Author Topic: Scale Speed (Read 2008 times)

Douglas Ames · « **on:** May 05, 2012, 09:03:16 AM »

Out of curiosity -
Do judges look at scale speed according to model size? The rules state no more than 60 mph. So with 60 ft lines plus 2 ft for half span (60") and 3 ft for your arm to ctr. of the circle you have about a 65' radius. A 5 sec lap would be about 50 mph? or 350mph scale speed for a 1/7th model. (approximation)

Does this matter?
Does anyone have the formula, `cause I just winged it in my head?

A scale Bug Smasher, aka -Piper Cub would fly considerably slower to get a scale effect vs. a WWII Fighter.

John Rist · « **Reply #1 on:** May 05, 2012, 03:08:22 PM »

Speed is judged by how long it takes an object to travel it's own length. So reason dictates that if you are modeling a slow airplane make it big. If you are modeling a fast airplane make it small-(Or not

). An Extra 300S flies faster than a piper cub. In judging realism of flight all that counts is that it look about right. By the way speed does not scale. A piper cub flies at 60 mph. A 1/5 scale piper cub will not fly at 12 mph. The speed required to generate lift is the same for a given airfoil regardless of scale. So if you fly the scale piper cub as slow as possible and the Extra 300S as fast as possible - to the judges the Piper looks slow and the 300S looks fast And all is good!

Fred Cronenwett · « **Reply #2 on:** May 05, 2012, 04:13:01 PM »

Scale speed and other factors are always a confusing issue.

the wing span is linear so a 1/8 scale model is 1/8 of the size of the full size, that is easy
Scale horsepower, scale weight and speed is not linear.

I need to find an old article I have in my collection that has all of the factors for horsepower, speed, and scale weight. The last time I ran the numbers for a typical warbird the numbers were out of the ballpark, bottom line the scale horsepower was below the actual we need. Scale weight was above the maximum weight the model can fly and scale speed was not comparable.

I have no less than 5 articles in my binders discussing scale speed (mostly for RC, but it still applies for CL) and every article addresses the size of the model. A large model going the same speed as a small model will look different from a scale speed point of view.

No good way to address this, just have to fly what looks right for the type of model and size that you are flying.

Fred C.

Fred Cronenwett · « **Reply #3 on:** May 05, 2012, 05:07:30 PM »

There was an article in the Jan 1978 issue of model aviation titled "about the size of it" by Bradford W. Powers.

He lists the following factors for models

Catetory Scale Factor 1/8 scale model factor (divide the full size data by this number)
Dimensions K to the 1 power 8
Areas K to the 2 power 64
weights K to the 3 power 512
Velocity K to the .5 power 2.828
horsepower K to the 3.5 power 1448.1
wing loading K to the 1 power 8

K is the scale factor, so a 1/8 scale model would take the 8 scale factor to power listed above to get value for the model

Fred C.

Will Hinton · « **Reply #4 on:** May 05, 2012, 05:59:13 PM »

The speed of a scale ship is something to consider, but don't forget to enter the sound level into the "equation" as well. If it's flying a reasonable speed for realism of flight to be good but sounds like a huge B-17 when it's John's 300, oops, now how do we score it? Get the picture? Do what YOU think looks and sounds right and let the judges decide on the score. They'll usually be right in your neighborhood with their opinions.

Fred Cronenwett · « **Reply #5 on:** May 05, 2012, 06:15:31 PM »

Let's run the numbers for a 1/8 Scale P-51

full size 1/8 model
Wing span 37 feet 55.5"
loaded weight 9200 lbs 17 lbs
horsepower 1490 hp 1.0 hp
Max speed 437 mph 154 mph
stall speed 100 mph 35 mph

Of course some of these numbers are maximum and are never static but least gives us a starting point.

