radfordc

My Feedback: (14)

Correct on the math for a 14" prop at 9000. Theoretical speed is 119 mph.

A 10" prop would need to be turning 12600 rpm to go at 120 mph. Do you think your engine unloaded to over 12000 rpm in the air?

larry@coyotenet

My Feedback: (21)

Too many unknown variables for an answer. The speed of the Kadet checks out pretty close so I am thinking the radar gun is pretty accurate.
The 47 was clocked from a dive then pulling out and running for about 500 feet level so that may account for the difference. Also a wood prop will flex in the air allowing the engine to unload more than you might think. Don't remember what muffler he had, I think it was a pitts style so maybe a little boost there.
Two years ago one of our groups tf-p-40's was clocked at WBOTR going over that speed with a g-62 and a Menz 20x10 prop.
Larry

Desertlakesflying

My Feedback: (28)

Had the old 60 size Hangar 9 Mustang Marie with an OS 120 Surpass II pumped with a 15X7 3 blade and it was clocked with a radar gun at 125mph on a flat line pass, no dive field at 5000' ASL cold morning

jeffEE

My Feedback: (5)

If digital means I had to count on my fingers, then that's what it was. How would you factor in the drag of the airframe into the prop size x rpm equation? A P-51 would be different than a Sopwith camel with the same engine and prop set up. Wow....lots of factors to get this correct. But just how close to the exact speed does it have to be? For bragging rights? Or a contest?

Ram-bro

My Feedback: (101)

We are looking at best numbers. O% drag. So our simple calculations are just based upon the known numbers. Rpm x pitch will givr you the distance traveled in a minute. I see what i did wrong on my calcs.

radfordc

My Feedback: (14)

Wow, I did not know that a 120 four stroke could turn over 17500 rpm in the air!!

Pelle Gris

What most people forget, when calculating pitch speed, is the airfoil of the prop. The pitch is only measured as the physical twist of the blade, but no allowance is made for the airfoil lift itself.
As I see it, that could explain some of the "weird" speeds when compared to the basic data.
I have used a simple Excel spreadsheet for some time, and it´s pretty damn accurate when compared to the pitot tube system installed in the model. If this is accurate is an open question though!!!

I fly a home made towplane 103in span, and weighing in at about 44lbs with fuel.
This have had 3 different engines, with varying propellers, installed, and the calculated airspeed have been very close to the numbers supplied via telemetry. Below I have listed some of the data.

Engine / Propeller / Static rpm / Calculated speed / Actual speed
King 140RV / SEP "Wilga" 32x10x2 / 5000 / 57 mph / 61 mph
King 140RV / Fiala 28x16x3 / 4600 / 82 mph / 90 mph

Valach VM-210 / Fiala 34x16x2 / 4800 / 86 mph / 87 mph
Valach VM-210 / Menz 32x18x2 / 4600 / 93 mph / 93 mph

This spreadsheet assumes that the rpm increases about 20% in the air, and this seems to be quite close on the one setup I have been able to get airborne rpm. In this case it´s the Menz 32x18x2, which goes from 4600 rpm on the ground to about 5500 in a full throttle level pass. This matches the predicted 20% increase in rpm.

On a less draggy airframe I would expect the rpm increase to be slightly higher which then translates into higher speed, but I have not tested this yet.

Pelle Gris

I was unable to attach the spreadsheet, so for those that are interested I have added the formula below.

To get estimated speed in km/h:
((Pitch in inches*2,54*static rpm*unloading factor*slip)/60/100)*3,6
"unloading factor" is set to 1,2 and "slip" is set to 1,0

To convert to mph divide result by 1,609.

I have also attached a screen shot.

Greg Wright

My Feedback: (6)

It all depends on what kind of warbird we are talking about.

Prop or Jet? That's my Question...

Lifer

My Feedback: (1)

Since every post in this thread has referred to propeller pitch and r.p.m., I'd say we are not talking about jets.

Greg Wright

My Feedback: (6)

Karl hibbs,

Not GPS.

With Spektrum radio technology we actually use a Pitot tube that is attached into the nose or the leading edge of the wing and has an electronic sensor in it the gets plugged into out telemetry module that sends the info back to the screen on the radio to give you the true reading of how fast the plane is going.

karl hibbs

My Feedback: (6)

Greg it would be interesting to have both on board and compare, I have used the eagle tree pitot tube but it didn't have telemetry and it worked good.

Hydro Junkie

Greg, that gives you airspeed, not ground speed as they can be very different, depending on the direction the plane is flying when compared to the wind. With a 20MPH wind and an IAS of 90MPH, your ground speed could be as low as 70 or as high as 110. This is why it's preferred to land into the wind, it lowers the ground speed. It's also why an aircraft carrier turns into the wind during flight ops. The ground speed is lowered by the headwind AND the ships speed lowers the ground speed further at the point of touchdown, reducing stress to the arrestor gear on the ship and the aircraft landing on it

radfordc

My Feedback: (14)

Correct on the wind affecting ground speed. I was making the assumption that all reported speeds were the average of two runs....one into the wind and one with the wind. However, that may not be a good assumption.

radfordc

My Feedback: (14)

I compared your formula to the formula that I used in post #16.

For an engine turning a 7" prop at 10000 rpm static (unloads to 12000 rpm in the air) your formula gives a speed of 128 kph or 80 mph. My formula gives a speed of 79.77 mph. Pretty good agreement.



Still makes me wonder how that guy was able to get 125 mph from a 120 four stroke engine turning a 7" pitch prop. Would have to turn the prop over 18000 rpm in the air.

Fly2XS

My Feedback: (65)

Jeff, I thought you used a Sundial!
Are you going to Michigan Warbirds?
Scott

jeffEE

My Feedback: (5)

NO sun in Minnesota. I am trying to get to MICH. for the event, but if its more than a week away, I have been known to forget things.

TimD.

My Feedback: (207)

I have run G-62's for 20-25yrs my favorite gas engine. I normally turn mine at 7800 or so on the ground and they unload to 9000-9500 in the air. the engine can take it all day long at those speeds. So I would not doubt 10000-11000 just maybe not for long.

TimD.

My Feedback: (207)

I use the old Zinger 22x8-14 moves them good

dubs1946

There has been some discussion on the other site on pitch and airspeed etc. What was suggested and it seems correct to me, apart from the down wind , prop slippage etc. The prop has an airfoil shape and there is considerable lift forward from the airfoil and that naturally shows up as airspeed.
Not everyone agrees but i do.
Dave.

karl hibbs

My Feedback: (6)

dubs, what other site? or maybe a link, Thanks

dubs1946

RCSB, you have to join to se the threads. Its free. This is a dicussion between Mick Reeves and Detlef Kunkel on the subject.


http://www.rcscalebuilder.com/forum/...6&PN=0&TPN=102

Take the time to look at his whole thread on his Spitfire he is a master craftsman and engineer. He also makes his own engines have a look at his V 4 engine.


http://www.rcscalebuilder.com/forum/for ... PN=1&TPN=1

The V4 engine test.

https://www.youtube.com/watch?v=EuVtW9Rg5FQ

karl hibbs

My Feedback: (6)

Thanks for the info.