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Old Aug 17, 2012, 08:09 PM
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Tupolev model 91

I have not posted in quite a while and never in this particular forum, but, hope that possibly I could get some input or let me know if this is the wrong sub-forum.

While flipping through one of my Russian aircraft books (actually in a couple of my books...I like Russian stuff...) I came across the Tu-91. For several years I had wanted to build one of these http://ram-home.com/ram-old/tu-91.html, but, just haven't had the motivation until recently. My thoughts were that this design had broadly pattern plane-like proportions, but, also had some dihedral in the outer wing panels for stability as I am a competent but not awesome pilot.

My plan was for a wingspan of around 850mm to about maybe a meter. This would be about 1/16 to 1/18 scale. Construction is to be hot wire cut foam (the only thing I have used for scratch building). I was further planning to use low temp covering film over the foam for a stressed skin construction with carbon fiber reinforcing in critical spots such as the wing spar.

I have thought about enlarging the tail surfaces by 15 percent and using a high lift airfoil on the main wing along the lines of the A-10. In fact I had planned to just copy the GWS A-10 airfoil. I want something that has the definite "flavor" of the full scale and still slows down pretty well. The contra rotating props should cancel out any torque as well.

At this point I have most parts ordered, but, am waiting on the motor system. This is to be a contra-rotating brushless setup; really only because it looks cool as there is no efficiency to be gained that way. The first motors I ordered http://www.hobbypartz.com/88e-mc351-...e3-1550kv.html did not come with the nice spinners like it was supposed to so I have some on order from EBay http://www.ebay.com/itm/AEO-Model-15...11%26rk%3D2%26 as well as a lower KV version with two blade props http://www.ebay.com/itm/AEO-Model-11...item1e6ec99125. Both these come with nice spinners and that is important to me that it looks sorta scale. It might be overly difficult to try and make something totally scale, so, this is an acceptable compromise for me.

I could really use any input/suggestions that the group here could offer as it has been several (5 or more...) years since I last scratch built anything. If this one works out I may consider building a larger one around a contra-rotating setup Hobby King sells which would be around 1.8 meter span.


Thanks in advance for any help you can provide.
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Old Aug 17, 2012, 09:22 PM
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Very cool!
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Old Aug 18, 2012, 11:36 AM
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[QUOTE=turbonut;22478103]Very cool![/QUOTE

Thanks! Maybe I need to move to scratchbuilt foamies section? I just am not sure whether this post/topic fits here since I don't plan to use traditional balsa and ply construction.

Really this is a project being designed around a powerplant. Much in the same fashion as things have been done in full scale coństruction. The engine in the Tu91 is the same as those used in the Tu95. It is unfortunate that Kruschev did not like it as pilot reports were very positive and the Soviet Navy felt they needed it as a shore based aircraft after the decision was made to not expand into carrier ops. Additionally; I think it is such an uconventional/ugly plane that it is cool! Hats off to the Tupolev design bureaux!
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Old Aug 18, 2012, 01:13 PM
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[QUOTE=DR. JOY;22482220]
Quote:
Originally Posted by turbonut View Post
Very cool![/QUOTE

Thanks! Maybe I need to move to scratchbuilt foamies section? I just am not sure whether this post/topic fits here since I don't plan to use traditional balsa and ply construction.

Really this is a project being designed around a powerplant. Much in the same fashion as things have been done in full scale coństruction. The engine in the Tu91 is the same as those used in the Tu95. It is unfortunate that Kruschev did not like it as pilot reports were very positive and the Soviet Navy felt they needed it as a shore based aircraft after the decision was made to not expand into carrier ops. Additionally; I think it is such an uconventional/ugly plane that it is cool! Hats off to the Tupolev design bureaux!
Fear not that the building material of choice might be inappropriate, as a little lurking in this forum will reveal quite a few foam-based builds:

http://www.rcgroups.com/forums/showthread.php?t=1669382

http://www.rcgroups.com/forums/showthread.php?t=1672171

http://www.rcgroups.com/forums/showthread.php?t=1661701

http://www.rcgroups.com/forums/showthread.php?t=1252568

A thorough search will dig up even more. It's an interesting scale subject, and while you may not see a butt-load of comments, rest assured the scale folk hereabout will be looking in to see how you deal with any design dilemmas!