Based upon experience we know that a 56" span P-51 would probably get a 46 size glow engine and would need to weigh 6 lbs to fly correctly. The 46 glow is more horsepower that the numbers suggest and the overall weight is significantly less than the scale weight. Personally I wouldn't want to fly a CL scale model that weighed 6 lbs at 80+ mph!

If our model was at the scale weight we would never get off the ground. we actually have to be lighter than scale weight since our wing loading that actually works for models airplanes is a function of the reynolds number, which is a function of wing span. The larger the wing the higher the wing loading can be. This is why larger models fly better

Maximum scale speed is way out of range for us. We can actually fly slower than 35 mph for landing so our stall speed is less than the scale stall speed.

Everytime I run these numbers I come to the same conclusion that you have to fly what looks right based upon the type of model and and the overall size of the model. Bottom line is that the scale speed is a judgement call by the flight judge.

Fred C.

Douglas Ames · « **Reply #6 on:** May 05, 2012, 08:25:33 PM »

Quote from: Fred Cronenwett on May 05, 2012, 06:15:31 PM

Everytime I run these numbers I come to the same conclusion that you have to fly what looks right based upon the type of model and and the overall size of the model. Bottom line is that the scale speed is a judgement call by the flight judge.

Fred C.

OK, Thanks!

john e. holliday · « **Reply #7 on:** May 06, 2012, 12:14:22 PM »

Go read 2.9 of the control line sport scale rules.

Fred Cronenwett · « **Reply #8 on:** May 07, 2012, 06:12:39 AM »

2.9: The line sizes, line lengths, and pull test are designed for a model flying no more than 60 mph. Any contestant entering a model with a speed exceeding 60 mph shall provide the Event Director with documentation specifying the line diameter, line length, and pull test to be used to ensure the model meets the same standards of safety established for slower models.

To enforce this rule the CD must mesure the line length of each model and then time each model to make sure all models are flown at 60 mph or less, Most CL scale pilots are not speed demons so that has not been a problem.

Fred C.

Paul Smith · « **Reply #9 on:** May 07, 2012, 06:29:40 AM »

The 60 MPH "speed limit" only applies to contestants who take advantage of the exceedingly weak line size and pull test rules.

I've never witnessed judges actually timing speeds to verify the 60 MPH. I've heard a few make a comment like "he's going too fast", but they never do anything about it. In reality. some of the heavier models with small wing area NEED to fly faster than 60 MPH.

They only fly 10 laps of level flight, so in the absence of a separate speed limit judge, there isn't much opportunity to verify speed. To get good points you need to throttle back during level flight. Full power is used just for high flight & loops.

My planes could probably do better than 60 MPH, but my lines are big enough to meet the rules of Stunt and Carrier, so I'm covered.

Paul Smith · « **Reply #10 on:** May 07, 2012, 04:49:18 PM »

As far as "scale speed" goes, I agree with the earlier post.

Nearly ALL CL models are way too SLOW. We're off the low end of the chart. We should be going a lot FASTER. With the exceptions being Piper Cubs and other ultra-slow planes built to big scale factors. But with subjects like the Mustang & Bearcat, you can't be fast enough, let alone too fast.

Clancy Arnold · « **Reply #11 on:** May 07, 2012, 08:50:29 PM »

Paul
You saw my 1914 Jeannin Stahltaube fly at the Brodak Fly-In last summer. It is built to a scale of 6.2 to 1. The actual aircraft flew at a max of 71 MPH and cruised at 60 MPH and my model cruses at 37 MPH. 65 Ft lines and 7.5 seconds per lap. How would you rate that on "Scale Flight Speed"?
Clancy

Jim Fruit · « **Reply #12 on:** May 10, 2012, 08:17:02 AM »

Quote from: Clancy Arnold on May 07, 2012, 08:50:29 PM

Paul
You saw my 1914 Jeannin Stahltaube fly at the Brodak Fly-In last summer. It is built to a scale of 6.2 to 1. The actual aircraft flew at a max of 71 MPH and cruised at 60 MPH and my model cruses at 37 MPH. 65 Ft lines and 7.5 seconds per lap. How would you rate that on "Scale Flight Speed"?
Clancy

