Does anyone know what the smallest inner pipe diameter I could run on a k-45 ( this would be a 70 cm long pipe)?

Thanks

The Wren 44 tailpipe size is on this diagram and should suit a K-45.

https://drive.google.com/file/d/0B4u...NDdVA1MmM/view

Wren make a 66cm long pipe as standard, but any required length can be ordered Tail Pipes & Insulation Sheet

Not a chance. The tail cones are different sizes, and the wren pipe is a diverging design. You almost need a "60" sized pipe.

That pic makes the K-45 look a lot larger in diameter than the 44 whereas there's only 1mm difference. What are the actual cone exit diameters? If the K-45's exit dia is a lot bigger, what will that do to the exhaust velocity? If the K-45's exhaust velocity is significantly lower as a result of the bigger exit area, will that mean that the K-45's top speed will be slower than that of the 44 in the same airframe?

I'm not sure what you are asking/saying with the "1mm" comment... but the K45 is MUCH fatter than the mw44. The K45 is shorter. Overall... the K45 is a heavier engine on it's install. AND... Kingtech changed the ECU and now it needs a 3s Life battery. (hard to find high current LiFe, so the required ecu battery is bigger too)


In the purest sense... yes... the lower RPM of the rotor, and the larger nozzle on the K45 will produce a lower velocity, and in theory, could produce a slower flying model. But you have to remember that we are dealing with velocities upward of 1000 mph coming out of a small turbine... so it's really the drag of the airframe that will dictate the top speed of the model. SO... on a very slippery model... the 44 MAY be faster.... but on a draggy model... the k45 may be faster. But, without a radar gun, and someone willing to swap engines in a couple different airframes... who knows.


As a final FYI... the K45 is a great running engine, and I would recommend it to anyone wanting to get it... but I was a little disappointed once I got it into my hands, and put it beside my wren. (I've stuffed my wren in some small models)

Once a get the kids off to school, and I get some coffee into me... I'll measure the nozzles. (I did it before, but don't remember what the sizes were)

Tony

Gordon is right the Wren is 75mm dia and the Kingtech is 76mm dia... a 1 mm difference. The Kingtech is shorter making installation potentially easier.
Both engines produce good performance and reliability and the only noticeable difference is that the Kingtech is thirstier.
They will both happily use the same exhaust tube.

The really big difference is the cost.

The Kingtech is VERY much less at $1450 plus any import duty.
The Wren 44 is £1650 plus VAT making it £1980 to UK and European buyers
For those buying at Dollar prices the £1650 converts to about $2205 plus any import costs.
In fact you can get a Kingtech 70 for less price than a Wren 44 and have a lot more power.

Dealing with the battery issue is a minor problem.

John

OK.... 1mm on the od of the can. But still doesn't sound right. But, OK. (can't get a good measurement on that since they are mounted)


I was able to get calipers into the nozzles, and the wren is around 32mm.... and the K45 is 38mm. With that info... I guess the wren pipe could work. Just a little less room for error. (it's in the low 50mm range in the bellmouth)

Thanks for your replies Tony and John. I don't really think that there'll be a worthwhile difference in top speeds with the engines in identical jets. Probably more important is the relative turbine acceleration and deceleration figures, and the amount of residual thrust affecting the landing approach onto a short runway in a tree-bounded sheep field. My 44s are a bit slow on both accel and decel so I set up a flight idle of 100k rpm which helps during manoeuvres, but is far too high for landings. So I program the normal 55k rpm idle to be active when the gear is extended, but the resultant slow "slam" time can make an overshoot and go-around something to be avoided.

The Wren pipes are claimed to slightly increase available thrust due to their divergent first section, a feature other pipes don't seem to have.

Gordon

5.4kg through a 46mm pipe.... often people go too big....

Henke... as always, love your work/data you post. For comparison to the pipe... do you know what the nozzle opening is? (I wish Kolibri would come to the USA)

Nozzle opening usually is linear with thrust. ...or used to be, but now days with higher internal pressure we see more oumpf through smaller outlets. K70 has exactly the same nozzle as K45, I compared at Jetpower. If the K70 was the real deal I do not know though. I know some engines do not like back pressure such as Frank, while others do fine. My friends flies the Hawk 240 on a 75mm pipe. (Might have been the 190R too, but still 200N+ on 75mm tube.)

T50 is 33mm on my CAD model, engine installed at the moment so can not double check. But I know I had engine plus calipers when doing the model.

My bottom line is that people should test more. Even though grumania and Tamjet make awesome tubes I think they are not testing enough and selling shelf parts. I run a 46mm tube for 5.4kg thrust with video to proove I gain 200grams with the tube while Grumania suggests the 65mm pipe as it's listed 40-80N.

Agreed, I am actually in contact with Gumania for a pipe, but given what I am trying to do, the Lambert is my next best option, K45 is off the table due to the size and pipe requirement. t that can size I might as well go K70 and that is too big

The Wren diagram suggests a pipe inlet diameter of 1.5x the turbine nozzle diameter. Would that be a decent ballpark figure for all turbines or is there more to it than that?

Gordon

Hi Henke,
how long was the pipe in that video?

Also, I sent you a PM, thanks

Don't remember, it was for my Victory race jet. Now flying it in the Super Viper. 11lb including 9min of fuel LOL. I can check both lengths for you later today.

Seems resonable. Remember the setup is tuned for an all round maximum performance. testign static is only just static. I have seem pipes implode as speed as they were too big. Pipe size, and air inlets need to be tuned for the speeds you expect. I'm not a wiz at it but doing some basic testing usually gets you there - and often way of the standard charts.

The Wren pipe sizes were developed from testing that I carried out when the prototype 44 was first made. It was not an easy engine to start and I found it was not possible to start the engine if the pipe was too small. I did exactly what Henke did with his tests and mounted an engine on a test bench with its tail pipe and a thrust scale. I found that the pipe made with the divergent section leading to a straight section gave the best results which was a small gain. I tested with a 54 and a 66 size engine and all the results were published in RCJI. Before I did this work pipes would usually lead to a substantial loss of power. Very few people have been prepared to do the basic research work which has always surprised me. I did find the results were scalable and were applied to a twin P200 powered plane with good results.

I did play about a bit with a pipe that was divergent and convergent sections and was secured to the back of the engine. It had a very small outlet and was intended for a Me163. I also did work on bifurcated pipes which are often badly designed and loose power and make a fearful noise. All this was a long time ago now.

It is good to see that at least one person is prepared to do basic testing, well done Henke.

John