Opinions please on an aeromodel design!

[Air Challenger]

Hello Everybody

We are students of aeronautical engineering from Portugal.

We really would like some opinions on preliminary design pictures of our RC model for a contest of weight lifting around 10cc engine, wing span around 1.82m

Here go the pictures

Thank you beforehand

[Tom Harper]

What's your goal?

What is your area, aspect ratio, power?

[Sparky Paul]

Your tail moment arm is excessive.
The horizontal tail area would be adequate for a shorter tail length.. as it is, it's too large.
The vertical isn't large enough.
If you can go to a tricycle landing gear, takeoffs will be easier.
This link will take you to several years of design efforts for weight lifters..
http://www.angelfire.com/indie/aerostuff/sae2000n1.htm
Here's an SAE lifter I made to the 2003 rules, similar to your design..

[mars_ubi]

Hi, I'm the advising teacher of the students above and as they could not answer you sooner I am replying to you requests and thanking you guys for your interest.

The horizontal tail volume coefficient is 0,6, indeed it is too much, they have been instructed to put only 0,4, and so the horizontal tail area or the horizontal tail arm length should have a reduction of 33%.

The vertical tail volume coefficient is 0,03855, it is approximately what it should be, they have been instructed to put 0,04. Notice that the tail arm length is big, to minimize weight and c.g. location problems, that is why the tail area is small.

Wing area is 1,098 m^2, aspect ratio is only 3. This is because it must take-of in a 60 m run, with a power of a O.S. .61 FX engine.

The design goal is lifting of with maximum useful load in the form of lead rectangular plates in the cargo bay showed in red in the drawings. Take-of weight is estimated around 20kgf.

Tail dragger landing gear is for minimising empty weight since a nose leg would carry a significant landing ground load.

The main difficulty of the design is to take of so heavy in the required length.

[Sparky Paul]

My designs use a tricycle landing gear, adjusted to have the wing at the alpha needed for the Cl at the takeoff point and airspeed. This has the least drag configuration while acclerating from the dead-stop in the permitted distance.
The landing gear then is short, and stiff.
A stalky bicycle gear has problems with the wheel alignment side-to-side as the plane's weight shifts during the takeoff run.
And the requirement to rotate the tail up at the higher weights makes it difficult to -stop- the rotation at the desired pitch angle... the plane's typically continuing to rotate until the prop hits the ground..
The planes when properly flown don't land hard. Soft landings are not unusual.
The wheels I use come from inline skates. They're solid plastic with ball-bearing hubs. The rolling friction is practically zero.. so little that the plane can't taxi without accelerating too quickly.
And it should be taxied as little as possible anyway, to minimize wear on the wheels.
My 2003 plane had no ailerons. Rudder, elevator, motor and flap.
I've seen too many aileron caused tip-stalls on these heavy airplanes to feel they are needed.
This one has tip-stalled on takeoff with flaps, and self-recovered, something an aileron plane most likely would not do.
The flaps aren't all that great an idea, as the plane tends to lift off at too slow a speed for reliable lateral control. Ailerons would be a definite hazard.
On landing at the high weights, it's typical to use ALL the runway length, with enough energy left to do a 180 at the far end and still come back some considerable distance. A tail-dragger would tip over doing the turn.. or when it runs off the pavement onto the grass.
The Fort Worth Thunderbirds who host the event this year have a DVD of last year's event, well worth getting to see the environment there.
And the crashes are amusing... :)

[dayhead]

I've heard or read somewhere that slats/slots are not effective at model sizes,anyone care to comment?

[Sparky Paul]

Never tried slats or slots.
Andy Lennon has. His book goes into lots of stuff like this.
"The Basics of R/C Model Aircraft Design"

[Air Challenger]

Thanks for the opinions and help on the RC model as we are first year students.
We are still thinking about the idea of using the slats even if we have never tried it before on an model of this size
Thank you

[fhhuber506771]

Slats/slots... the added structural weight and construction complexity is not really worth it on small wings.

What are your design limits by the contest this time? I've seen some very interresting requirements listed from other contests. (break down in a limited time to fit in a given size suitcase... and reassemble as quickly for example)

[Air Challenger]

Our design limites are wing span of 1.83 m and a 10cc engine, the model has to lift off in 60 m with the most weight it can carry

[vintage1]

Biplane then. If your span is limited, use more wings!

You will get a lower flying speed and more energy into lifting than into maintaining speed.

Shorter takeoffs too as less flying speed needed.