Clancy:

(6.2 to 1)? Are you saying that your Taube is approximately 1/6 scale? I know that it is over 7' span, which would make the original approximately 42' in span. If this is so, that would make your 37 MPH cruise speed represent 222 MPH scale speed! (37 x 6). My C45J is 1/12 scale. I expect it will fly about 40 MPH. That would make its scale speed 480 MPH. (40 x 12). That plane could not reach 480 MPH except in a terminal dive. This is what I believe Paul is struggling with. The different scales at which we build our models mathematically has a great effect upon its represented flying speed. What we cannot scale is Reynolds Numbers. The model is flying through the same relative density of air as the original plane did. It will usually have to fly at a much greater scale speed just to stay in the air due to its smaller size. I believe the secret is to be able to fly at a speed that appears believable and controllable, not so slow as to stall out of the sky and not screaming around the circle. The expressed rule is somewhat unverifiable in that there is no mechanical means to measure perception. We just need to put in a flight that is believable in the judges opinion.

Jim Fruit

Fred Cronenwett · « **Reply #13 on:** May 10, 2012, 05:43:33 PM »

to calculate the scale speed for our models you divide the speed of the full size airplane by the square root of the scale factor that you built the model to. The only feature of our models that is divided by the scale factor is linear dimensions such as wing span and length.

for example:

1/8 scale divide full size speed by 2.828 (square root of 8 is 2.828)
1/7 scale divide full size speed by 2.646
1/6 scale divide full size speed by 2.449
1/5 scale divide full size speed by 2.236

A full size airplane that cruises at 120 mph built to 1/6 scale would fly at 49 mph to be scale speed.
The P-51 that can fly at speeds approaching 400 mph would have a maximum scale speed of 163 mph (1/6 scale), which is not pratical for CL scale
The piper cub which is very slow, maybe 80 mph max would top out at 33 mph at 1/6 scale.

This why there is lots of confusion with this issue, but in the end we have a rule that says we can't fly faster than 60 mph so the fighters should fly towards the top end of that range and the piper cubs should fly at the low end of the range to maintain scale speed.

Fred Cronenwett

Clancy Arnold · « **Reply #14 on:** May 10, 2012, 07:05:24 PM »

Jim
My 1914 Jeannin Stahltaube is 6.2 to 1 Scale. The original is 45.5 Ft (13870 mm) and my model is 7 Ft 4 In. (2235.4 mm). 13870 / 2235.4 = 6.2047.
Clancy

Fred Cronenwett · « **Reply #15 on:** May 10, 2012, 07:30:28 PM »

With a rated speed of 63 mph for the full size Taube, the 6.2 scale model should fly at 27 mph to be scale speed.

Fred C.

Jim Fruit · « **Reply #16 on:** May 11, 2012, 08:03:20 AM »

Fred and Clancy:

Maybe I am missing something, but this math escapes me. We are not dealing with the area of the circle, but rather with the distance around the circle. So in the 1/8 scale example that was used, 1" of representation on the model represents 8" on the real plane. This would relate to the distance of the wingspan (model x 8 = the real plane) AND the distance of the fuselage centerline (model x 8 = the real plane). So, when the model moves forward one model length, in scale length it has moved forward 8 model lengths (model x 8 ). When that model moves 1 MPH, in scale speed it has moved 8 MPH (model x 8 ). When it moves at 30 MPH, in scale speed it has moved 240 MPH. Again, this assumes the 1/8th scale model. It all changes with a different scale. What am I missing? At my age, it is easy to miss things.

Jim Fruit

Jim Fruit · « **Reply #17 on:** May 11, 2012, 11:02:59 AM »

Fred and Clancy:

Not really sure what happened above where the text reads "model times smiley face". Not being fecitious, I was trying to repeat " model times 8". Sorry. See what I mean by the age getting in the way.