James
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Old Aug 18, 2012, 01:53 PM
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Duplicate post. Darn Kindle........
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Old Aug 18, 2012, 01:55 PM
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Duplicate post. Malfunctioning Kindle operator.
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Old Aug 18, 2012, 01:56 PM
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[QUOTE=Deuce;22482771]
Quote:
Originally Posted by DR. JOY View Post

Fear not that the building material of choice might be inappropriate, as a little lurking in this forum will reveal quite a few foam-based builds:

http://www.rcgroups.com/forums/showthread.php?t=1669382

http://www.rcgroups.com/forums/showthread.php?t=1672171

http://www.rcgroups.com/forums/showthread.php?t=1661701

http://www.rcgroups.com/forums/showthread.php?t=1252568

A thorough search will dig up even more. It's an interesting scale subject, and while you may not see a butt-load of comments, rest assured the scale folk hereabout will be looking in to see how you deal with any design dilemmas!

James
Thank you!

I can foresee some problems but its the ones ya don't see that get me. I did just get back from picking up my thin ply for templates and man has it gotten expensive since my last purchase. I make my templates from ply covered in aluminum tape to give a smoother surface and cut. I adhere the templates to the foam with the UHU Por like glues on the template only. It holds it in place securely but can be easily removed.

As far as covering the foam I have to decide between my original plan of low-temp covering or silkspan with thinned polyurethane varnish, odorless mineral spirit and gorilla glue suspension. That gives hard finish to the foam, but, is heavier than using the econokote. On the plus side it is lighter than fiberglass and gives a smoother surface.

At any rate; I have the butcher paper, ruler and marker out and am going to go ahead and start finalising some plans (no CAD for me!). I will use the motor I have on hand to get started with thrust testing etc. I plan to post some pics of the equipment I have on hand later.
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Old Aug 25, 2012, 04:57 PM
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I got the drawings enlarged. These are photos of the lateral and 50% top view.

I was slightly worried about component placement. I don't think there will be a problem with that given the space available. Now I am getting excited about this build as I have the majority of the equipment on hand. At this point I am waiting on my new transmitter that I am getting for my birthday (JR x9503). The motor(s) have arrived and the speed controls are in transit. I will begin with the wing and empenage. My tail surfaces will be enlarged 15% over scale. I cannot decide if I will enlarge the wing area or not., but, that has to happen before I can really begin.

So; time to cut out the tail feathers.
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Old Aug 26, 2012, 04:39 PM
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Horizontal and vertical stabilizers completed. I did these first so I could say I got something done. I went ahead and covered them so I could see how viable my chosen construction method would work (mostly due to the fact that the covering film was ancient).

I am going to enlarge the wing area. I think I will go with about 15 to 20%. I am just going to play with increasing the chord and seeing if the proportions are adequately maintained. For airfoil I was thinking Clark Y with a few degrees of washout. I am going to stick with the scale amount of dihedral in the outer wing panels.

Next step is plan-view templates for the main wing.
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Old Aug 26, 2012, 05:18 PM
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Interesting aeroplane Dr J. I have no experience with foam being a balsa junkie but trying any new techniques on the minor parts seems a good way to go
I'll be interested to see how the Contra rotating props work out as there are quite a few post-WW2 designs that are appealing.
Keep the pictures and commentary coming - as James said, you may not get a truck load of input as you are 'outside the box' a little but we will be observing

Pat
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Old Aug 26, 2012, 06:53 PM
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Quote:
Originally Posted by maltone View Post
Interesting aeroplane Dr J. I have no experience with foam being a balsa junkie but trying any new techniques on the minor parts seems a good way to go
I'll be interested to see how the Contra rotating props work out as there are quite a few post-WW2 designs that are appealing.
Keep the pictures and commentary coming - as James said, you may not get a truck load of input as you are 'outside the box' a little but we will be observing

Pat
Thanks for the encouragement! Here is a picture of the power plant, but, BPHobbies has some new contra-rotating motors in stock that deserve a look as well. Given that I am going to stay at about 1 meter wingspan the setup I have planned should be good. However, it is good to know there are some alternatives.