Use a fine pitched prop on the motor of larger diameter than normal too.

[Ollie]

"You will get a lower flying speed and more energy into lifting than into maintaining speed.

Shorter takeoffs too as less flying speed needed.

Use a fine pitched prop on the motor of larger diameter than normal too"

Right on, vintage1!

Tandem wings (2 x area). Airspeed x ~0.71. Use vert tail on boom. No ailerons. Simple! No slots or slats. Use ground effect to shorten the take off! Use -90 degree flaps to shorten the landing run touch down with big drag and neg. lift. Mount very short landing gears (3 or 4) on wings. Low wings for just clear of ground. Put the thrust between wings and above wings. Prop just clear of ground. Vert tail in prop wash but with ground clearance.

[Sparky Paul]

All good stuff, Ollie, except for the flaps. Probably not needed, with all the drag a biplane has anyway.
The 60" span limit is supposed to bring out the multi-winged things, I expect. Last year the span was 72", and a couple bipes and tandems showed up.
For a first year's attempt though, simpler is better. A good running motor, a -flyable- airplane, and as few gadgets as necessary, always works.

[Sparky Paul]

Biplane then. If your span is limited, use more wings!

You will get a lower flying speed and more energy into lifting than into maintaining speed.

Shorter takeoffs too as less flying speed needed.

Use a fine pitched prop on the motor of larger diameter than normal too.
.
At max weights, Vint, these things use ALL the available runway.. What a bipe would do is up that max. weight. It is an intense competition with some of the best and oddest things showing up.

[adam_one]

Biplane then. If your span is limited, use more wings!

You will get a lower flying speed and more energy into lifting than into maintaining speed.

Shorter takeoffs too as less flying speed needed.

Use a fine pitched prop on the motor of larger diameter than normal too.Yes, a biplane could be an interesting alternative but, one should bear in mind that a biplane has higher parasite drag, also that the flow fields from the two lifting surfaces "fight" with each other, causing something called "interference drag".
In addition, there is an extra parasite drag from the supporting structure required to keep those two wings in proper alignment.
Furthermore, two wings are heavier than a single one...
If carefully designed a biplane may have some advantage, but it may also turn into a "no-win situation".

[vintage1]

I don't disagree, BUT I feel that at very slow speeds a biplanes downsides - drag mainly - may be more than offest by the loss of high speed parastic drag.

I WOULD use ailerons, as they are I feel better able to control a model to within an whisker of a tip stall.

But I could easily be wrong on both points.

[Sparky Paul]

I don't disagree, BUT I feel that at very slow speeds a biplanes downsides - drag mainly - may be more than offest by the loss of high speed parastic drag.

I WOULD use ailerons, as they are I feel better able to control a model to within an whisker of a tip stall.

But I could easily be wrong on both points.
.
Vint these things don't quibble about tip stalls.. They do it instantly!
Ailerons are the primary cause at the speeds they have at takeoff.
100% differential.. no down at all, if you must use ailerons.. otherwise design for rudder control only.

[Ollie]

"Use -90 degree flaps to shorten the landing run touch down with big drag and neg. lift."

I mean 90 degree up flaps at touch down to kill lift and big drag to shorten the landing run.

[mars_ubi]

Actually, the other team of our university is working on the tandem concept with no slats but with Junkers Flaps on the front plane and the same on the rear upper wings but working as elevons as well. Like Michel Columban´s Cri-cri, the world’s smallest twin engine aircraft.
We are going to put a slotted wing of the monoplane on the wind tunnel to check for the lift at this small Reynolds.
Both machines will chare components like the wings, and possibly flaps as well. The theoretical calculations indicate that the tandem takes-off better, but the monoplane will climb better giving a better margin for the flying part.
About the landing gear, thank you Sparky, you’re right. What propeller do you think that will work better for the static thrust of the O.S..61FX?
Thank you all!

[mars_ubi]

Last time we built two radical designs and they ended up with too large empty weights, 22,5 kgf for the triplane, and 12,5 kgf for the monoplane. But the most radical design actually won, flying with only 3kg of useful mass because it was the necessary weight for winning against weak competitors in the design report results. It has flown with 7 kg useful mass but the take-off run was too large for a few meters, about 15m. The triplane flew only a straight line giving unsolvable problems in the reduction gear.

[fhhuber506771]

take a lesson fromthe flying Lawnmower, Pizza box flyer, flying car, flying tank crowd... You have a span limit and no area limit? One big square wing with end plates.