Jim Fruit

Fred Cronenwett · « **Reply #18 on:** May 11, 2012, 03:11:55 PM »

The only feature that is ratio of the scale factor is the linear dimensions, speed is calculated by taking the square root of the scale factor and divide the full size aircraft speed by that number. The article I am referencing is in the Jan 1978 copy of model aviation. speed is distance divided by time. The linear dimensions are divided by the scale factor, but time varies as the square root of the scale factor. Because of this scale speed is calculated by taking the full size speed and dividing by the square root of the scale factor. The article discusses all of the math that supports this analysis.

for most of our models we will never reach a scale speed without violating the 60 mph rule. Very slow airplanes like Taubes and piper cubs will do better if flown very slow.

Fred C.

Jim Fruit · « **Reply #19 on:** May 11, 2012, 04:27:43 PM »

Thanks, Fred.

I have not saved Model Aviations as far back as 1978 (too many moves) so I cannot reference the math that they are using in that article. I still challenge the logic, however. I understand the logic that speed (MPH) = Distance/Time. I also relate to the linear dimensions being divided by the scale factor. But dividing the full size speed by the square root of the subtrahend of the scale factor makes no sense. The unit of measure remains at a mile, and the unit of time remains at an hour. To incorporate the square root is arbitrary. Maybe that is what is they are trying to do in the article, that is to try to come up with something that is useable rather than having a rule that is somewhat unenforceable. I am not trying to be just argumentative, I just wish there were more finite rules (or none at all).

I want to thank you for your help with the plans for the Rearwin Speedster. It turns out that the plans that you sent to me were of the first Speedster made. Then, I obtained an additional set that were of the second Speedster made. The second used the engine out of the first for its construction and those two were the only units made with that type of engine. So, they were very unique because of the engine and number two was the one that I took multiple documentation pictures of at the 2011 Oshkosh Airventure. It is this sort of research that makes this scale hobby so much fun.

Jim Fruit

Fred Cronenwett · « **Reply #20 on:** May 11, 2012, 06:20:07 PM »

The math tells us one thing, but in reality our scale models fly nothing like the full size airplanes. Our models require more horsepower and must be lighter per square foot of wing area than the full size. As the model gets smaller the wing loading that you can tolerate gets lower, as the model gets bigger the wing loading can increase. We know this from experience.

I have at least 5 articles that talk about scale speed that have been published over the years (I did not write them) and all of them have the same difficultly that you have had with this topic.

In CL scale we are limited to 60 mph maximum and in the end we need to make the model look realistic for it's size and type. As I have said before the overall size of the model and type really affect the image the judge has. Piper cubs need to fly slow and P-51's need to fly fast.

Fred C.

Trostle · « **Reply #21 on:** May 11, 2012, 07:31:03 PM »

Quote from: Fred Cronenwett on May 11, 2012, 06:20:07 PM

(clip)

I have at least 5 articles that talk about scale speed that have been published over the years (I did not write them) and all of them have the same difficultly that you have had with this topic.

(Clip)

Fred C.

This "Scale Speed" subject was the basis for many long and passionate arguments in the RC community for years. This was reflected in the articles such as those Fred mentioned above, letters to the editors of the magazines, including Model Aviation, its predesessors, and RC Modeler as well as extensive correspondence to and among the Scale Contest Board. I think many people just gave up arguing whatever their different positions were. Over time, the Scale Contest Board has accepted provisions that "most" or "many" in the scale events seem to have accepted based on the square root factor explained above.

For our CL models, scale speed really boils down to a subjective evaluation by a judge. It is in that same category as "Realisim of Flight" and another one that is almost as bad at being able to judge is "realistic sound". At least for the turbines, they can boast "realistic smell". And as far as I know, the required pilot figures is not to be actually judged except that it is to be of appropriate size and shape when the figures can be seen in flight. At least the Contest Board has not allowed the scale events to become Doll contests.

Keith

Keith