Avaiojet

It's not at all that difficult to figure wing loading and engine choices once a model is built. All measurements are in front of you and you can just weigh it.

I'd like to do this "before" I start my build. That is, determine the model's weight.

In all honesty, I've never really given any thought, to the weight of any model I've designed or built. I've taken a "whatever it turns out" attitude.

My next project is special. It's a life long dream and a ten year effort, with what I can honestly say, has so far been with little results.

I've been patiently waiting for someone else to do my work for me. That is, design, build and offer retractible gear for the F3F-1. Please, and I'm begging, DON'T suggest or even mention the Wildcat. Any conversation about the gear used for the Wildcat would be a waist of time and spirit. They are absolutely not in any way the same, but it's obvious they retract in the same configuration.

I'm not waiting any longer. I've decided to design and build the gear myself at 1/5 scale.

To do this correctly, I should have a "close" approximation as to the weight of the model. The last thing I need is to "over" design and overbuild the gear, creating unnecessary weight and work. I also have to maintain the scale size, look and operation.

"Has anyone else ever attempted to determine the approximate weight of a model "before" the build?"

This is what I'm asking and I'm not quite sure how to go about it? Weigh all parts and pieces?

Certainly I'll provide measurements. and a dicsription.

Grumman Gulfhawk G-22 at 1/5 scale. Same aircraft as the F3F-1 with mods.

Top wingspan and cord: 69.5" X 12" with 44 ribs. +/-, Alerions top wing only. Warbird style.
Bottom wing and cord: 64" X 10" with 38 ribs +/-, no ailerons.

Fuselage length: 55"
Cowling diamiter: 11" +/-
Fuselage height at cockpit: 13.5" +/-
Fuselage thickness at cockpit: 9.75" +/-

Stab span: 25" at scale but I will increas it slightly.
Stab and Elivator cord: 10" tapered to 4.75" oner 11" cord to cord.

Rudder and Vert Stab" 13" tapered to 4.5" over 9.75". This will be slightly increased.

Tire size: 5.5"
Track 15"
Prop: 22" minimum, max. 24", hopefully 3 blade with hub.
Engine: Considering options because I'm researching, but it appears as though the Robart R780 is currently the best choice.

If anyone needs more information I'll provide it.

As you can see, this is a serious and special project which needs special considerations.

I would appreciate any and all help.

Thanks in advance.

Charles

rmh

You have your work cut out for you
The model will be about 1300 square inches total- so a really good flying weight is 15 lbs 18 tops .
If you follow the typical "warbird" construction -you will be heavier than that .
I have built warbirds for others -using th kits -and all of em were really structurally way overweight - not over strength - just overwight
Try this - lookat th latest best of the ARFS from China - the competition aerobatic ones
study how they eliminated weight
THEN design your structure and IF possible do it for cutouts with laser .

pmw

My Feedback: (1)

Charles,

Forgive me for saying this, I don't mean to sound insulting, but you may be "over engineering" a little. If you wanted to know the weight to the ounce, then your only way would be to determine the weight of each part. But if you are using wood, there are too many variables in the structural components to determine the weight before you make the piece. The density varies too much.

My suggestion would be to estimate the uncovered airframe weight from weighing the approximate amount of wood you will use. Then add the known weight of the other components. Those are really the heavy items anyway, such as covering, engine, radio, prop, wheels, etc. The final variable is the paint. Paint weighs a lot more than most people think. You can get pretty accurate numbers by weighing an empty container and comparing it to a full one. But even this is not perfect because if you spray, much of the paint is blown by the airplane. If you know about how many containers of paint you will use, you can get close.

Regarding the strength of the gear, remember the loading on the gear during landing can be tremendous, depending on the sink rate at touch down. So knowing the weight of the plane is only a starting point when trying to determine the strength of the gear. I would build a simple prototype gear to experiment with structural issues and then build the final version after determining the strength needed. Of course, you would want to test the gear on a spare plane of the same weight as your model, not the model itself.

I wish you luck. I've always loved that plane and only the landing gear has stopped me from building it.

Paul

Avaiojet

"Forgive me for saying this, I don't mean to sound insulting, but you may be "over engineering" a little. If you wanted to know the weight to the ounce, then your only way would be to determine the weight of each part."