The "joke planes" big drawback tends to be the stuff added on for appearance (that fake engine housing and the huge handle on the flying mower add tons of drag) wit a real airfoil, a "Pizza box" (typically they use flat plates for those) turns into a heavy lifter.

For getting heavy stuff off the ground.. wing area wins.

A little steeper learning curve for a new pilot with the square wing's lack of roll stability... But hang the fuselage under and it reduces the problem a lot.

[Sparky Paul]

Actually, the other team of our university is working on the tandem concept with no slats but with Junkers Flaps on the front plane and the same on the rear upper wings but working as elevons as well. Like Michel Columban´s Cri-cri, the world’s smallest twin engine aircraft.
We are going to put a slotted wing of the monoplane on the wind tunnel to check for the lift at this small Reynolds.
Both machines will chare components like the wings, and possibly flaps as well. The theoretical calculations indicate that the tandem takes-off better, but the monoplane will climb better giving a better margin for the flying part.
About the landing gear, thank you Sparky, you’re right. What propeller do you think that will work better for the static thrust of the O.S..61FX?
Thank you all!
.
When I tested the available .61 sized motors for SAE, looking for suitable substitutes for the then mandated K&B .61, using the APC 14x5N prop gave the best static thrust on any of the test motors, in the vicinity of 8 pounds.
It's my opinion that the K&B is still the best choice, having just a bit more horsepower than the OS. .61FX. Next would be the 14x4, then 13x6.
But firstly the motor must be run in properly.

[Sparky Paul]

Ubi, do you have any other photos or links to photos? I hadn't heard of much European interest in SAE type events, and would like to feature any you might have on my websites.

[mars_ubi]

Paul,

Here is one link to a photo gallery of the first competition that was held here in Portugal, Air cargo Challenge 2003,

http://aero.ist.utl.pt/portal/modules.php?set_albumName=AirCargo&op=modload&name=Gallery&file=index&include=view_album.php

And this one is the link for the 2005 event that will take place in September,

http://aero.ist.utl.pt/~apae/aircargo/

But both sites are in Portuguese.

[mars_ubi]

take a lesson fromthe flying Lawnmower, Pizza box flyer, flying car, flying tank crowd... You have a span limit and no area limit? One big square wing with end plates.

The "joke planes" big drawback tends to be the stuff added on for appearance (that fake engine housing and the huge handle on the flying mower add tons of drag) wit a real airfoil, a "Pizza box" (typically they use flat plates for those) turns into a heavy lifter.

For getting heavy stuff off the ground.. wing area wins.

A little steeper learning curve for a new pilot with the square wing's lack of roll stability... But hang the fuselage under and it reduces the problem a lot.

From the theoretical calculations there seems to be an optimal wing chord for a certain wing lift coefficient where the possible take-of weight matches the weight that the aircraft cake have in flight with the available power. But if we use a chord just a little bit lower than the structure can be lighter.

[mars_ubi]

Thank you Paul for the propeller advices. We were using a APC12,25X3,75 and an APC13X4 with the objective of making the engine reach the maximum power rpm's, but we didn't measure thrust.

[Sparky Paul]

Thank you Paul for the propeller advices. We were using a APC12,25X3,75 and an APC13X4 with the objective of making the engine reach the maximum power rpm's, but we didn't measure thrust.
.That is much too small a propellor.It will over-rev the engine.
Those are good sizes for a .46 Fun Fly, but not for a .61.

[Sparky Paul]

Paul,

Here is one link to a photo gallery of the first competition that was held here in Portugal, Air cargo Challenge 2003,

http://aero.ist.utl.pt/portal/modules.php?set_albumName=AirCargo&op=modload&name=Gallery&file=index&include=view_album.php

And this one is the link for the 2005 event that will take place in September,

http://aero.ist.utl.pt/~apae/aircargo/

But both sites are in Portuguese.
>
Ubi.. thanks a LOT!.. There's little difference between the things we design over here and there!
But some of those are more "interesting" than anything I've seen in the 4 years we had the evern locally...
What are the specs and how did this one perform?
And which was the winner?

[Sparky Paul]

...

[mars_ubi]

That one didn't fly, Paul. It was an interesting concept from Instituto Superior Técnico(http://www.ist.utl.pt/),Lisboa, but I don't remember the estimated payload or empty weight. Span was 3m, engine OS.61 FX, propeller was APC 15x6, estimated with a software that I don’t know as the best for static thrust. These guys were very inexperienced in modelling so this machine had a very poor construction in carbon fibre reinforced plastics. It had lack of rigidity all over and the rear plane was completely twisted along the span, my guess is that it couldn’t fly like that anyway.

