ticketec

My Feedback: (2)

Hey Guy's

Starting a new project and can get my hands on a thrust tube that is originally from a CARF eurosport (external diameter is 90mm). I'm using a Wren 54 (15lb) thrust turbine in my model.

Euro's are designed for much larger turbines and I know having too small a thrust tube is a problem, but is too big a tube also a problem, or can I use the tube safely??

Thanks

dave

Dr Honda

My Feedback: (4)

There's always a optimal size and shape. BUT... a big tube will work, without an issue. I guess the only thing you will have to watch for, is proper draft around the outside, so the fuselage doesn't get too warm.

Dave Wilshere

Yes it can be too big and it will be in your case.(unless you are cutting to less than 5" long!
It makes a huge difference, most installs are not optimised anyway, but a big tube on a 54 will give a big deceleration on the exhaust gasses...they expand to fit the void and you will have rubbish static thrust, it will take an age to get off as it needs the ram affect of the incoming air. That and unless you new model is very nose heavy the CARF pipe is a heavy brute.


Dave

ticketec

My Feedback: (2)

Ok, thanks for the info gents, makes sense.

I did look at Wren's divergent/convergent duct the used to make for the small wrens back in the day but they don't make them anymore and it would be over $500 landed, I guess I'll have a proper sized one made for it locally then.

Thanks

dave

Jgwright

Dave

Wren do still make pipes for most sizes of turbines on the divergent then parallel principle. You can also get them from Grumania jets in Germany. They are simple to make locally especially if you can get the bell mouth sent to you. You will need the correct Stainless sheet and a spot welder with accurate control of the weld time as it does not take much to make a hole in the sheet.

John

Henke Torphammar

I found that our 40mm pipe for Kolibri turbines runs cooler then the competitors 46mm tube. The 46mm tube get colored by the heat where the 40mm keeps it's shine. If the setup run to hot the default answer will be larger pipe, but that is often wrong.

Shaun Evans

Hi,

It's true that there is an optimal size for a given thrust rating, but try what you have first. I had a Starfire with a BVM pipe and bypass designed for a Ram 750, but I retrofit a Simjet 1200 in there. Everyone told me it wouldn't get off the ground because the pipe and bypass were too big. Funny thing, it flew fine. Hauled major butt to be exact.

ticketec

My Feedback: (2)

Thanks john and henke

Hey Shaun, great to hear your slowing getting better!

I did see a table once from Wren regarding turbine size vs size of thrust tube but haven't been able to find it since??

I have a mate here who has a spot welder and makes thrust tubes so the equipment is available, just need some ideas on diameters. I'll also have t9 make one for my 1/4 me-163 when I get round to building that one so this will be a good learning experience.

thanks

dave

TJK

My Feedback: (69)

A sincere question about the theory of thrust-loss due to oversizing the exhaust tube: expanding on this theory, wouldn't exposed turbines, such as those on the Kangaroos, Hotspots and BVM Cats, then be considered to have infinitely "oversized" exhausts leading to maximum thrust loss? Obviously, that is not the case, just interesting to think this through, especially considering Sean's real-world experience.

Tom

ticketec

My Feedback: (2)

Interesting point,

If thrust loss is not a major concern (Shaun's experience), then the major concern would be to ensure you cause enough of a venturi effect to cool the pipe down, which lines up with Henke's experiences?

But, We also know that Wren's divergent/parallel design of thrust tube can add almost another pound of thrust so even though the exhaust gasses have exited the turbine, the thrust tube will play a part in the overall thrust output...

More info/data needed...

JohnMac

Some years ago a club member had an original SM hawk fitted with a Wren 54 and a divergent pipe, also from Wren. It flew OK but was a bit marginal on take off. The model came with a bigger pipe for a 120 size engine. I bought the model and fitted a Wren Supersport. It flew much better.
I decided to try to see how much thrust would be lost by going back to a bigger pipe. The result was no thrust loss but an increase.
Bigger seems better to me, If you can fit it in.
John

rfbenn

Don't know if this helps but I just measured the Wren thrust tube I have on the bench for a 54/jubilee/ SS etc.
Diameter at the turbine end is 61mm and expands to 70mm over 175mm the rest from there on is parallel.
(The bell mouth is spot welded to the thinner end)
6thou /.15mm thick steel.

Rob

Henke Torphammar

It's a fact that you can easily increase the thrust with a proper tube. Static thrust are not the only factor though and calculating the whole setup are difficult. It depends on what you want. My foamy conversions have so much power at speed we really need extra drag to slow things down, still they are only up to the stock 6S EDF while static, so we try to optimize the static thrust as they already go like ballistic at half throttle while airborne. Not much calculating, just static testing.

It's easier to visualize if we take it to the edge with a turboprop. It uses the exhaust energy to make thrust that are way more then the turbine.

ticketec

My Feedback: (2)

GOLD!

Thanks Rob!!!

After reading your post I went digging through my emails from wren and actually found this in my emails which lines up perfectly with your measurements! They show the size of the bellmouth opening as 68mm but nothing for the internal diameter so your figure is the missing link!

Thanks

dave

Vettster

Hey Ticketech... Good to see your starting on your ME-163

ticketec

My Feedback: (2)

Yeah, that's one of the two projects I have going ��

have to to stop damaging models though!

thanks

dave