Paul,

Thanks for the reply. Without trying to be a comic, I'd be happy with weight to the pound!

I'm hoping other guys with biplanes can offer weights and wing loadings. Most of this is about engine choice and gear strength.

"The final variable is the paint. Paint weighs a lot more than most people think."

Paint is something I have a lifetime of experience with. Did painted graphics and airbrush work professionally plus I restore sports cars. I do know tricks to keeping weight down with paint. I've painted planes for years and sometimes for customers.

"Regarding the strength of the gear, remember the loading on the gear during landing can be tremendous, depending on the sink rate at touch down."

Absolutely! I have five aviation ratings and my last airplane was a Lake LA4-200T. I know exactly of what you speak.

"I would build a simple prototype gear to experiment with structural issues and then build the final version after determining the strength needed."

I built the prototype out of balsa, 2/3 the scale. I really didn't want to test anything flying because I haven't flown in years. I'm far from set-up to do that. But it is a good idea. I'll get a guy to work with me on the test. For testing, I was thinking more of attaching the finished gear to a 5 gallon barrel, a stick out the end with a tail wheel, and dropping it at various heights. I can control the weight with sand. BTW. Some people refer to this model as "The Barrel." I don't like that.

They actually test "real" gear that way, by dropping.

"Of course, you would want to test the gear on a spare plane of the same weight as your model, not the model itself."

I'll take that advice. Shouldn't be to difficult a set-up because the gear would only have to be in the down position and not working as retracts. Getting it to retract and deploy is another issue. I'll talk to the manufacturers for assistance with that. I'm not shy.

"I wish you luck. I've always loved that plane and only the landing gear has stopped me from building it."

I can say the same, and I did. Now I'm changing that.

Please keep an eye on me as I progress, I'll need advice.

Thanks for the reply.

Charles

rmh

My Buckers weighed 17 lbs at 1800 sq inches -
there is a target weight for you -
These were high performance models tho here is a pic of one a customer did.
Hope you figure out a good strength to weight setup for yours -

Avaiojet

I made an error with the wingspan.

It's actually 69.9" on the top wing.

I corrected it.

Dick,

Great model. You always do great work.

Charles

rmh

thanks - the originals were TOC models 2nd place in 1990 flown by friend Steve Rojecki
I had to make em as light as practical the airframes were 9 lbs complete. inc a shock strut gear I designed
The F3F is a project -for certain - I would do it with sheeted 1 lb foam flying surfaces -cap stripped to look like ribs - best weight strength -I know this -tried em all
fuselage two half shells planked like a ship then joined -you end up with a big "eggshell" no real interior structure except as needed to support engine/gear and wing attachments . I do this on super light pattern models they look like glass structures when done.

wellss

If you take the weight of the full size airplane, with "reasonable" construction, you should be able to get a weight that is close to

model weight = full size weight / ( scale)^3.5
model weight = 4800 lbs / 5^3.5
= 4800 / 279.5
= 17.2 lbs

If the real plane weighed 4800 lbs, then 17 lbs is reasonable.

Avaiojet

Wells,

Run that by me again?

How does that math work for a model that is 1/8 scale or 1/6 scale or 1/4 scale. Your math isn't being told the size of the model?

But 17lbs sounds great!

I'll take it!

Charles

.

wellss

Sorry, edited...

If the plane is 1/8 scale, you would divide by 8 to the 3.5 exponent, etc...

Avaiojet

Dick,

I'm probably putting the "short" on my own thread, but I gotta tell ya, what do we see in these two photos?

Can anyone guess the year? Who are these guys? Someone has a nice maple field kit?

The model on the left is a Dalotel. Designed by Dick Hanson. 68" in span and flies the FAI or AMA pattern.

The model on the right is the Zlin. Possibly a little smaller but a great performer. I don't have the specs and should know more about that model because I purchased one from Dick Hanson Models in 1986! 1986!!

Yipes, has time flown by! No pun intended. God am I getting old or am I getting old?

In all honesty, the closest I ever got to flying any pattern was reading the illistrated articles written by Dick himself. I still have all that stuff. Gee, ebay memorabilia? I'll be stinkin rich!

Dick, was it you who designed the Tipo or modified the Tipo? The Tipo was a great looking model. I have a pattern ship styled after that design.