The winner was our UBI2 from Universidade da Beira Interior(http://www.ubi.pt/), Covilhã which was built as a proof of concept configuration. It worked and has a spectacular low speed flight capability, but beside that, and what I have told before, it seemed to suffer from Dutch roll for every speed above slow. We also had problems with main wheels alignment, and the tail was heavier then it was predicted, so, this made us increase the nose length.

The best flying machine was from Universidade do Minho(http://www.uminho.pt/Default.aspx?lang=en-US), Braga. It weighted only 5,5kg empty, compared with 12,5 kg for UBI2, it has flown with 6 kg of useful load. They tried with 7 kg but it didn’t seem like it wanted to take-off. I heard that it crashed after the competition.

[mars_ubi]

.That is much too small a propellor.It will over-rev the engine.
Those are good sizes for a .46 Fun Fly, but not for a .61.

We measured 13,500 rpm for the highest revolutions mixture setting with the 12,25x3,75 it doesn’t reach the 16,000 rpm specificated by OS for the max. power?

The competition and the engine test were performed at 500 m altitude. Design take-off weight for UBI2 at that altitude was 21,5 kg.

[Sparky Paul]

You want the motor at the torque peak, not the horsepower peak.
I got out my latest SAE plane with the OS 61 FX on it, but find my tachometer is broken.
If memory serves, with the APC 14x5N, rpm was in the 9,000 -10,000 rpm area.
I also had the plane hooked up to a scale, so I could measure the static thrust.
This showed about 8 pounds at the best rpm for the motor with that prop.
The 14 inch prop provides a lot more disk area than a 12 inch.
I can't find any sensible information on the OS 61 torque characteristics, on the 'net.
But look for the 14 inch props, 4 to 6 inch pitch, instead of the smaller ones.
Wood is better than plastic, it's lighter.
I had a whole lot of 14 inchers, but gave them all to the Embry-Riddle Daytona Team in 2001, when they ran out of theirs.
They had the best plane, but a taildragger.. over-rotating at takeoff took out most of their props. Their last flight at a potential winning weight failed when the control horn on the elevator broke right after takeoff.

[BMatthews]

The interplane gap on a biplane has a lot to do with the reduction of that interference drag. Keeping the interplane spacing more than one chord is good. IIRC about 1 1/2 chords is the best bang for the buck. Minimal drag and minimal problems with wide spaced wings. More spacing is better but other stability factors may come into play.

But on the whole I think a biplane is a viable alternative to super wide chords. If some research on using solid and wide interpane stuts located at the tips could be done I think there's some gains possible through the control of tip vortices as well.

[BMatthews]

No wonder it over rotated. The tail looks far too small for the job.

[Sparky Paul]

Note the splay on the landing gear.. That's a hand-laid up autoclave result.. they broke two of them!

[Sparky Paul]

The interplane gap on a biplane has a lot to do with the reduction of that interference drag. Keeping the interplane spacing more than one chord is good. IIRC about 1 1/2 chords is the best bang for the buck. Minimal drag and minimal problems with wide spaced wings. More spacing is better but other stability factors may come into play.

But on the whole I think a biplane is a viable alternative to super wide chords. If some research on using solid and wide interpane stuts located at the tips could be done I think there's some gains possible through the control of tip vortices as well.
.
It really boils down to how much plane can the motor accelerate to flying speed in 200 feet.. Wide chords tend to have thick profiles, which aren't good.
That poor little motor can do just so much. :)
The guys from Canada do well with biplanes, extremely well designed and engineered. Lots of gap, and span-loaded with quite thin wings.

[BMatthews]

Ya know Paul, since you've obviously had your hands deep into a few of these SAE efforts, I've often thought it would be good to help keep the university teams honest and provide a comparison if one or two modeler based teams were allowed to compete. We have the background for all the disciplines and would probably win the first few times but it would help teach the students a bit about compromise designing. A factor that some of the whimsical flights of fancy seem to have ignored.

What do you think?

[fhhuber506771]

We measured 13,500 rpm for the highest revolutions mixture setting with the 12,25x3,75 it doesn’t reach the 16,000 rpm specificated by OS for the max. power?

The competition and the engine test were performed at 500 m altitude. Design take-off weight for UBI2 at that altitude was 21,5 kg.

