haul fight like that, lets say that 25% of the passengers use this service at a cost of 44.00 USD each. With a typ. 747 seating of 440 people, that's 100 passengers buying WiFi service totaling 4,400 USD in income for the carrier. It's well worth it for them! • Great answer. I think you're widely optimistic about wifi utilization though. Of all the passengers, maybe half will use their laptop or tablet. Of those, maybe one fifth optimistically might buy the \$40 internet pass. But half of that is maybe given up in licensing fees. So would be 440 * .5 * .20 * \$40 * 0.5 = \$880. But I guess that's still \$836 in profit. – RoboKaren Nov 18 '16 at 1:21 • @RoboKaren If you only need to "save" 220 USD per flight, you can just shop around for a better deal on the champagne you serve in business class ;) – alephzero Nov 18 '16 at 2:20 • @alephzero, you could do that without installing a WiFi too and come out even better. No, to make sense, it is the WiFi that has to make up for the 220$ per flight. – Jan Hudec Nov 19 '16 at 15:35

Anything new attached to the outside of the airplane will change the aerodynamics and efficiency in someway; that is unavoidable.

The only question is if it will disrupt the airflow in a way that has a significant negative impact on the plane.

You'll recognize that the antenna is installed very close to the CG, so the new air resistance has virtually no pitching moment.

It is far from any control surface, so those all work the same.

The increased air resistance affects fuel economy a very tiny amount, but paying customers have enough preference for in-flight WiFi that its worth it.