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Need help choosing engine size

Old 01-13-2004, 03:35 PM
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iflyrc82
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Default Need help choosing engine size

I am doing a buildup of a B-29. The person i got the plans from said that the model was designed for 4 .15 to .25 engines.. This plane has an 82.5 inch wing span and is 58 inches long. I feel that a smaller engine would work, i was considdering using 4 Norvel .74 engines. If anyone has any sugestions, please e-mail me at [email protected]


THanks
Adam
Old 01-13-2004, 03:59 PM
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CafeenMan
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Default RE: Need help choosing engine size

what's the wingspan?
Old 01-13-2004, 04:02 PM
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CafeenMan
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Default RE: Need help choosing engine size

PS. If the plan is an older plan, then you may be right - engines have gotten much more powerful over the years. I haven't built any 4-engine bombers, so take my thoughts for what they're worth here. What I've seen is that many of these kits would appear to be way over-powered with the suggested engine range - especially toward the high end that is suggested. That's assuming it isn't a lead sled from a lot of detail.
Old 01-13-2004, 04:12 PM
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Default RE: Need help choosing engine size

It has an 82.5 inch wing span and is 58 inches long. I know engine requirement mostly depends on weight, and i am not sure what this plane will weigh when finished, but it looks like a fairly light design, then again, i would like to have retracts and a bomb bay, so this might be to much weight for the .74. I dont really know
Old 01-13-2004, 04:14 PM
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CafeenMan
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Default RE: Need help choosing engine size

I think those .74's are going to be too small. It sounds like the old Royal kit.
Old 01-13-2004, 04:40 PM
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Default RE: Need help choosing engine size

Yeah from what feedback i have gotten so far, it looks like im going to have to go with a family of .15's
Old 01-13-2004, 05:00 PM
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Default RE: Need help choosing engine size

Definately go with the .15's. There are a couple of 125" B-17's here and they are all flying comfortably on 4 x .25's
Old 01-18-2004, 03:48 PM
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Default RE: Need help choosing engine size

I have seen the magnum 15's used a lot for combat. I hear they work well and are reliable. It is only what I read so take it with a block of salt. I agree that 4 15's or 18's would do the job.

Mark Shuman
Old 01-18-2004, 04:20 PM
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Default RE: Need help choosing engine size

Others in another forum have sugested that i use the .25's even, i am greatly considdering this, i dont want to sacrafice the ability to keep this model looking scale by using such a large engine, but im afraid that the weight of this aifcraft is going to be to much for the .15's. With the retracts, and the bomb bay that i would like to have a payload capability for. Any thoughts?
Old 01-18-2004, 04:37 PM
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Default RE: Need help choosing engine size

If it is going to be that heavy you would be better building it to 125" span and fitting your 4 x .25 engines. That is what the guys here are flying the .25's in. Remember, 4 good .15's will pull a greater weight than you think.

If you want the extra power then why not meet in the middle and put in 2 x .15's and 2 x .25's.
The .25's would go inboard on either side and the .15's on the 2 outboard motors. A lot of people do this to get a nice medium power range
Old 01-24-2004, 02:41 PM
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Default RE: Need help choosing engine size

I sort of missed this one floating around.

I gather from the amount of discussion that the decision has pretty much centered on a size by now but I'd like to offer a few ideas.

I find it helps to think about multi engined models in terms of a few smaller normal models flying in VERY close formation when choosing what size engines to use. From there we can break down the model into separate models for consideration of power and weight based on how we want our multi to fly. This isn't exactly a new idea. The Dehavilland Mosquito was designed to fly like a Spitfire based on the concept of twice the power, twice the weight, twice the wetted area and a bit less than twice the wing area of the Spitfire. And if you look at some of the dive bombers and full bombers of the same eras I think you'll see similar parallels. Take a Douglas Dauntless and multiply the weight and wing area by 4 and I'll bet you come up with something sized close to a B17. Both aircraft performed in roughly the same envelope given the subtle differences to allow for the scaling effect.

So with this in mind let's look at some multi engine ideas for our models. For example, take your B29 plans and find the wing area, divide by 4 and consider this "single" in it's own world. This "single" will have it's own size of wing area and weight based on how you want the overall model to fly. With me so far? So let's consider what sorts of models fly with the engines you've discussed so far.

The engines we use will carry a LOT of weight up if we don't expect them to loop and snap roll from level flight For example I've seen the lowly Cox 049 fly a hot little 16 oz skyrocket with fine style. But I've also seen the same engine pod launch a 2 1/2 lb glider with a solid, if unexciting, uphill glide. Each engine has a similar range of weights it will be happy with.