It is not suprizing to me that the 12.25X3.75 would not turn up to rated HP RPM.. They often rate the horsepower at some unatainable rpm by use of a far undersize propeller. (rated BHP sells engines... They'llrank one up and blow it up to get the rating... as long as they can prove the number.) Its mighty hard to swing a 12 inch prop that rpm even with a low pitch... calculate the tip speed and you will understand.

The group that used the 15X6 on thier engine... used way too much prop load. A .91 4-stroke would have troubles with that prop. A .91 2-stroke might be OK with it (but it would be close to overload...)

Aim at 13X4, 13X5 or 14X4 for a .61 engine when seeking maximum static thrust. (fastest accelleration for take-off) do a few test runs of the engines with these sizes, using a fish-scale to measure the pull the engine provides. (the simplest way to test static thrust.) Just tie the scale to the back end of a model.. and rev to full power.. read the "weight" as the thrust. The biggest number wins. Use that prop. My bet is.. the 14X4 will win. (APC "14X4 W" if you can get it) If you can get Bolly "clubman" props.. look at the 13.5X4 and 13.5X5 (harder to find around here than the APC Master Aiscrew and Zinger brands)

Note.. for your purposes a "flixible" prop is NOT detrimental. They flatten out under heavy load, autiomaticly reducing pitch. This gives you a bt of a "constant speed" prop effect, maintaining load n the engine as airspeed increases. (meaning you are still at high torque and getting high thrust because the prop doesn't really "unload" the same as a more rigid prop would) This is why I recommend the Bolly Clubman. They have a bit of this characteristic. APC is very rigid with essentially no flattening under load. (And is better for most other purposes)

You do not need more than 11,000 rpm out of the engine, measured durring the static thrust tests. But if you can't get 10,000 out of it... you need to reduce prop load. (.61 2-stroke ball bearing supported crankshaft.. typical of OS.61 and the K&B .61)

[Sparky Paul]

Ya know Paul, since you've obviously had your hands deep into a few of these SAE efforts, I've often thought it would be good to help keep the university teams honest and provide a comparison if one or two modeler based teams were allowed to compete. We have the background for all the disciplines and would probably win the first few times but it would help teach the students a bit about compromise designing. A factor that some of the whimsical flights of fancy seem to have ignored.

What do you think?
.
It's tempting.. and some groups do get a lot of help with experienced modellers in their group.
SoarNeck is one I've met. Paul ?? is another.
But it's an academic event, and sometimes a faculty advisor will direct the team into the unworkable unknown, following his pet theory.
There's also the requirement to present the aerodynamic basis for the design, and predict its performance. And present this to a panel of experts who rate both the design itself and in an oral presentation expect reasonable defense to pointed questions.
Many good modellers who can whip this stuff out because they know what works don't always know why it works, in the scientific view.
But the most important team member generally turns out to be the pilot. Most of the designs can fly, but they're so close to unflyable guys with normal sport and even competition experience can't save the plane.
Tony Frackowiak and John Redman out here get many teams into the air and back safely, when no one else can. If these two can't fly the plane, no one can!
I built my things just to see what I could do, especially seeing what horrors showed up at the 2000 event. An experienced modeller on a team would not have done the mechanical setups some of these guys used.
That's why I devote a fair amount of space on my site to the equipment installations, noting cable runs, sensible positioning, ease of access.. things a math major wouldn't consider. :)
The best airplanes generally stand out from the crowd anyway, usually thru extremely good design and engineering, along with accumulated experience in the team.
Ecole Technique Montreal always fields superlative designs, flown well.
The Cederville guys, obvious modellers and very good designers present minimal airplanes, but they're always contenders for first place. (They use K&Bs, obviously splendid fellows. :) )
I think the way the events are structured are OK.. it's one of the few such in academia where a subject isn't just talked to death, but has to be proven in the real world.
The East Coast competition is this weekend in Deland, FL. Be interesting to see what is used there.

[mars_ubi]

“They had the best plane, but a taildragger..” -Paul,

I thought of an taildragger because of reputation for short take-offs in real scale planes, and because of the savings in weight and drag. Actually the will was in the trying. I’ve made a taildragger with the main wheel too much to the front and it was a challenge to take-off with that thing. If we want stability on the ground run then we must put the main wheels further back, I think that’s the origin of over-rotation when the tail gets up. In the real planes the instability is always easer to control because the period of the unstable movement is much bigger.
“…when the control horn on the elevator broke right after takeoff.” –Paul
Was it a flying tail?

