# Scale Models

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#### HeliDev

##### Well-Known Member
Im working on a design for a two place chopper and want to start with a scale model. I havent built a full size aircraft before and want to get an idea of how it would handle.
My question is what scale would you recomend to start with. I am reasonably familiar with the work of mister renolds and how difficullt he can make things, so I want the model to be at least partially represetative of the aircrafts characteristics.
I have some ideas that I think would work quite well, but I dont want to be deluded by the scale effect.
One more question is anyone familiar with the company "Aircraft Designs Inc". They have some books which appear to be very good, but they dont respond to my emails, and calling the US is expensive.
Thanx

#### orion

##### Well-Known Member
Scale model testing is a tricky field and unless you are versed in the technology of doing so, the results you get may be misleading. This is especially true for helicopter operation.

In general, it is best to keep the model as large as possible so the Reynold's Number effects are minimized. For aircraft configurations, we generally recommend keeping the model at least 1/3 scale, although useful information can still be gained form a 1/4 scale model too.

Keep in mind that the information you are going to get will be more qualatative rather than quantatative. This means that you may get an inkling of handling issues rather than that of perfomance.

Regarding Aircraft Designs, keep in mind that they are our comptetitor so my comments should maybe be taken in that context. My company is Orion Technologies, at www.oriontechnologies.net.

I've looked at their software and books many years ago and have found them to be of very limited use. The work tends to be very generic and incomplete, and from time to time, contains significant errors. The software may be useful for playing around with very preliminary numbers however I would not depend on any of it for practical design information.

For better design programs, check out daVinci Technologies as well as other links found throughout the internet and at this site.

BH

#### HeliDev

##### Well-Known Member
Thanx for taking the time to reply.
I appreciate your honesty reguarding the books available from Aircraft designs inc, and will keep what you said in mind.
The book I was aminly after was Modern Helicopter design by Hollman. I know him to have produced a number of rotorcraft books over the years so for preliminary design purposes I am satisfied that prelim calcs should be OK.
As a side not about myself, I dont have a formal education in Aerodynamics. I was studying a space sciences degree, untill university politics destroyed it leaving us without a faculty to speak of and unqualified lectures. Paying $4000 per year didnt make much sense to me if I wasnt recieving an education. I applied to another uni to study aerospace engineering, and would have done it if not for two factors. 1) The course was only available full time, which made it nearly impossible for me to work. 2) The cost for the couse was$20000 approx per year. Which for the amount of money I could get back as an engineer in Aus seemed ridiculously expensive.
So I decided to do my CPL(H) instead and study aerodynamics in my spare time.
I have built a number of RC gliders and rocket powered gliders, which worked better than I would have thought, and surprised the crap out of all those who said they wouldnt work.
One thing I noticed immediately when starting to fly choppers is that there are a number of areas which I think could be easily improved but are continually over looked, because thats the way its always been done. The layout of helicopters has changed little since Igor designed the VS300, despite the fact there seem, to me aleast, better ways to do it.
Now that I sound like a know it all (which Im not, infact I know far to little), there are quite a few things for me to figure out.
All my designs differ markedly from the traditional layout, so the performance is not the issue, right now, as much as how it handles.
My main areas of interest at the moment is the compound design. I think that there would be a good market for this type if it could be demonstrated to work. The problems lie with the impingement of the rotor wash on the wing, as well as control aspects.
This is why I want to make the model first, to establish the control and handling, as opposed to performance and strength.
Where Im at now is desprately short of information equations ect....
I also hope to use the modle building to gain experience in composite construction.
Right now I want to get a flying model, and get an idea of what works, if its good enough hopefully Ill hve enough money to pay your company for the finer points of design (structural strength ect).
Sorry for the length of post but I felt that some background may help with understanding my problems.
Basically Im after design books and programs.

PS I went to your site and found it to be very informative, I especially enjoyed the articles.
PPS I really like the fact that no one here says that stuff cant be done. I get really sick of hearing that every time I tell some one what I want to do.

#### HeliDev

##### Well-Known Member
Sorry as an extra would there be any point in stating with a 1:5, just as a handling test, or would the difference in scale be to great?

#### orion

##### Well-Known Member
1/5 scale might be a bit too small for any practical information however, for getting a general idea of function, you should be OK.

#### Jman

##### Site Developer
Orion,

When building the scale model, will it be important to build it 1/5th the weight as well or is it ok to build it as light as is practical? The reason I ask is that most RC aircraft are no where near the size to weight ratio with their full scale counterparts. Will this affect data? I guess my real question rephrased is when building a scale model to explore the flight characteristics of a design, is it common practice to make it scale in weight as well?

Jake

#### HeliDev

##### Well-Known Member
Orion, Thanx for your input, Im going to start with as large as I can, money and space will be a driving factor.

Jman, Unfortuantely scale is really decieving. Heres an easy way to look at it.
Take a 1:5 scale model;
The linear dimensions are 1/5 of the full size.
The surface area is 1/25 of the full size.
The mass is 1/125 of the full size!!!!!!
So to make the model to scale weight would be very hard, and probably very hard to get airborne: .
Other problems arise with the airflow over the model, being fairly different to that of a full size.
Thats just the basics the way I understand it, feel free to shoot me down in flames if I got something wrong.
THe area I was unsure about, in particular was whether the handling of the aircraft would be similar would be to different to gain any knowledge from, which orion was kind enough to answer for me.

#### mogren

##### Active Member
My test have all been with 44in or less models. The full scale stuff will of course take a lot more pounds per sq/in( foot) due to the scale affect. BUT. If it will fly as a model, it will fly as a full scale plane,99% of the time.The small stuff is a lot more unstable and a lot harder to fly.
The bigest differnence and loss at model size is the form drag at the airfoil joints, (wing root, etc). and the tail needs to be larger than the full size critter.
I like the idea of a PLOPPER< a chopper at low speeds that stops the rotor at flying speed and cruises on the small wings, .
I am testing a FLYING FLEA at model size now for CG testing and the incidence of the rear wing to get it out of the front wing wash. The CG and planform stability has been consistant from very small (18" span)to full scale . If you try to fly it at scale loading, it needs to fly at faster than full scale speeds!!! very fast
MM