Dick, thanks for holding my interest in model aviation.

rmh

Holy cow - I ain't old enough to remember those models
tho I still have the glossy of that Tipo "Avenger
The red n white Zlin-I am doing again same scale scheme too
The Dalotel - That one is a twin of the one Chip Hyde flew to 1st place in 1984 Nats FAI- and thoroughly pied off the "pattern plane junkies . They thot a semi scale model was incapable of flying as well as the "pattern" designs.
guess again
It's been fun to watch the "Theory" Fans and their ideas get gently scuttled , a piece at a time
ah luv it

Avaiojet

Wellss,

OK math guy your on! Center stage!

I never got my GED so help me with this. OK?

Dry weight of the real F3F-1 is: 3285. And that's really dry, static and with no pilot.
The size of my model will be 1/5 scale.

What do you come up with?

Charles

Avaiojet

Dick,

Hey! I just remembered, you published that photo!

What a guy!

BTW. Still cutting foam wings? You brought it up.

Charles

rmh

I did?
anyway no - no more of that .

otrcman

I'll try to expand on Wellss' use of the 3.5 power. First, do you want to simply build a model of this airplane and take whatever weight that comes out or do you want to determine a desirable weight and then shoot for that weight? If you want a realistic looking model you need to shoot for a specific weight range based on the weight and performance of the full scale airplane and the scale size of your model.

Here's why you would choose a target weight: The original F3F had a known speed range. Stall speed, cruising speed, and top speed. In order to look realistic in flight, your model should visually mimic the speed of the real airplane. But wait, that doesn't mean that your model should cruise at 160 mph just because the prototype cruised at 160 mph. Visually, your model will look realistic if it flys by at a speed consistent with its size. What we see in flight is the airplane covering so many times it's own length per second. Let's say the prototype was 25 feet long and cruised at 160 mph. This means the prototype travelled about 9.4 times its own length every second. So if your model is 5 feet long, it should have a cruise speed of 5 X 9.4, or 47 feet per second that's 32 mph. You can make this calculation a whole lot simpler by simply multiplying your scale by the cruising speed of the prototype, so 160 mph X 1/5 scale equals 32 mph. (surprise ! Same answer).

Unfortunately, a model scaled to look just right in level flight will be capable of very small circles when it turns. And will do incredibly tiny loops for its size. In other words, if the real airplane at 160 mph made circles in the sky of 500 foot diameter, that would be 20 times the fuselage length. Your model would need to make 500 X 1/5 or 100 foot circles to look realistic. But a model scaled to go scale speed will be capable of circles much smaller and will look funny when maneuvering.

It turns out that scale sized circles, loops, and rolls look about right if your model is quite a bit heavier and faster. To make proper maneuvers, the model would need to fly about 71 mph.

Skipping over the math, it turns out that a model will look realistic going by if its weight is scaled to the 4th power. But it will make scale sized maneuvers if its weight is scaled to the 3rd power. You can't have scale flight speeds and scale maneuver sizes in the same model unless your model is 100% scale and weighs the same as the prototype. Unfortunate fact of physics.

So what Wellss is suggesting, and is accepted practice, is to compromise between 4th power scaling and 3rd power scaling. It's 3.5 power scaling. That way your visual speed is slighty fast and your maneuvers are slightly small, but it's the best you can do and most people never notice.

Down to the numbers for weight: If you had scaled your model for scale speed, you would multiply your size scale ratio by itself 4 times to get weight. With a 1/5 scale, that's 20% or .20 of full size. Multiply .20 X .20 X .20 X .20 and you get .0016. Multiply .0016 times, say, 4000 lbs. and you get a model weight of 6.4 lbs. Too light ? Well, that's what you would have to do to make your 1/5 scale model look perfectly realistic in level flight. If you want scale size maneuvers, you would multiply your 1/5 scale by itself 3 times, or .2 X .2 X .2, and then multiply that answer by the 4000 lbs, and you have a model weight of 32 lbs. Too heavy ? Well, that's what it takes to make your 1/5 scale model do scale sized maneuvers.

You'll need a calculater to take 1/5 scale to the 3.5 power, but it works out that the model should weight 14.3 lbs to look about right in flight. Any heavier will begin to look too fast in flight, and any lighter will make unrealistically small maneuvers. Is 14.3 lbs cast in stone? No, you can go above or below that weight. But that's a good number to shoot for.

