# Aerospace/Aviation/Propeller speed

Question
Could you tell me at what speed does the propeller of a single seater light aircraft turn. Does this differ in a twin prop aircraft?

I understand the propeller of a light hovercraft has to be reduced to around 3,600 rpm by using a reduction gear or pulley. Why is this so?

How is the air pressure in a hovercraft culculated?

Many thanks.

Speeds vary with aircraft models. Cruise RPM's can be from 1800 to 2800 RPM. The diameter of the prop is the limiting factor. The tip of the prop is going faster than the hub. So the prop tip should never get close to the speed of sound. Gear boxes are used in small airplanes to allow the engine to run faster for a more smoother running engine. Example the old Cessna 176. In large airplanes the gearbox is needed to reduced the prop RPM to avoid the tip speed reaching the speed of sound. In the old prop airlines the engine were limited to 2800 RPM but the props turned only about 1100 RPM to keep the large prop tips from reaching the speed of sound. For better result four blade props were used. Your question about the air pressure for hovercraft is more complex because it involves several parameters. The aircraft weight to begin with. Lets look at pressure this way The water pressure at your house is 50 lbs. Open the water valve with an open garden hose and the pressure is only a few pounds. Put the nozzle on to restrict the water and the pressure in the hose increases. close the nozzle and the pressure reaches 50 lbs. The initial air pressure must over come the  weight of the hover craft. As in rises and the  air pressure bleeds off from the space between it and the ground yet must be high enough to support the craft. Now the pressure alone is not the problem but the volume becomes the issue to sustain the craft. In buying an air compressor not only is the pressure important but at what volume. A 100 PSI compressor may not be able to supply your air tool at 50 PSI if it requires 6 CFM. and the compressor rating cannot do it. So measuring pressure is an arbitrary thing. Like electricity, one value can be meaningless. Or engines ie 100 hp @ 5000 RPM. Hope I answered your questions.

Aerospace/Aviation

Volunteer

#### Marc Ebelini

##### Expertise

Basis aircraft and engine maintenance. Avionics and other electronic questions related to computers and radio communications.