The Norvel 074 is a hot rod of a mill that can easily fly a hot and highly stuntable model of up to about 30 oz. But With a bit more wing area it'll easily fly a trainer type of model of probably 45 oz quite nicely if in an unexciting manner. No loops from level flight for example. I'll table the engines and weight ranges I think they can fly for models from hot to mild below....

[ul][*] Norvel 074 30 to 45 oz---- 37 oz is probably a good target for a B29 type model[*] Max 15LA 32 to 50oz------ 42 ----------------------[*] Mas 25LA 40 to 65oz------ 55 -----------------------
[/ul]

To carry these weights with a nice flying model that does not need to speed around the sky takes wing area. The engine makes it go fast and climb and the wing area lets it fly slow and take off sooner in the acceleration run. Simplified for sure but true IMHO. But there is no avoiding the issue that a lighter wing loading makes for a nicer flying model that stalls less violently and flies in a more scale like manner. Based on the "nice" target weights I've suggested above and considering that this model SHOULD fly with a rather stately nature in the air I would say you're looking for a wing loading of no more than 16oz per sq foot. But others will probably differ with my what may be seen as a rather glider like wing loading. So let's say 18 oz/sq ft as a compromise. I would consider this to be as high as you would want to go unless you want to see your model flying around the sky looking like a high speed Mustang all the time. And let's not forget about slowing down for landings and still having enough lift to prevent lawn darting.

So based on 18 oz/sq ft and the weights in the table above we can do the math....

(NiceWeight) x 4 / 18 oz/sqft = (wing area)

so.....

[ul][*]Norvel @ 37oz/engine = 8.2 sq ft or 1180 sq inches[*] 15LA @ 420z/engine = 9.3 sq ft or 1330 sq inches[*] 25LA @ 55oz?engine = 12.2 sq ft or 1760 sq inches
[/ul]

Doing it a bit backwards lets look at what we have. The 25 is a well know engine size and based on my numbers I see that each engine is carrying a !QUOT!single!QUOT! model of 440 sq inches and 55 oz or 3 1/2 lbs. These are really nice numbers that produce a good trainer type model that can still dance when called upon. I like what I see here.

The Norvel would be a 295 sq inch model flying at just over 2 lbs. Not great for the take off and landing speeds but still a fairly happy model I'd like to see a bit more wing area or less weight for this one. Comparisons show that the Norvel 074 puts out about the same power as an OS 10LA.

So how does this fit in with your 84 inch B29?
Old 01-24-2004, 03:18 PM
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Default RE: Need help choosing engine size

The wing area of my B29 due its tapered leading edge is only around 600 In square.. seems small to me, but this is what the plans call for.. i really dont know how much the plane is going to weigh, but i do plan on having retracts and a bomb bay, so to be on the safe side im guessing around 12-14 lbs. what do you think, i think im going to have to use the 25's just due to the weight.
Old 01-24-2004, 05:09 PM
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Default RE: Need help choosing engine size

Please go back and re-read what I said about wing loading. It's not about adding power to fly. It's about building light enough to avoid the lead brick syndrome.

Please believe me when I say I'm not trying to be cruel but no one would ever consider flying a single engine model that weighs even 10 lbs on only 600 sq inches of area. Adding more engines won't change that fact. Adding more power to a bad situation will just speed up the inevitable disaster.

To achieve a decent flying model with 600 sq inches I would not want to see a weight of more than about 5 pounds. Some more charitable folks may suggest that 6 lbs would be OK. I seriously doubt you'll find many folks that would enjoy flying a 600 sq inch model that weighed more than 6 lbs and most would prefer the 5 lb limit.

If it was me trying to do a 600 sq inch B29 I would go for 4 Norvel 049's or 061's at most and use construction techniques that were more in line with these smaller engines. Fuselage planking from 1/16, wing sheeting from 1/32, nacelles planked with 1/16 and then the whole model finished with dope and tissue with a final light spraying of silver and some LIGHT detailing. Such a model could be kept within the 6 lb limit. Retracts that are light enough and strong enough to handle this sort of model on paved surfaces are available. Using mini servos would ensure the radio weight was kept down. The bomb bay could even be equipped with LIGHT bombs that would drop slowly but add a lot of visual appeal. There would still be lots of room to add operating but simple and light flaps. The 4 Norvels would easily handle this magnitude of weight and the model would probably still be able to loop either from level flight or with only a slight dive.

I noticed in this and the other thread that you never talked about the wing area of this plan. If you had I think you would have gotten much more accurate feedback concerning the engine selection. The one proper bit I saw here was Team MPP's bit about making the 125 inch version.

