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Scale speed

Old 04-24-2020, 09:46 AM
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bentwings
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Default Scale speed

A number of us gathered with social distancing to talk about scale models. Almost the first topic was scale speed it turned into a shouting match as we tried to maintain the distance and keep tempered in the aneling range. So assume we are watching an F8F Bearecat practicing for the Reno air races. He flies by at 500 mph or 4000 feet per second. One of the guys has a 1/4 scale model of the plane. How fast in real world terms does this model have to fly at to represent scale speed?
Old 04-24-2020, 12:12 PM
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speedracerntrixie
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Obviously the math says 125 MPH. However there is no correct answer as evidenced by what you have already encountered. The foundation of the answer lies within the realm of perspective. Unfortunately in a group setting you have multiple perspectives, no two people see things exactly the same. We run into this often with new guys taking up pattern. In most cases the new pilot feels he has flown better then what the scores reflect. As the new pilots perspective aligns closer to the judges his scores improve.
Old 09-22-2020, 05:44 PM
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astrohog
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Originally Posted by bentwings View Post
A number of us gathered with social distancing to talk about scale models. Almost the first topic was scale speed it turned into a shouting match as we tried to maintain the distance and keep tempered in the aneling range. So assume we are watching an F8F Bearecat practicing for the Reno air races. He flies by at 500 mph or 4000 feet per second. One of the guys has a 1/4 scale model of the plane. How fast in real world terms does this model have to fly at to represent scale speed?
Suppose a full size Cub flies at 60 miles per hour or one mile per minute. You would want a quarter scale Cub to fly one quarter scale mile in one quarter scale minute. Since in quarter scale distance is divided by four and time is divided by two our model will fly at one half the speed of the full size Cub or 30 miles per hour. Like scale time, scale speed varies as the square root of the scale factor. To fly at scale speed our models need to fly slower than the real airplane by an amount equal to the square root of the scale factor.
In your bearcat scenario, the 1/4 scale model would need to fly at 250 mph.
Hope this helps!

Regards,

Astro
Old 09-29-2020, 04:39 PM
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speedracerntrixie
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Looks pretty darned realistic to me, way slower then 250mph.




Old 09-29-2020, 07:22 PM
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Originally Posted by astrohog View Post
Suppose a full size Cub flies at 60 miles per hour or one mile per minute. You would want a quarter scale Cub to fly one quarter scale mile in one quarter scale minute. Since in quarter scale distance is divided by four and time is divided by two our model will fly at one half the speed of the full size Cub or 30 miles per hour. Like scale time, scale speed varies as the square root of the scale factor. To fly at scale speed our models need to fly slower than the real airplane by an amount equal to the square root of the scale factor.
In your bearcat scenario, the 1/4 scale model would need to fly at 250 mph.
Hope this helps!

Regards,

Astro
The idea of "scale time" is rediculous. The math clearly calls for 125mph.
Old 09-30-2020, 09:06 AM
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Originally Posted by allanflowers View Post
The idea of "scale time" is rediculous. The math clearly calls for 125mph.
LOL. Well, if you say so.....

And BTW, it's ridiculous.

Attached Files
File Type: pdf
scale speed.pdf (75.9 KB, 43 views)

Last edited by astrohog; 09-30-2020 at 09:16 AM.
Old 09-30-2020, 11:14 AM
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You guys are over my head for the most part. But i'll open myself up for ridicule and mockery. My issue with trying to do scale speed is as follows. A 25% scale model and a 50% scale model fly at pretty much the same speed. If they both have the same wing loading both will pretty much have the same stall speed also. Yes there are other variables but I think we can all concede the actual airspeed of the 50% model is not double that of the 25% model.

david
Old 09-30-2020, 11:53 AM
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Originally Posted by daveopam View Post
You guys are over my head for the most part. But i'll open myself up for ridicule and mockery. My issue with trying to do scale speed is as follows. A 25% scale model and a 50% scale model fly at pretty much the same speed. If they both have the same wing loading both will pretty much have the same stall speed also. Yes there are other variables but I think we can all concede the actual airspeed of the 50% model is not double that of the 25% model.

david
The original question was about scale speed. Now you've introduced wing loading? That is a completely different topic.
In your last sentence, you claim that, "we call concede that ACTUAL airspeed of a 50% model is not double that of a 25% model.". WHAT ACTUAL AIRSPEED? A given model will have a range of actual airspeed during its flight regardless of what scale it is.
If you are talking SCALE speeds, I would agree that scale speed for a 50% model is not double that of the scale speed for a 25% model. For example, a 50% scale model trying to replicate a 100 MPH airspeed of its full-scale counterpart would need to fly at 71 MPH, whereas the 25% scale model of the same plane would need to fly at 50 MPH to accomplish the same scale speed.

