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How to Determine Engine Placement?

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Old 10-26-2015, 09:42 AM
  #1  
ZTrap
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Default How to Determine Engine Placement?

When there is absolutely no indication on the plans, how do you determine how far forward to place an engine? The cowl is NOT drawn on the plans and there is not one notation of the distance between the firewall and the prop. HELP! [and thanks].

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Old 10-26-2015, 11:21 AM
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RBACONS
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If you have a cowl, and you have the plans, then you should be able to figure out where the cowl attaches to the fuselage on the plans. Place the cowl vertically on a flat surface and measure through the thrust washer hole to determine how long the cowl is. You should then be able to add this measurement to the front of the fuselage plans to get a total distance from the firewall to the front of the cowl. Add 1/16" or so for the thrust washer to stick out the front of the cowl. Then measure your engine (and mount) and see if you need to recess the firewall further than the plans show or build it further out to accommodate a short engine. Don't forget to allow for any right or down thrust required which will effectively change the amount of room you need for your engine by a smallish amount.
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Old 10-26-2015, 01:27 PM
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Thanks for the reply, RBACONS! Your recommendation makes sense. Still, the cowl is not drawn on the plans, and I can move the length of the cowl forwards and backwards on the fuselage a good 1". I don't have any parameters or measure of how much cowl to allow.

Thanks again!

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Old 10-30-2015, 11:58 AM
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At that point, it might be wise to make attaining your desired cg the determining factor.
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Old 10-30-2015, 03:16 PM
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What plane and who's plans are we talking about?
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Old 01-24-2017, 01:13 PM
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It's the Texas Twins Dust Devil ... picture of the fuse plan attached
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Old 02-02-2017, 03:31 PM
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A year and a quarter delay? You getting interested in finally starting the build?

That's a VERY short nose shown on the plan. If it is not a heavy engine for the size of the model I'd suggest you mount it so the cowl ends up closer to the front limit for a sturdy mounting and position the engine so the prop is a decent gap in front of the cowl. If you're planning on a 4S engine or a larger size 2S engine for the size of model then you can go with it sitting a little closer to the firewall. But with either option there does not seem to be any need to crowd the firewall. So be generous with room for fuel lines and the like.

As to where it SHOULD be from an aerodynamic standpoint? There's really no serious advantage one way or the other when the mounting point has to be within the range of the cowl fitting over that one inch spacing. So do what you need to do to aid in avoiding as much nose weight as possible.

I'd also suggest using really light wood on the tail surfaces and rear half of the wing and fuselage. That nose is short enough that you'll need all the help you can get.
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Old 02-02-2017, 05:14 PM
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B!

Thank you for your follow-up; you made me smile! Yeah, it's been longer than I care to mention for my building activities. Consider that I had this kit since ~2003!! It was kitted by a long gone small company called Texas Twins (RGS Enterprises), and the plane is the Dust Devil. It resembles Cessna A188 AG Wagon / AIG Air Tractor - love that look. It has a unique "Reflex Airfoil." What can I say, three children, high school years, college years, etc. etc. finally getting back to the hobby again.

Anyway, yes ... I started building again! Just finished the wings last weekend and "they are magnificent," as was said on the Seinfeld show. 87"! Pictures attached. I am starting the fuselage this weekend. I'm also deciding between a new 2S OS .75-AX or a 4S OS .91 Surpass (only ran twice). Would love to use the 4S .91; but the engine is no longer made and I probably can't get parts if I need any service.

Thank you for the advice on the engine placement and, yes, my question was very much related to aerodynamics/physics. Good to know your thoughts on the 'flexibility' available as long as the CG balance point accurate.

By the way, I also have a finish, but uncovered, Midwest Stearman, that has been hanging in my workshop since 2000!!! I read all sorts of horror stories about the plane being extremely tail heavy and needing much weight in the nose. Kept me from finishing it, and searching for a solution. Even thought about a conversion to electric and packing the batteries in the nose.

