how to make your tj 5%faster - RC Groups
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Dec 14, 2002, 05:23 PM
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Peter W's Avatar

how to make your tj 5-10%faster

i went to work on those chunky nacelles on my twinjet with a small rotary sanding drum. reduced the thickness alround by 3
much more air gets to the prop and as a result the noise it makes is lower in pitch and sounds different in air.
however it is noticeably faster. my 480's are almost shot to pieces but combined with totally flat elevons, you can go a bit faster.

Last edited by Peter W; Dec 15, 2002 at 08:42 AM.
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Dec 14, 2002, 07:48 PM
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RCMustang's Avatar
Notice any difference in motor vibration or nacelle sturdyness? Did you reduce the all around thickness or did you just make the back of the nacelle thinner?

If you can post a picture, that would be great. I'm considering this mod too.
Dec 15, 2002, 08:38 AM
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Peter W's Avatar
at first i was only going to reduce the thickness toward the back of the nacelle, but thinking about it, why would that make any difference if the air first has to go over a thicker portion of foam? so i just went crazy with the sanding drum and reduced the thickness of basically the whole of the nacelle.
so it is not as strong anymore so no crashing.
vibration i have not noticed anything.
however the noise is less in pitch and so i guess more power from the prop.
i reckon it could make 10% difference actually. 8-10% difference of 65mph is around 70mph.
pic, it's pretty ugly and i don't have a digi camera tho i can borrow someones occasionally.

Dec 15, 2002, 02:01 PM
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RCMustang's Avatar
I got to thinking about your modification a bit more this morning. I'm curious what other effects the modification will have. Essentially, you have increased the effective area of the propeller. You could probably achieve similar efficiency by increasing prop diameter. That said, you probably have decreased battery life a bit by increasing required current. The TJ nacelle is about 3/8 in wide. Assuming you carved it down to half that size, you increased the effective area proportionate to a ring with an inside radius of 5/8 in (motor radius plus half the nacelle radius) and a outside radius an inch. That means you added (1)^2 *3.14 - (0.8125)^2 * = 1.06 square inches of area. Incomparison, a 5.5 inch prop has a total area of 2.75^2 * 3.14 = 23.75 square inches. You'll get only about 4.4% more area. If you can equate area to thrust, you may get about 4.4 % more speed at the cost of current. You can get the same kind of thing by increasing prop diameter to a six inch prop. That area will be 28.26 square inches or 19% more area than the 5.5 prop.

Now we get into the thrust vs speed issue. That goes way beyond my simple anaylsis. I only did this to examine you assumption of 5-10% more power. You are close, but I'd rather just change prop size and keep nacelle strength havin run some numbers.
Dec 15, 2002, 04:05 PM
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Peter W's Avatar
sure it draws more current but i can still get good flight times and a friend watching me fly reckoned it was a bit faster too.
the flight did not seem any shorter than normal. tho i only flew it twice cos it was real cold, round 4 degrees C or so.

Dec 16, 2002, 03:05 AM
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You CANNOT (!!!) equal prop-area to thrust!!!
ever looked at the pitch difference along a prop?
===> much slower speed on the inside, therefor higher pitchangle!
adding that area to the outside, as you suggested, would incrase total prop drag and thereby current-draw!

a prop area īhidden by e.g. an engine nacelle isnīt just "as not there", but still rotating, still influencing overall airflow and, since it is partially stalled,producing quite a bit of drag (and that may not just be in the stalled area, since flow laminations are able to extend along an airfoil!!!) an area along a prop, which is stalled, is a drama for overall performance!!!

therefor itīs actually quite a good idea to sand those nacelles and "clean" the airflow in this area!

keep in mind structural integrity!!!!! of course, no nacelle at all would be least draggy, just like NO Twinjet has not much drag either, but itīs only half the fun, isnīt it?
trying to say: no need to cut or sand it all, but making an egg from the box sure is a good idea. think of wheelcovers of a fixed gear plane, for example.
what you aim for is a thing, thatīs "easy" with airflow, that doensīt accelerate it too much, too sudden at any points, therefor doesnīt produce bigger areas of very low pressure or even flow laminations (both equal drag!)

by the way:
sanding the trailing edges of that thing (including the fins!) does the same thing - smoothen airflow, were it has only negative effects.

both combined do quite a bit a job for maxspeed
that is without increased draw, since all you did was take out something, that was decellerating your plane before!!!!!

I know this is the foamies area (and I fly them too!), and not a discussion forum about aerodynamics, but still, they follow the same rules as a 1000 bucks performance-glider, and if we can use that for our advantage, why not do it! we donīt have to be the goofies in the air, just because we like the advantages of a relatively quickly built aircraft!!!

streamlined holidays, from Europe, to all of you,
Sir Lui
Dec 16, 2002, 01:00 PM
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Peter W's Avatar
Thankyou Sir Lui,
I knew i was right
and as for the sanding of the trailing edges? I'm right on it!