Astro
Old 10-01-2020, 07:59 AM
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I was oversimplifying I guess. Point being a model of a Corsair for example will fly at pretty much the same actual speed as no matter the size of the model.

David
Old 10-01-2020, 08:33 AM
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Originally Posted by daveopam View Post
I was oversimplifying I guess. Point being a model of a Corsair for example will fly at pretty much the same actual speed as no matter the size of the model.

David
Speed of any model, regardless of scale, is relative to the throttle position.
If a 1/4 scale corsair is flying at 60 MPH and a 1/5 scale corsair is flying at 60 MPH, then yes, they are pretty much flying at the same actual speed. If they are not, they are not.
Still not sure what point you are trying to make, or how it relates to this discussion?

Astro

Last edited by astrohog; 10-01-2020 at 08:36 AM.
Old 10-01-2020, 12:36 PM
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“Scale Speed” is apparently a term that invites differing interpretations.

If one is talking about the impression – that ‘looks’ realistic – then one should probably start with the mathematical answer (divide the full scale speed by the scale factor – a very simple computation). However, the speed that ‘looks’ right might vary from this simple answer, depending on the observer, environment, etc. Nonetheless, the math answer is very straightforward and a very good starting point.

If one is talking about aerodynamics (Reynold’s numbers, wing loading, etc.) then we are off into the weeds. Nothing wrong with that but it is not the same thing as a purely subjective answer.

One person here tried to introduce the totally obscure idea of ‘scale time’, then excoriates another person for talking about introducing ‘wing loading’. Really? At least some arguments are still based on some science, not science fiction.

Finally, to the original poster, 500 mph is NOT 4000 ft/second (which would be over 2700mph).
Old 10-01-2020, 01:24 PM
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Originally Posted by allanflowers View Post
“Scale Speed” is apparently a term that invites differing interpretations.
The OP’s original question is very clear in the context he presented it. No room for interpretation there. He asked for MPH in scale speed. Period.

Originally Posted by allanflowers
If one is talking about the impression – that ‘looks’ realistic – then one should probably start with the mathematical answer (divide the full scale speed by the scale factor – a very simple computation).Nonetheless, the math answer is very straightforward and a very good starting point.
Except for the fact that simple division does not get you to the true dynamic scale speed. Your equation is simply incorrect. One must factor the square root (see chart I attached in my previous post).

Originally Posted by allanflowers
If one is talking about aerodynamics (Reynold’s numbers, wing loading, etc.) then we are off into the weeds. Nothing wrong with that but it is not the same thing as a purely subjective answer.
Of course we are talking aerodynamics here! We are in the aerodynamics forum and the OP asked for a scale speed in MPH, nothing subjective there at all. It would have been silly for the OP to ask for a subjective answer as he would’ve gotten the same results during the incident that caused his post in the first place!

Originally Posted by allanflowers
One person here tried to introduce the totally obscure idea of ‘scale time’, then excoriates another person for talking about introducing ‘wing loading’. Really? At least some arguments are still based on some science, not science fiction.
excoriates? (At least you used spell-check this time!)
I simply stated that wing loading was a separate issue! LOL
You, on the other hand, ridiculed me for providing the CORRECT answer.


Regards,

Astro


Old 10-01-2020, 02:38 PM
  #13  
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As for what is “correct”, that is debatable. I would not agree.

So WHERE are you finding any spelling errors in my posts? Or is this just another example of your argumentative manner and sloppy logic?

Old 10-01-2020, 04:06 PM
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Allan, I guess a 1/4 scale Rare Bear reaching race " scale speed " is virtually impossible. I've seen 1/4 scale unlimited Pylon warbirds with 280cc Herbranson engines fail to hit 250mph. I mean he does have a graph and graphs don't lie right?
Old 10-01-2020, 10:05 PM
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Originally Posted by allanflowers View Post
As for what is “correct”, that is debatable. I would not agree.
Disagree all you want. That is certainly your prerogative. Care to justify your position with some facts?