Anyway, I'll post more pics of the Dust Devil build - I hope to complete it before flying season 2017.
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Old 02-02-2017, 06:18 PM
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....Even thought about a conversion to electric and packing the batteries in the nose.
But these days a motor and pack that has about the same power as an engine is only the same weight or LIGHTER.

If it's not covered yet and if the wood in the tail surfaces is built from "lead balsa" that would barely float it might be worth re-doing them from much lighter wood. Everything you save at the tail reflects at the nose 4 to 8 times over for a total saving of major proportions.

I would not worry too much about the engine. Shy of a crash the only stuff that will fail over time will be the bearings. And those are easily replaced with non OS parts. Use it and enjoy it. That's what it was made for. Or would you rather wait and die only to let some collector have it for pennys on the dollar? Live a little, enjoy your engine.
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Old 02-21-2017, 08:53 AM
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... making some progress

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Old 02-22-2017, 01:39 PM
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Looks like it has a strong crop duster influence from that canopy....
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Old 02-24-2017, 05:01 PM
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making progress and parts taking shape. Engine installed ... using the Os .91 4S Surpass.

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Old 03-24-2017, 11:37 AM
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Today's update. Finished the wing and keeping progress going. More pics attached.

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Old 03-24-2017, 11:00 PM
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Very nice build indeed.

I would not worry too much about spare parts for that engine, it's perhaps one of the most sold 4s engine in the world, meaning thousands (mmm, at least some) of the model shops around the world have spare parts for that engine at the shelf.

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Old 05-23-2017, 09:21 PM
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Default Dust Devil

Hi,
just curious as to how you like the Duster? I built one in the 90's and enjoyed it more than I have any other airplane that I ever flew. That says something, as I've been flying R/C for more than 50 years. I used an OS .70 4 stroke in mine an it was all the power I needed. It really is an amazing design. It got shot down a few years ago in the only interference problem that I have ever been involved in. My fault ;-(. I have a kit squirled away hope to get it built this fall.
Very nice job of building and covering by the way.
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Old 05-26-2017, 07:09 AM
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Hi James - thank for the note on your Dust Devil from the 90's. Good to know it was a good flyer. I have not maiden it yet ... still working on it, bending wood, and working out the controls. I configured a contraption for the split elevator; I think it'll work OK. Some new pictures attached.

I do have a question for you: Do you recall if you added any right and down thrust? The plans and instruction don't mention anything about that. I also noticed in a YouTube video of a Dust Devil that there is considerable pull to the left upon acceleration and take-off. Really appreciate if you can recall.

-- Z-trap
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Old 05-26-2017, 11:42 AM
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reply below, James ... do you remember adding any right/down thrust?
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Old 05-26-2017, 12:33 PM
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Default Duster

Hi,
there was no downthrust or right thrust used on my copy. Like most tail draggers, some right rudder was used for take off. I wonder if he might have had a wind gust that caused the swing? It does seem extreme right at the first. I thought that you must have been finished because of the pic of a completed yellow and black duster at the beginning of the messages.
One of the most fun things to do with the airplane was landing backwards. In a mild breeze you could actually set up an approach into the wind. When the Duster got 20 ft or so past you you could throttle the engine back to the point that it would fly back to you. Moving to full idle and letting the nose down would let you land at almost no forward speed, right in front of you.
James
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Old 06-05-2017, 06:21 AM
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Good progress on the Fuselage this weekend lower planking installed, upper decks installed, empennage squared and installed, firewall fuel proofed with epoxy, tank fitted-in, pushrods fitted. Will review my check-list, but I think Ill be ready to cover the fuselage soon.

Picture from this weekend:
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Old 06-11-2017, 08:50 AM
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A note for next time. The nose of the movable surfaces should be more of a ">" shape instead of a "C" shape. When you round it too much you risk the nose coming into contact with the other surface due to how the shape rotates in the arc of travel. If you look at how you angled but then rounded the nose of your elevators you'll see what I mean. As you angle the surface the rounding causes the hinge line to close up in close to the hinges instead of closing evenly across the whole face as it would if you had made the surface look more like a flat faced ">" shape.