I know this is certainly not what you wanted to hear but you'll be putting a whole lot of work into this project. To accept the idea of adding power to try and bandaid a very bad wingloading issue would be a real shame. Granted my logic above may not be totally in line with what some folks consider acceptable but I don't think you'll find anyone that does not agree that even 10 lbs is way overboard for a 600 sq inch model.
Old 01-24-2004, 05:43 PM
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Default RE: Need help choosing engine size

Wow, this is a bad deal for me then, i already have the main section of the wing built and dry fitted together... I never thought that the wing area for this aircraft would be that far off. The plans i have are very old and were orignaly for a control line aircraft. I really have no idea of what this plane will weigh when complete, but the plans call for 3/16' balsa or bass wood ribbs for the main rib sections whitch is 5 ribs on each side, the remaining ribs are light ply, and the twomain spars are made from 1/4 bass wood. the main keel's of the aircraft are 1/2" x 3/16" for the top and 1" x 3/16" for the bottom, a material is not calld out, i was going to use bass wood for that as well. There are about 18 ribs of the fuse and they are all constructed from 1/8" lite ply. The plans do not show any planking for the outside of the fuse, but i was going to plank around where the wing meets the fuse with 1/32" balsa. The fuse is 58" long.

Still using the parts that have already made, i could possibly get 700 SQ IN out of this wing, but that would be it.
Old 01-24-2004, 09:53 PM
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Default RE: Need help choosing engine size

What does it say it will weigh as a Control line model with the listed (in plans) engines? Do the old plans assume the engines will have mufflers?

Subtract the weight of an appropriate engine multiplied by 4. (If the plans expect un-muffled engines.. you need to NOT subtract the muffler weights!)

Add the weight of your .15 multiplied by 4. (or whatever engine you are going to use... Include the mufflers as you will use them.)

Add the weight of your planned radio equip and the retract mechanisms. (since i assume the CL version had fixed gear)

You now have your estimated finished weight.

NOW you can get an idea of if its going to fly for beans.
Old 01-24-2004, 10:01 PM
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Default RE: Need help choosing engine size

These plans do not call out any information of that kind... Not the best set of plans in the world
Old 01-24-2004, 11:21 PM
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Default RE: Need help choosing engine size

For control line it may have worked out. But then a lot of older control line plans came out seriously heavy. And flying a control line model does not have the same landing requirements as an RC model.

What you describe for the structure is way over the top for a CONTROL LINE model of this size let alone an RC model. I hate to say it but it would be best to cut your losses now. The plans could serve as outlines for parts that are cut from more applicable size and selected wood and the parts skimmed on their internal outlines but that's about it.

I'm truly sorry to be the messanger on this one as I could see the enthusiasm in your earlier posts but it would be worse to let you carry on in great expectations only to have them dashed into toothpicks. Or end up with a model that survives it's first flight despite the nervous breakdown of the test pilot and ends it's days hanging from a ceiling because it's just not worth the risk to fly it ever again.

If you're interested in persuing this project see if you can post up sections of the plans one pic at a time and I'm sure you will find folks to aid you in your efforts.

For myself I won't be back here for a week. Off on business.

Good luck.
Old 01-27-2004, 09:16 PM
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Default RE: Need help choosing engine size

After reading all of the above posts I think B Matthews is right on. Stop where you are and junk what you have built. I agree that the wood selection is off. I would look at 1/16 balsa for ribs and sheeting. i would punch ligtning holes in the ribs and any fuse formers. Look to build up the tail feathers. Maybe 1/32 balsa sheeting may even be a possibility. Balsa spars and stringers for sure. i would cut out all unnessecary wood where I could. Remember you need to rig up fueling and throttles for 4 engines. If I may make a suggestion. Maybe some of the others can kick it around some. How about 2 15's inboard. Just dump the outer 2 engines. It would simplify construction. I also believe you could save some weight. i do agree that about 6# is your absolute max. I would also like to know how you plan to handle wing removal. Plug ins or full width and bolt on wing? It would help to plan out the fuselage structure. I

I would take the plans and spread them out on the kitchen table. Grab a note pad and a cup of coffe and give them a hard look. Think of how you are going to lay out your radio installation. look to swap lighter wood or construction. Plan all you want to do. Choose engines and fuel components. I would like to say if you did not have retracts try looking at fixed gear. Remember a set of retracts of any sort including mechanicals will weigh around 1#. If you must have retracts look at some mechanicals if the wing is 1 piece. If you go 2 piece then I would suggest pneumatics are the only option. Make all of your notes and then sit back and plan out what you need to do. I would also use a monocoate or similar finish. I am not familiar enough to advise on a doped tissue finish. Maybe someone out there has more info than me on a lightweight painted finish. I wish you well and just remember to think light.

