Scale prop problem.

[Wing Zero]

I am looking at building a model which uses a prop arrangement similar to that of the TU-52 Bear. The original model calls for a counter rotating four blade prop set. At scale, the prop is only about 6 to roughly 6.5 inches in diameter. However, there seems to be nothing out there that is 4 bladed in a size of 6~6.5 inches. Even then, counter rotation that small would be so difficult for such a demanding airframe. So I plan to use a dumb prop and let it free wheel in the main props thrust. I know this isn't the best idea, but I am trying to keep scale-ish. The prop needs to also be able to put out about 2 pounds of thrust. So it would seem pitch is going to be an issue. Can anyone offer some advice - positively. I thought about milling down a center on a high pitch small prop, and gluing the two halves together, adding a tube in the center and a sleeve to help help reduce stress. I'm not sure if I could get a 6x prop to put out like a 11x7 or so. I thought perhaps if I could find like a 6x6, making it a 4 blader would be close to equal? I'm having real difficulty trying to find a prop set up. Motor and batteries haven't been purchased, so there is room for changes.

[vintage1]

I can certainly do laser cut 4 bladers to that size..contras too.
What's the model size, wing area and weight?..need to work out what you need as you say, pitch and power wise.

Then start looking for suitable props..surely the bear has about 4 engines..?

2lb of thrust from 6" blades sounds infeasibly high RPM to me..id guess around 30K+..or is this a VTOL project you want?

[JetPlaneFlyer]

Why are you thinking abot such a course pitch prop?

Generally lower pitch gives more (static/low speed) thrust but a course pitch gives higher top speed potential. If you want maximum thrust as low/moderate speed then a course pitch is the very last thing you want.

Steve

[Wing Zero]

The bear does have four engines... this only has one. The model is a A-90 Orljonok. I have a full set of plans I plan to put to use. The model is roughly 70" long, with about 40" of wing span. I realize that anyone who is completely oblivious to how ekranoplans work, will instantly say it can't be done or won't work. I would assume the model would be no more than 7.5 pounds. I intend to keep her as light as possible. Once the dynamic air cushion has been created, the prop is the cruising engine and will need to be able to deal with a high end of speed. Static thrust does nothing in the way of a moving model, static thrust is good for some calculations, but shape, size, weight, and such play a move major role in the end. I realize I am asking for alot, but if the real craft can be done, a model should be doable. As far as less pitch... with more pitch the throttle can be backed off. With a prop with too little pitch, spinning it any faster won't do much good.

[HugePanic]

why not drive both props???
there are two-motor-assemblies available, or you could build your own one.

this would also help creating more thrust....

http://www.hobbycity.com/hobbycity/store/uh_viewItem.asp?idProduct=6565

[JetPlaneFlyer]

Wing Zero,
You maybe need to try a program like Motocalc which will be able to calculate what prop will give you optimum thrust at various flying speed for your chosen motor... I'm sure you will find that for a 6" diameter prop it comes up with a much finer pitch than 6" .
If you use too course a pitch the blades will stall just like a wing flown at too high an Angle of attack.. stalled blades don’t generate much thrust ;) . Any prop selected has to give enough static/low speed thrust to move the model through the water and get it up to flying speed.. Sizing a motor/prop only for cruise is no use, the model would never have enough 'oomph' to move more than a walking pace through water. Or are you planning to fly the model from the land rather then the water?.. Even if you are then the prop still needs to be able to accellerate the model from stationary to flying speed in a reasonable distance.
Note that the real A-90 Orlyonok had two jets mounted in the nose which blow onto the wings to increase thrust and lift to get the 'aircraft' in the air... So even the full size could not take off just using it's prop therefore your assumption that the model will work on the prop alone because the full size did, is fundamentally flawed.


BTW... you need to read up on your ekranoplan theory... They don’t 'ride on a cushion of air' (are you thinking of a hovercraft?)... They are Wing In Ground effect (WIG) vehicles, they fly just like a conventional aircraft but take advantage of ground effect which reduces induced drag making them more efficient. At high speed there is also some ram effect that increases the air pressure under the wing and therefore increases lift but Im not sure this would be effective at model flying speeds.
http://www.aerospaceweb.org/question/aerodynamics/q0130.shtml

[vintage1]

The bear does have four engines... this only has one. The model is a A-90 Orljonok. I have a full set of plans I plan to put to use. The model is roughly 70" long, with about 40" of wing span. I realize that anyone who is completely oblivious to how ekranoplans work, will instantly say it can't be done or won't work. I would assume the model would be no more than 7.5 pounds. I intend to keep her as light as possible.

I would have said no more than 2lb myself. Built light.

Once the dynamic air cushion has been created, the prop is the cruising engine and will need to be able to deal with a high end of speed. Static thrust does nothing in the way of a moving model, static thrust is good for some calculations, but shape, size, weight, and such play a move major role in the end. I realize I am asking for alot, but if the real craft can be done, a model should be doable. As far as less pitch... with more pitch the throttle can be backed off. With a prop with too little pitch, spinning it any faster won't do much good.