Other than that the progress is looking fantastic.

On the down and right thrust I'd certainly put in some. Two or three down and 1.5 to 2 of right thrust should do the trick.

Model trimming is a highly variable and highly personal sort of thing too. Some like the model to be balanced well back and to use very little angle between the wing and stabilizer so it's neutrally or only marginally positively stable. Others like to have a higher amount of stability and so they tend to put the CG somewhat forward compared to the first near neutral camp. The thing is how the model responds to adding power and how much downthrust you find you need is tied into this selection of the balance point and associated elevator trim. So it's hard to say "use this much of X thrust".

For my own sport flying models I tend to like a stability amount that is quite close to neutrally stable. And for models that spend more or less equal amounts of time inverted as they do upright I like them to be dead on neutral. And the little bit of 3D flying I've done even to have the CG located a touch behind the Neutral point where the model is slightly negatively stable. (it's not as bad as it sounds. No worse than what it's like to constantly correct the car as you drive down the road if the negative thing isn't taken to extremes on the wrong sort of design). And the closer to neutrally stable you like the less need there is for downthrust.

You still want that degree and a half or two of right though. Works well during takeoff to avoid a strong pull to the left.
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Old 06-13-2017, 10:29 AM
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Hi BMatthews: Thank you for the reply, and the notes on the "nose of the movable surfaces." I did deviate from the normal " < " profile and rounded the edge as per the article in Model Aviation, where the author stated a that rounded nose provides better airflow/dynamics, and less drag. However, I do agree with you, and you are correct about about the "<" shape providing better overall movement.

Also, thanks for the note on the 'down' and 'right' thrust. With the torque from the OS .91 4S I expect there will be a good amount of 'left' tendency.' I intend to use washers behind the engine mount, and I can always take the washers out if I don't need the correction.

I am well on the way to complete covering - latest pictures attached. Covering is not my favorite activity, especially around the empennage - pain the the @#!*&&. Also, I think I have a good color match for the plastic cowl - using 'tractor paint' in 'school bus yellow.' They don't make matching paints anymore for the Ultracoat. I'll keep posting on progress.
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Old 06-16-2017, 08:17 PM
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The thing is when folks like that say to round the edge they mean the outer edges exposed to the airflow. But down in the middle of things away from the air flow you still want the hinge line to have a nice sharp "V" shape. One is an aerodynamic thing and the other a simple mechanical need.

Other than this little niggle I'm quite enjoying watching this beast take shape.
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Old 06-17-2017, 05:20 PM
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More progress today. Fuselage covered and some markings applied. I'll work on attaching the canopy tonight/tomorrow, and I'm thinking to use 5-min epoxy. It's a big canopy and I don't want to take a chance on it flying off. More pictures attached ... of the 'beast!'
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Old 06-20-2017, 07:43 AM
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Canopy epoxied and taped. Making cowl adjustments and working on split elevator alignment ... pictures attached.
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Old 06-30-2017, 06:28 AM
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Default Dust Devil Project Completed

Well folks, the Dust Devil is completed. Had to switch around some internal hardware (battery location, etc.) to get the balance point correct, and decided to forgo the black Du-Bro spinner ... their 2.5" spinner doesn't fit the large prop and, frankly, the quality is poor. All surfaces are trimmed to neutral at this point, and I learned to create/allocate a 'Throttle Cut-off Switch.' Will review check-list again tonight to minimize surprises - there are ALWAYS surprises..

I ran the engine a few times on the plane and it probably needs some more break-in time to get a lower idle; but, I think it's ready for the maiden. A few picture are attached of the finished "beast." It's been a while since I've been in the air and will look for experienced assistance for the maiden - maybe this weekend.

I'll provide a report on the 1st flight. Send good thoughts, fellow pilots.
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