How fast?
 

I was flying my plane this past weekend and some kids came up to me and asked, "how fast does your plane go?" I didn''t know how to answer that question...so I thought I''d poll the forum.

I have a range of planes...from a simple electric park flyer to a .46 size Stix to my .82 saito low wing, Venture 60. Any way to measure airspeed?

Is there some sort of device I can install that will tell me my air speed?

RE: How fast?
 

A police radar gun works well. A mathmatician with a stop watch and you flying in between 2 fixed points will work or you can do what I do, take a stab in the dark and say 30 MPH. Good Luck, Dave

RE: How fast?
 

[link=http://www.amazon.com/Mattel-J2358-Hot-Wheels-Radar/dp/B000EHLB0M]Hot Wheels Radar Gun[/link]

$26.95

Ken

RE: How fast?
 

30 MPH??? What kind of ninny aircraft do you fly anyway? :D:)

RE: How fast?
 

Okay maybe 50 miles per hour. Just a quess. Dave

RE: How fast?
 

You''d be surprised how fast some planes are. 70-90 mph isn't uncommon on a lot of sport planes.

http://www.amazon.com/Bushnell-10-19..._sbs_t_title_1

RE: How fast?
 

Sure. There is a unit called "How Fast" sold by these folks http://www.wingedshadow.com

Might want to buy the "How High" also so when the kids ask you the height question you can answer that too ;)

RE: How fast?
 

A good place to start is to calculate your pitch speed. Many pieces of software, such as ThrustHP can do that. The pitch speed is just the speed that the propeller throws the air back at, basically the prop''s pitch x the rpm x a conversion factor for mph or kph.

That gives you a good number to start with. My AX46 powered mustang has an 11x6 prop. It turns it at 13,000 rpm, give or take a couple hundred for the weather. That returns a pitch speed of 74mph. That''s at static ground RPM.

A plane in flight is gonna achieve a different speed than this for a few reasons. First, in the air, the engine "unloads" and achieves better RPM''s. So the pitch speed goes up. We think the ''stang''s engine is turning about 15K in the air, and that would up the pitch speed to 85mph. However, the plane has drag, so it won''t likely achieve quite that. The draggier the plane, the less it''ll achieve. You know you''ve got a draggy airframe if you put it in a full power dive and the engine barely speeds up, it still has to work to pull the plane down. Slick airframes will wind their engine right out, quite an impressive sound. Some are slick enough to over-rev the engine and can occasionally blow it up.

When someone asks me how fast my planes are, I give them the static ground pitch speed as an idea of things.

I have that hotwheels radar gun to try this summer, though. We''ll see how close my guess was (''course, it''s a toy, with poor range and accuracy, but trials in teh car have been promising.... If it can see the plane, it should be within about 5%)

J

RE: How fast?
 

Oh, forgot to mention. When I clocked my Atlanta I did this. Go to Google Earth and display your field. I found two prominent landmarks (trees, etc) and measured the distance with their ruler tool. I then flew through them with a stopwatch. If you know distance traveled and you know trip time you can easily work out speed. That''s how many speed trials for real racing aircraft were often done back in the golden age.

P.S. I came up with a speed of 138 mph coming out of a shallow dive with the engine full screaming open turning a 12x12 APC. [8D]

RE: How fast?
 

Use the formulae Speed = distance/time. Measure the distance between the two points, time it and then break out the calculator. e.g. 200 metres in 5 seconds gives you 40m/s. Multiply that by 3.6 to convert to km/h and you get 144 km/h (89mph). For over there this would be 656 feet/5 sec = 131 ft/s multiply by .68 gives you 89 mph.

cheers

RE: How fast?
 

There's also a great computer program out there that will tell you the speed of your plane based on the doppler effect on the engine. You take a video of your plane making passes, then feed the video through the program and out pops the ground speed.

The "How fast" is also quite cool.

RE: How fast?
 

next time someone asks "how fast?" tell em: "damned fast at full throttle!" and save your money!

RE: How fast?
 

When some kids asked me the same question some time back I answered without hesitation 743 mph!
They where duly impressed.

RE: How fast?
 

If you belong to the revvers club anywhere between 100 & 200 mph!

RE: How fast?
 

I remember a day in Rocky Hill, CT in the early 80's when an officer with a radar gun stopped by to watch us fly. My retracts weren't working that day, but my plane, gear down, level flight was at 112 mph.

RE: How fast?
 

Oh yeah! :D