I do plan on using some balsa. The cowling is going to be balsa. I am going to have to modify the scale outline such that it will match up with my power-plant. I have developed a sensitivity to balsa over the years of building kits and have to wear some protective gear (respirator and goggles) in order to work with it. I particularly hate the respirator, but, that is the way that goes. When I started building a few little scratch designs foam was the way to go.

As far as other contraprop aircraft the Breguet Alize (I don't know I spelled that right) and the Skyshark look intriguing. The Westland Wyvern is also cool. I saw a review if an ARF Wyvern in one of the paper mags recently. The author of the article was lamenting there were no scale power options for it. I think he is wrong as Maxx products markets several motors. Perhaps one of those would be adequate. The Fairey Gannett (again probably spelled wrong) would be pretty unique. If I were to build any of the above it would be the Skyshark. I like the Skyraider with its' boxy look and the Skyshark is similar enough to make me interested. Finally; I do have a couple of motors en route for the next project. It will (may) be a fantasy scale type creation based somewhat on the lines of another soviet aircraft with a TV2 derivative for power. This is strictly on the hush hush mind you and is entirely dependent on my attention span.

Progress wise:

I also got the elevator hinged. Plan to work on templates next. I have got to settle on an airfoil. The airfoil selected will dictate whether I will install the retracts that I have on the way. It is all a matter of compromise while keeping it somewhat scale. The work week beckons, however, so things will slow down some. Maybe it will go smoothly and I will get more done than I think.
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Old Aug 29, 2012, 03:40 PM
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I at least got the main wing plan view templates cut out today. I have also (after some input from the more scientific minds on the forum) come back to the Clark "Y" airfoil section. Now I just have to scale them and cut them out. I will try and get some of the wing blanks cut tomorrow, but, have family obligations and that comes first.
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Old Aug 30, 2012, 01:11 PM
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I am looking for some more opinions. I have retracts ordered. These are the ones http://hobbyking.com/hobbyking/store...idProduct=7701. I also have these struts on hand from a different vendor http://hobbyking.com/hobbyking/store...in_Trike_.html.

As I am still waiting on the retract units I am faced with the question of whether it is worth adding them or just go with fixed gear and save them for a different model. Essentially; is the lowered drag from the retracts offset by the weight of the system (87 gm for the struts and 88 gm for the retract kit if we believe the HK website). This is about 6 ounces more or less: I would say not a trivial amount of weight at this scale. This does not take into account the mounting plates for the wings etc. Granted it would be more scale and look quite cool to have the landing gear suck up out of sight with fancy sort of scale struts..........

I want some opinions. Would you go with a simple lighter fixed gear or go for the whole setup? Maybe I could use the struts in a fixed gear arrangement or alternatively just use the retracts without the fancy oleo's.

Despite the fact that I have been flying for about 17 years or so I have limited experience with retracts and only have used them on ARF's where they are already installed.

As always I really appreciate any ideas or thoughts. I do not think I am getting out of my depth here, but, I do like to try new things. In my previous planes I have just used simple fixed gear, so, this is a little bit of a departure. I do have some really small Robart mechanical gear here somewhere. Perhaps that would be a better plan? I do have to find them first.
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Old Aug 30, 2012, 05:23 PM
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I have the airfoil templates scaled and have called in a consultant. Katness has put down her bow and picked up a ruler.
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Old Aug 31, 2012, 10:41 AM
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laminating the balsa blocks for the cowling/cockpit area. Additional pics of several contraprop units. Just got two from HK today and assembled them. They are for a future project. Also time to do a mid project workbench cleanup. Also carving and sanding away all the balsa that doesn't look like an airplane.

Still interested in any suggestions about retracts versus fixed gear.

Slowly making progress.
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Last edited by DR. JOY; Aug 31, 2012 at 06:28 PM. Reason: Additional random thoughts
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