How fast are our war birds?
03-13-2017, 07:32 AM
#1
How fast are our war birds?
How fast are we going? That has been a topic of discussion for years. Recently our club bought a radar gun and I did some tests at our field on a nice winter day. We are located in Pueblo, CO. elevation 5,000 feet. The temp was 70 ( winters here can be mild) light breeze under 3 mph. The plane was a TF Giant P-47 ARF weighing about 26lbs. Two motors were flown on the same plane. First was a Saito 60 radial and the 2nd was a G-62. Same fuel(probably more oil than the g-62 needs) same prop, a Zoar 21/10. RPM Saito 6500, G-62 7200.
Here are the speeds, an average of several readings.
Saito-82 mph
G-62 110 mph
Here are some other speeds for comparison
Electric pattern plane, speed between maneuvers 55mph
Sig Kavalier with OS 45 glow 87 mph
Sig Kadet with OS 45 glow 77 mph
New Tower tilt rotor electric foamy 50 mph
Speed 400 type electric speed plane with an inrunner brushless and 5X5 prop 118 mph
So my conclusion is that the large war birds fly faster than we thought
Larry
Here are the speeds, an average of several readings.
Saito-82 mph
G-62 110 mph
Here are some other speeds for comparison
Electric pattern plane, speed between maneuvers 55mph
Sig Kavalier with OS 45 glow 87 mph
Sig Kadet with OS 45 glow 77 mph
New Tower tilt rotor electric foamy 50 mph
Speed 400 type electric speed plane with an inrunner brushless and 5X5 prop 118 mph
So my conclusion is that the large war birds fly faster than we thought
Larry
03-15-2017, 10:47 AM
#16
10" x 7200 rpm = 72000 inchs per minute
720000/12 = 6000 ft per min or 1.14 mile per min
1.14 x 60 = 68 miles per hour
How do we get almost twice as much actual speed as measured by radar?
03-15-2017, 11:00 AM
#17
As a check here are the numbers for a full size aircraft that I use to own...Sonex with VW engine. Prop pitch 48" and rpm 3200.
48 x 3200 = 153600 inch per min / 60 = 12800 ft per min / 5280 = 2.4 miles per min x 60 = 145 mph
The actual speed of this aircraft was 120 mph. The actual speed is below the theoretical speed as expected due to prop efficiency being less than 100%.
I still don't see how a model prop can pull a plane at nearly 200% efficiency? Would make sense if the rpm in flight were nearly twice the static rpm on the ground.
48 x 3200 = 153600 inch per min / 60 = 12800 ft per min / 5280 = 2.4 miles per min x 60 = 145 mph
The actual speed of this aircraft was 120 mph. The actual speed is below the theoretical speed as expected due to prop efficiency being less than 100%.
I still don't see how a model prop can pull a plane at nearly 200% efficiency? Would make sense if the rpm in flight were nearly twice the static rpm on the ground.
03-15-2017, 12:24 PM
#20
When I raced pylon years ago, I was told to use a formula: RPM divided by 1,050 times pitch. We had Como .40's turning around 20,000 on the ground and used 8.75x 6.5 Rev-Up props. So, 20,000 divided by 1,050 equals 19.047 multiplied by 6.5 equals 123.80. The formula assumed less than 100 percent efficiency and gave an estimated speed of 123 mph.
I am not a math expert but this was darned close.
I am not a math expert but this was darned close.
03-15-2017, 04:00 PM
#21
The prop was a JC 22x12 from aircraft international and a good tuneup I haven't checked the rpm, it sounds like the engine is loaded at full power on the ground but it unloads in the air. it is much faster than the 22x10. I tried a 22x14 but it couldn't pull it . I have used radar on lot of planes that was suppose to be a lot faster than they were .
03-16-2017, 06:42 AM
#24
Something you all might not have accounted for in your RPM/Pitch computations is engine loading.
When you run an engine on the ground, it's loaded based on a static location.
When you run the engine in the air, the loading is different, based on the fact the air going by the prop exerts LESS DRAG than it does on a prop running in a static location. This is something we found with boat props when testing them in the water. Due to the density of water, the engines exceeded the top RPM due to the lack of water when held in a static location when compared to what they did while running normally. One can only assume the opposite with an aircraft engine due to the thinner nature of air on the blades
When you run an engine on the ground, it's loaded based on a static location.
When you run the engine in the air, the loading is different, based on the fact the air going by the prop exerts LESS DRAG than it does on a prop running in a static location. This is something we found with boat props when testing them in the water. Due to the density of water, the engines exceeded the top RPM due to the lack of water when held in a static location when compared to what they did while running normally. One can only assume the opposite with an aircraft engine due to the thinner nature of air on the blades
03-16-2017, 07:27 AM
#25
Radar guns have to be used correctly. There is something called "cosine effect" that will give you inaccurate readings. Basically it means you have to try to have as little off set from the flight path as possible. The speed with the P-47 was taken with it flying right at me(obviously I trust the pilot
) and you can not
pan with the gun as the plane goes by. As far as the g-62 is concerned they like rpm. We prop them for higher rpm on the ground than most people. I think that 7200 rpm on the ground unloads above 8000 in the air. Theoretical speed never seems to correlate very well, there is something else missing in the calculations. According to a 2009 post by a well known engine guy(who does not post here anymore) the stock magneto g-62 with the apc AT-6 prop which is about a 20x14 will take a big 100" AT-6 over 120 mph after unloading to 9,000 in the air. That jives pretty good with the math. I am thinking that the plane I timed was unloading in the air more than we think. Need an on board tach with telemetry to verifiy.
Larry
) and you can notpan with the gun as the plane goes by. As far as the g-62 is concerned they like rpm. We prop them for higher rpm on the ground than most people. I think that 7200 rpm on the ground unloads above 8000 in the air. Theoretical speed never seems to correlate very well, there is something else missing in the calculations. According to a 2009 post by a well known engine guy(who does not post here anymore) the stock magneto g-62 with the apc AT-6 prop which is about a 20x14 will take a big 100" AT-6 over 120 mph after unloading to 9,000 in the air. That jives pretty good with the math. I am thinking that the plane I timed was unloading in the air more than we think. Need an on board tach with telemetry to verifiy.
Larry