[mars_ubi]

The interplane gap on a biplane has a lot to do with the reduction of that interference drag. Keeping the interplane spacing more than one chord is good. IIRC about 1 1/2 chords is the best bang for the buck. Minimal drag and minimal problems with wide spaced wings. More spacing is better but other stability factors may come into play.

But on the whole I think a biplane is a viable alternative to super wide chords. If some research on using solid and wide interpane stuts located at the tips could be done I think there's some gains possible through the control of tip vortices as well.

I think you are right. It's a fact that the induced drag is the 1st enemy of this contest. My only and important note is that it gives almost twice of the construction work. That triplane was a hard pain work. That box wing idea is something that I sure would like to try.

[biber]

I'm not familiar with that kind of competition, but to reduce the empty weight to the minimum seems to be quite important. For that purpose a biplane should be a very good choice. In addition very thin high lifting airfoils are possible with a framework of rods, cantilevers and wires used to make the thing stiff and stable. Or is a high cruise speed demanded? For that a biplane certainly wouldn't be a good recommendation. I wonder if going without ailerons for saving weight could give a benefit too.

[mars_ubi]

Paul, …still about propellers. That triplane had a nice looking MENS, wood, 34x10, via a reduction gear that we got from a hand-drilling machine that looked robust. I recall now that we have made a rudimentary thrust test with a self-made rubber dynamometer and it was showing around 20 pound. We only did it once but I think that this value was only available at the beginning of the 1st test flight, before the reduction gear broke for the first time, after that we just got in troubles.
The APC 12,25x3,75 was used because I heard from a hobby shop guy that it was used for 3D with the .61 engine.

Why do you say that the wood propeller is better? Did you measure greater static thrust with any wooden propeller? Why do you use APC?

[mars_ubi]

I'm not familiar with that kind of competition, but to reduce the empty weight to the minimum seems to be quite important. For that purpose a biplane should be a very good choice. In addition very thin high lifting airfoils are possible with a framework of rods, cantilevers and wires used to make the thing stiff and stable. Or is a high cruise speed demanded? For that a biplane certainly wouldn't be a good recommendation. I wonder if going without ailerons for saving weight could give a benefit too.

Those are all good points Biber!

[Sparky Paul]

This is the SAE site..
http://students.sae.org/competitions/aerodesign/east/
Rules for each year's competition change based on the current year's experiences..
Here's a compilation of 4 years at SAE Design West..
http://www.angelfire.com/indie/aerostuff/sae2000n1.htm
.
There's also a very active competition in Brazil
I've lost the URL, or the site is down.
.
AIAA also promotes a challenging event for electrics.
Their site appears down right now also.
It's usually a series of tasks.. the plane has to fit a box, be assembled rapidly, fly, land, pick up a payload, fly and land again. A bit more challenging than SAE which is pure grunt. :)

[Sparky Paul]

Paul, …still about propellers. That triplane had a nice looking MENS, wood, 34x10, via a reduction gear that we got from a hand-drilling machine that looked robust. I recall now that we have made a rudimentary thrust test with a self-made rubber dynamometer and it was showing around 20 pound. We only did it once but I think that this value was only available at the beginning of the 1st test flight, before the reduction gear broke for the first time, after that we just got in troubles.
The APC 12,25x3,75 was used because I heard from a hobby shop guy that it was used for 3D with the .61 engine.

Why do you say that the wood propeller is better? Did you measure greater static thrust with any wooden propeller? Why do you use APC?
.
I ran a propellor test just now, using my 2003 airplane and the OS 61 FX. Wind was variable and gusty, which bothered.. used a Zinger 13x6 wood, APC 13x7, and APC 14x5N. The best prop for these was the APC 13x7, turning 10,200 and pulling about 8-1/2 pounds.
With the 13x6, I got 10,100 rpm, 7.1 pounds.
The 14x5N, 10,700, 7.2 pounds.
I have a 14x6 Master, but it would have required major surgery to fit the spinner, and didn't feel like doing that today.
The 12.25x2.75 is what I use on my OS 46 FX in my 3D Top Cap. I'd never consider a prop that small for a high powered .61. The OS 60 FP, a modest motor might like it, but it's too small for the more powerful FX.
Your geared motor is for the Open class, I have no experience with that.

[joao]

Hello all,

Mars UBI, I think a flying tail means an all moving horizontal tail surface, and that can easily translate into a too sensitive "elevator" .