Mark Shuman
Old 01-27-2004, 10:13 PM
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Default RE: Need help choosing engine size

What i have done so far is draw most of the wing section on Autocad (im a cad tech so this didnt take long) i plan on changing theplans i have to autocad, then posting them a section at a time to get information on what modifications i should do to make this project work. I am very disapointed that a flyable plane can not be made from these plans. The person i baught them from (who happens to watch these forums) claims he built te same plane, and it flys great... i just dont know. I have thought about scaling up the aircraft to a 120" model and giving the wing more area. I have part of the scaling and changes done on a CAD file. But i just dont know now.. there is so much more to think about now. I hate that i paid $20 for a set of plans, then baught $60 in materials, then wated all of my materials because of crappy plans.
Old 01-31-2004, 06:02 AM
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Default RE: Need help choosing engine size

Scalling it up to 120 inches would provide a wing area of.....

((120/82.5)^sq'd )x600 = 1270 sq inches

Now you've got something you can work with. Also based on the fact that larger models tolerate higher wing loadings easier and still fly well I think you can push THIS size of wing up to 25 to 30 oz/sq ft which would make the target flying weight of your model....

8.82 sq ft x 25 (30) = 220 oz ( 264 oz) or between roughly 14 to 16.5 lbs. At the lower figure you would have Sig Kadet like flying charactaristics assuming a good airfoil and proper wing washout design to prevent tip stalling. And I suspect if the weight was up to 20 lbs it would still fly sort of OK but the fun factor would be greatly reduced by the need to handle it with kid gloves.

In any event I would strongly recomend cheating a little and use a more model specific airfoil for this project. A Selig 4233 has been used on a few models with good success and apparently offers a fairly friendly stall and good lifing charactaristics. Adding flaps would further enhance the landings and takeoffs by adding camber, lift and wing washout for these two critical times.

Based on my earlier engine to wing sizing this would still technically be too small for four 25's but I'm willing to grant folks that the scaling factors with the larger models comes into play and in this case 25 would be a nice selection provided the weight can be kept to the lower 14 lb limit. If it get's heavier then you may need a trifle BUT ONLY A TRIFLE more power. four 40 four strokes would be a marvelous selection with the sound they would have but in that case a slightly larger model up to 1400 to 1500 sq inches would be a better match I would suggest.

There was a big B29 that flew at, I believe, this years Joe Nall meeting. I'm not sure how big it was but it WAS big and the engines were quite small in size as I recall. It offered spirited performance that included loops and inverted passes. Hardly scale I'll grant you but any model that can perform that sort of flight will most likely be a kindly model to generally fly around and will take off and land without the dangers that overly heavy wing loadings can bring.
Old 01-31-2004, 02:42 PM
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Default RE: Need help choosing engine size

A bit small for .25 2-stroke... about right for 4-stroke?

The sound of four 4-strokes... swinging nearly scale size 3-blade props. Just thinking of that = [8D]
Old 01-31-2004, 03:12 PM
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Default RE: Need help choosing engine size

yes that would be nice, but the 4 strokes a so expensive... I am strongly considdering them though. I have read some posts of people running 4 strokes inverted, is this an acceptible practice, because any engine i use is going to be inverted. And any tips on things tt keep in mind when running engines inverted would be helpful, lke fuel tank location and such. I will have a set of plans for my wing design on here soon in the form of a cad file... I may start a new thread i dont know... i hope someone will be interested in looking at my design and giving me tips. I will need a lot of help, i have never built a plane this large before.. Also i will be needing information on using a wing tube design to join the two wing halves... Once again i have never done that as well, and really havent even had teh chance to study designs of any planes that do.

Adam
Old 01-31-2004, 03:13 PM
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Default RE: Need help choosing engine size

This C-130 stand-off uses yer basic Kadet construction techniques.. a LOT of air inside, a Kadet wing, and 4 OS 40FP's.
It's close to being over-powered at maybe 12 pounds AUW.
Flies well on inboards only, or outboards only.
A longer more scale wing wouldn't hurt the appearance nor degrade the performance.
As Ben mentions, lightness counts.
A good looking B-29 really shouldn't have the sticks showing as they do on the C-130 though.
Fully sheeted fuselage and wing, with the sheeting used as a monococque structure for the fuselage.
Just don't crash it!
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