Sorry for the long winded answer. But that's the short version.

Dick

otrcman

I just noticed after posting that my suggested weight is slightly different than Wellss'. That's because I assumed that the full scale weighed 4000 lbs and Wellss used 4800 lbs. Both model weights, 14 lbs and 17 lbs, would be correct depending on whether you are scaling your F3F at max gross weight or at a lesser flying weight.

Dick

wellss

yep, that's it. Thanks otrcman

You'll need to keep power loading the same as the full size though. From your target weight, figure out what power you'll need, then see if it'll fit in the cowl. You can take the weight of the full size engine and scale it down the same way to see if you'll need to add nose weight or not, depending on which engine you choose.

rmh

The numbers are fun
The situation which occurs in actual practice is that it is impossible to build the model too light
Physically impossible.
Anybody who has actually accomplished this - please let me know.
Again real world
99%of scale warbirds -not fun scale - are built far to heavy
The gag is that they fly "scalelike".
In actuality they are simply far too heavy.
The upshot is that they seldom are flown as a slighly bad landing will wipe out the gear etc..
Models must be able to fly in a manner that the flyer can see -then correct the flight path. On landings this becomes split second adjustments
If you want to enjoy th model for a while - do everything possible to build a light airframe
It will be heavy enough -

mjfrederick

I love that statement...

otrcman

Thanks Dick. You distilled my whole essay down into a single sentence.


Dick Fischer

da Rock

I've read through this thread, and not sure you've actually gotten an answer that directly addresses your question.

You want to know how strong to make your gear. Bottom line.

To know that, you need to know how much weight it'll have to deal with. OK, the assumption is that you'll also want an airplane that'll fly worth spit. You know your wing area. You know what wing loadings fly well. Working backwards from that gives the answer. You also know what engine you'll be using. That may or may not modify the answer you got from wing loading.

Isn't that all the theory that matters?

As for it being fun to watch the "Theory" Fans and their ideas get gently scuttled , a piece at a time, you just gotta start with the right theory, and ones that actually apply.

Design the gear to deal with the max weight that will fly worth spit, and it'll be perfect. If your build comes out over that weight and your gear fails, what have you lost? A lousy flying airplane. And you are gonna miss it? If your build comes out at or under that weight, you prove what an excellent designer you are.

Avaiojet

Da Rock,

Great observations.

Here's why it's difficult to determin weight "before" construction. In construction, with model airplanes, the designer can choose woods and chemicals. No two builders work the same?

Balsa, plywood, oak, and there's composits, fiberglass cloth, etc.

It's the calculated use of these elements that the builder can use to actually keep his weight down. My goal.

One example would be the use of balsa spars VS. hard wood spars, This model should have forward and rear spars. I expect to have D tube construction with one rear spar centered in the rib. Possibly hard wood for this spar but certainly hard balsa.

The design calls for twice as many ribs as needed, just for scale appearence. So, in actuality, I could use soft balsa for half of the ribs and hard balsa for the other half.

If I wanted to take it this far.

Each and every area of construction can be dealt with the same way. Built up stab and sheeted VS. solid.

So, with consideration to all these areas, the builder can build a really light model for it's size.

A table could be made from the "results" of other modeling projects. All a modeler has to do is supply the following"

Wingspan, wing area, engine choice and engine weight, and the total weight of the model with the engine in place.

Comparing the results of many models and determining wing loadings, could help me considerably.

Dick has a life long histry of building light. His efforts and results has proved this.

His Jungmeister at 17lbs with 1800 SQs. is a fine example. Thing is, will modelers offer this information and will it be accurate?

Charles

HighPlains

My Feedback: (1)

What you are actually doing with your ^3.5 scale coffiencent is achieving the same G loading in both turns by the full size and model. The reason the model turn then looks scale-like is that the wing bank in a level turn is the same angle.

But time is not scaled, since the model would complete the turn in less than half the time.

wellss

Using a wood like spruce for a spar will give you higher strength / weight. For a given strength, it will be lighter than balsa. 1/8 balsa ribs, 3 or 4 inches between and 3/32 sheeting should give you a decent wing...