Originally Posted by allanflowers
So WHERE are you finding any spelling errors in my posts?
In your snarky remark to me, where you misspelled, “rediculous”. LOL. You are so busy bloviating that you fail to actually read the other posts?
Originally Posted by allanflowers
Or is this just another example of your argumentative manner and sloppy logic?
You crack me up! Nowhere in this thread have I been argumentative, yet you accuse me of being so and using sloppy logic? It is considered proper etiquette to disprove someone with facts rather than your baseless drivel. You and speed are two peas in a pod! Ever heard of pigeon chess? LOL

https://www.mnbigbirds.com/Scale%20Factors%20Page.htm

Regards,

Astro
Old 10-02-2020, 04:09 AM
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speedracerntrixie
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Astro, if you are going to use the word " speed " as a noun you should capitalize it.
Old 10-02-2020, 06:38 AM
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Originally Posted by speedracerntrixie View Post
Astro, if you are going to use the word " speed " as a noun you should capitalize it.
Nouns aren't capitalized, unless they are proper nouns. In this case, I was referring to you, however you don't capitalize your screen name, so why would I?

And, once again, with no real content to add to a thread, speed goes on the offensive regarding punctuation, after claiming that other members here bring no value to these threads! LOL

How about adding some facts behind your snarky remarks and utter drivel nonsense.....inquiring minds would like to know....

Astro
Old 10-02-2020, 07:47 AM
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Originally Posted by allanflowers View Post
The idea of "scale time" is rediculous. The math clearly calls for 125mph.
CLEARLY! LOL

Scale Time; TIME has been made to run faster in our model than in real life. The model’s clock is ticking at a faster rate than ours. This is an exact truth, if a model clock were make at exact quarter scale it would tick exactly twice as fast as the original clock. This same scaling law explains why the humming bird’s wings beat so much faster than the wings of an eagle.

Pretty ridiculous, huh?

Astro
Old 10-02-2020, 07:52 AM
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astrohog, please refrain from insults. Your post would have been equally effective without "behind your snarky remarks and utter drivel nonsense".

Thank you, David
Old 10-02-2020, 08:13 AM
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Originally Posted by daveopam View Post
astrohog, please refrain from insults. Your post would have been equally effective without "behind your snarky remarks and utter drivel nonsense".

Thank you, David
More than happy to David! As long as the same rules apply to all : )

Apparently that is not the case on RCU anymore.

Regards,

Astro
Old 10-02-2020, 09:49 AM
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Astro, this is what the OP asked,



" How fast in real world terms does this model have to fly at to represent scale speed? "


He DID NOT ask for anything you have provided. I posted a video of what in my experience REPRESENTED a scale warbird at racing speeds. Having been to both Reno to watch the races and to the Southern California desert to watch the R/C Unlimiteds run gives me a pretty good perspective. The R/C Unlimiteds are currently running about 230mph. The visual perspective compared to watching the real thing live is that the R/C versions are way above scale speed.







Old 10-02-2020, 10:31 AM
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Oh, speed.....

The key word in the OP's question is represent.
The definition of represent is as follows: constitute; amount to.
"this figure represents eleven percent of the company's total sales"

It has nothing to do with your perspective or opinion of things. It is finite.

Words are also contextual. In the OP's question, he mentioned there were several people discussing this issue and that a shouting match ensued (safe to assume due to multiple opinions, perspectives or "experiences" with scale speed). The OP's question was clearly worded with the intent of receiving a definitive answer, not more opinions.

In order to find the definitive, correct answer to his question, one needs to simply do the correct math. In this instance, the math goes deeper than what you provided and consider the other factors that I mentioned.

You and Allan can scoff at it all you want, it doesn't change the facts, but then again, you have proven that the facts simply don't matter to you when stacked against your opinion.

I've provided all of the facts, you've provided no substance at all, but I expect you will continue to beat this dead horse and claim some sort of victory in order to try and save face.

Good Day.

Astro
Old 10-02-2020, 11:23 AM
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Astro, ever wonder why every thread you participate in goes straight to the toilet?
Old 10-02-2020, 11:32 AM
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Originally Posted by speedracerntrixie View Post
Astro, ever wonder why every thread you participate in goes straight to the toilet?
No, but for once I would actually be interested in your take on it.

I’m all ears....

Astro
Old 10-02-2020, 01:21 PM
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Dave asked nicely.....

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