Cheers, and keep the taildraggers for "sports" flying only ;)

João

[mars_ubi]

OK João, Taildraggers for fun only!

About that tail, looking at the picture again I can see what seems to be a hinge line. I just couldn’t see a stabilizer and an elevator at first, and it seemed to have a very negative deflection.

Sensitive control means too much control volume coefficient so it can mean also that it is a way to reduce the needed tail size and have less drag and weight. I once made a flying tail in a sailplane model that had, at the same time, a separated elevator so it was all moving and the camber was varying also. It was very effective and the control response was great. Thank you Joao because you have made me remind of that.

[mars_ubi]

About the propeller I think that the most radical would be a single bladed with counterweight, 15 inch diameter would have only Mach 0,6 at 10000 rpm, 17 inch diameter would have Mach 0,71 at 10000 rpm, it possibly needed a special blade tip and it would sure be noisy. But that is quite a challenge…

[Sparky Paul]

OK João, Taildraggers for fun only!

About that tail, looking at the picture again I can see what seems to be a hinge line. I just couldn’t see a stabilizer and an elevator at first, and it seemed to have a very negative deflection.

Sensitive control means too much control volume coefficient so it can mean also that it is a way to reduce the needed tail size and have less drag and weight. I once made a flying tail in a sailplane model that had, at the same time, a separated elevator so it was all moving and the camber was varying also. It was very effective and the control response was great. Thank you Joao because you have made me remind of that.
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A flying tail made a lot of sense when there was the total projected area limit.. wing, fuselage and horizontal all has to total less than 1200 sq. in. This led to the bulgy front pods, skinny tail booms and very small flying tails way back there.
With the change to a simpler span limit, the horizontals could be more sensibly sized and positioned.
But flying tails still are used. No reason not to, if the volume is proper, and the control sensitiyity isn't extreme.
And they can be made with less area than a split horizontal.
I tried a couple of vesions, one with a geared tab at the trailing edge attached to the fuselage, which assisted the servo in moving the horizontal off neutral. Didn't notice anything extremely bad or good about it.
An ordinary flying tail, with expo set for a soft response around neutral works quite well.

[Sparky Paul]

About the propeller I think that the most radical would be a single bladed with counterweight, 15 inch diameter would have only Mach 0,6 at 10000 rpm, 17 inch diameter would have Mach 0,71 at 10000 rpm, it possibly needed a special blade tip and it would sure be noisy. But that is quite a challenge…
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On the APC props, we've found the tips is quite important. With the 14x5N which we use most on our Kadet Squadron planes, if there's a ground strike and the last 1/4" or so is ground off, the plane's performance is noticeably diminished.
In our display flying, we do a cross-runway formation takeoff that uses the 40 feet available, with a pull-up to fly off to the right. With a shorter prop it isn't as reliable a manuver as we care for.

[Sparky Paul]

About the propeller I think that the most radical would be a single bladed with counterweight, 15 inch diameter would have only Mach 0,6 at 10000 rpm, 17 inch diameter would have Mach 0,71 at 10000 rpm, it possibly needed a special blade tip and it would sure be noisy. But that is quite a challenge…
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There are a lot of propellors in the 15 to 17 inch range.. which might work as single-bladed. The dynamic balance would be the most difficult situation to accomplish properly.

[fhhuber506771]

Hello all,

Mars UBI, I think a flying tail means an all moving horizontal tail surface, and that can easily translate into a too sensitive "elevator" .

Cheers, and keep the taildraggers for "sports" flying only ;)

João

Yes a "Flying stab" is where the entire stabilizer also acts as the elevator by pivoting the whole surface.

No, flying stabs are NOT overly sensitive... they are prefered for many high speed aircraft because it is easier to get VERY precise control movements. A 1 deg deflection of a flying stab does a lot more to pitch than 1 deg of deflection of a trainer's narrow elevator... but the linkages are easy to do to give higher angular precision.

Flying stabs are almost useless for 3D... because to get the throw needed for hover maneuvers, you have excessive throw for flight at speed. (aldo there's an effect of bending a surface that gives MORE control power than pivoting the whole surface)

Modelers don't have to worry about this one: A flying stab is effective at mach.. a hinged elevator can be blanked by the sonic shock wave pattern.

Flying stabs present no hinge line gap as a source of turbulence to initiate flutter. This make them desierable for high speed flight due to reduced flutter AND reduced drag.