On a 6" prop to absorb a couple of hundred watts..you will be up in the sort of 20k-40k area..speeds will not be high..I would guess a 6x3 sort of prop would be in the ball park. The photo I saw appeared to have 3 bladers on too. Pretty sure there is a 5x3 ore 6x3 three blader around.

[mnowell129]

Tbut if the real craft can be done, a model should be doable.
This isn't completely true, a scaled up bumble bee is fairly impossible, likewise a 1" wingspan 747 is fairly impossible. Air doesn't scale.

With a prop with too little pitch, spinning it any faster won't do much good.
Also not completely true, if you can spin it faster it will go faster, you only need coarser pitch when the motor has a limited RPM range.

I'll be looking forward to video of a 6" prop pulling a 7 pound model out of the water.

From this picture
http://www.militaryphotos.net/forums/showthread.php?t=84344
It looks like this class of aircraft had steerable thrust on the front engines. My guess is to lift the nose out of the water and break the surface tension so it could break out the water. This adds to JPF's skeptical position that the model will get out the water on prop power alone. Maybe a 2 pound model and V1 guestimated...

[JetPlaneFlyer]

It looks like this class of aircraft had steerable thrust on the front engines. My guess is to lift the nose out of the water and break the surface tension so it could break out the water.

I'm not sure the thrust was vectored (though I may be wrong on that).. I’m pretty sure that the nozzles were angled to act like wing blowers to increase airflow over the wing, thereby increasing lift, which raised the beast out of the water allowing the 'aircraft' to accelerate up to it's minimum 'flying' speed. The jets also gave considerable additional thrust during the 'take-off' phase. As I understand it once it was in cruise the jets were shut down.

Most of the Russian WIG designs featured similar jet engines in front of the wing.... they would never have got out of the water without them and I’m pretty certain a model would also struggle to do so.

[Wing Zero]

Okay... I have in front of me, a set of 1:32 plans. With hull sections, ducting details, etc. I also have two books, with a combined amount of photos reaching nearly 70, and over 50 pages of information alone on the plane, along with specs that match almost everything else I have ever seen. So...

I find it interesting, how people move from one simple subject into the more complex parts without considering that they have been considered. Why do people do this?

Jet engines - Yes I am well aware of this, as mentioned by someone, they are to assist in take off, and are shut down once air born. They assist in blowing air under the wings to help create the the ground effect. I have a neat little thing here just for this job called a EDF. I have a few different sizes as well. Some use thrust vectoring to acheive mixed results, such as the KM, others strictly use the them to aid take off.

Prop(s).... Yes, having the prop based on being a cruising method is well enough, as that is what it was meant for. Yes it is used during take off, and no, it isn't the only source of power. Yes, I could go with a larger prop, and it would look out of scale. Second, I don't know of many r/c models which use 4 blade props for flight, especially smaller models. I have a need for a small four bladed prop capable of good thrust.

HugePanic - Thank you. This might just be very useful. I had passed on counter rotating because of limits, but this would definitly be a huge help. Yes, thats also why the props on the A-90 are smaller, the counter rotating props provide alot of thrust in a small area, as well as help in a couple other areas. If it can be adapted to a small four blade setup, I'd be very happy.

Moving on... the A-90 has roughly a 1:3 thrust to weight ratio. The craft is NOT capable of flying outside the area of the WIGE. Which is high enough to pass over most ships (at that point). I only said 7.5 as a maximum. 7.5 pounds with 4 pounds of thrust seems it would be able to get airborne. I hope to get the model as light as possible.

"BTW... you need to read up on your ekranoplan theory... They don’t 'ride on a cushion of air' (are you thinking of a hovercraft?)... They are Wing In Ground effect (WIG) vehicles, they fly just like a conventional aircraft but take advantage of ground effect which reduces induced drag making them more efficient. At high speed there is also some ram effect that increases the air pressure under the wing and therefore increases lift but Im not sure this would be effective at model flying speeds." Really? Seems to me I understand the concept more than you. A hover craft uses a STATIC air cushion. I specificly stated DYNAMIC... as in forced, generated by movement. Some types of WIGS, are not able to fly outside of the WIGE. There is not enough thrust nor wing area to even be remotely possible in the real one. I am sure a well over powered model could. As far as not being able to use the WIGE on a model... how much would you like to bet? Go do a search on WhizzyWig. It was an intersting model. There are others out there who have completed home built WIGE ships as well. I built a WhizzyWig back when I had plans, and it was very unique.... The home page isn't working anymore, but you can still find images on it....

You know, over the past years I have found that the forums have become much like my LHS. Ask for help, everyone goes way beyond what is asked, and then they ASSUME the other person is clueless, and that what they are trying to do, is impossible.... I'm done ranting. I asked for what seemd to be a simple problem. Then the assuming of others had to start. I came for help and encouragement, I don't know why I bothered. Vintage - your were of somehelp, so thank you for that. Huge- huge help. Thanks.