I have used flying stabs on 130mph+ slope racers.. and I have a neat .15 ci powered 2.3 meter ARF motor-glider here (just arrived.. E-bay auction item) which uses a flyig stab for its ability to be set up to minimize drag while giving a smooth control response appropriate for a beginner.

[joao]

Hello,

My two gliders with flying tails also work both very well indeed with some exponential and the correct movement range.

Just wanted to remind the "Air Challengers" to check out for over-control.

Regards to all and thank you for your advice !

João

[mars_ubi]

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There are a lot of propellors in the 15 to 17 inch range.. which might work as single-bladed. The dynamic balance would be the most difficult situation to accomplish properly.
Paul, do you know FD3/64 small scale turbojet? The compressor and turbine are balanced by trial and error, holding, one at the time, by hand trough a ball bearing, adding tape to different positions while making it rotate with an air jet. When correct balance is accomplished with the tape, weight is removed from the opposite side. Maybe in this case you can just hold in you hand trough an electric motor and make it rotate at moderate speed applying a small power to the motor.
I might be wrong but the propeller is more similar to a disk than to a cylinder and that makes dynamic balance not very important. What do you think about that?
I think that a big perfect spinner can help the engine in working with a bigger propeller and in the case of a single bladed it would help in providing a bigger arm for the counterweight.

[Sparky Paul]

Mars, a single blade prop is quite different from the fairly uniform distribution of material a jet disk has, or even the conventional two-blade prop.
The only successful appliacation of a single-blade I'm familiar with is in rubber powered planes, such as Wakefield (or whatever the current FAI designation is).
These are large, but low revving, with the applied torque continuous as the motor unwinds, not pulsating as with a single-cylinder IC motor.
Getting a good static balance with the weight well secured to the hub, I'd have the prop shaft go thru whatever retains the weight, and then trying it for dynamic stability shouldn't be difficult, but might be "interesting" if the dynamic balance and the motor vibration cycle feed on each other. :)
Some control-line speed planes have used single-bladers, but not too successully, if their lack of use by most fliers indicates the practicality of the idea.

[fhhuber506771]

There were some successful uses of single blade props on full scale Piper Cubs in the late 1930's and early 1940's... the problem they ran into was premature bearing failures. (less than half normal life for the bearings) The thrust not being balanced around the prop shaft tries to bend the shaft... inthe cases of the full scale cubs, the bearings were weaker than the shafts.

[mars_ubi]

Mars, a single blade prop is quite different from the fairly uniform distribution of material a jet disk has, or even the conventional two-blade prop.
The only successful appliacation of a single-blade I'm familiar with is in rubber powered planes, such as Wakefield (or whatever the current FAI designation is).
These are large, but low revving, with the applied torque continuous as the motor unwinds, not pulsating as with a single-cylinder IC motor.
Getting a good static balance with the weight well secured to the hub, I'd have the prop shaft go thru whatever retains the weight, and then trying it for dynamic stability shouldn't be difficult, but might be "interesting" if the dynamic balance and the motor vibration cycle feed on each other. :)
Some control-line speed planes have used single-bladers, but not too successully, if their lack of use by most fliers indicates the practicality of the idea.

Take a look at the Silent motorglider
http://www.alisport.com/eu/eng/silent_a.htm#

In fact beside the mass static and dynamic balance, there is also the rotating thrust out of the axis, that can be solved through a flapping hinge, and there is the rotating pitch moment of the blade that is not annulated by the other blade, for this problem the only solution is a zero pitching moment coefficient airfoil in the propeller blade.

Your idea for holding the counterweight in the shaft is wise for a first try. It we want to add a flapping hinge, the counter weight must move with the blade flapping. Anyway great care must be given to any propeller test.

[Sparky Paul]

Mars, the link doesn't work.
A flapping hinge is a common feature on helicopter rotors.
But these are very low rpm large devices.
The wear on the bearings in a small device with a flapping hinge would be quite high.
Tolerances would be very tight, possibly unobtainable without a great deal of expensive machining in exotic materials.
I don't think the benefits would pay off relative to the cost of developing and maintaining the system in a model airplane sized device.

[humberto]

Sparky, the link work. Try again.

[joao]

The link again, (very human these digital devices, moody etc ;))

http://www.alisport.com/eu/eng/silent_a.htm#

João

[Sparky Paul]

thanks.. that's an interesting setup. I see a (toothed) belt drive between the motor and prop. Keeps the motor low. The single blade doesn't need as large an opening in the top for retraction.
That's the kind of glider I'd go for.