R/C Comet 4, DH-86, Javelin and Meteor

[britishtourer]

Very impressive!

[Chris Trott]

Are these not gas-turbine powered? (usually built from turbochargers) In which case, obviously they dont sound "just like the real thing" (mainly due to the small size and much higher rpm of the spool, but they dont sound unpleasant

1) No current R/C jet engines are built from turbochargers. They are all factory-built units which are specially designed for R/C applications. I don't know of any country's R/C governing body that currently allows "homebuilt" turbines to be installed on R/C aircraft because of the safety and liability issues.

2) No, not all "jets" in the R/C world are turbine powered. Many are powered by a 2-stroke engine turning a ducted fan. This is because the DF is much less expensive than a turbine (~$300 versus ~$1000) for the same thrust and fuel consumption. Turbine price is continuing to fall, so I think we'll see the number of DF's dwindle as they are converted to turbine, but it'll be a few more years. In 2001, a turbine was $2500+, so to drop $1500 in cost in just 5 years is a good indication of how well they're selling and how good the technology is getting.

3) The current generation of turbines actually operate at similar speeds to larger turbojets and low-bypass turbofans and sound like miniature versions of the big ones (in decibels and total sound produced). I have not been able to confirm it, but I believe that there is a high-bypass turbofan in development for R/C applications with the growth in popularity of civil aircraft and modern military transports. There is also a turboprop available that is a modification of a turboshaft for R/C helicopters, so there are powerplants for every application now if you have the money.

[John]

I saw that Comet do a display at Bruntingthorpe and it was simply beautiful, both in the air and on the ground

Regards

John

[TSR2]

I saw that Comet do a display at Bruntingthorpe and it was simply beautiful, both in the air and on the ground

Regards

John

Me too John. She had her maiden flight 2 years ago at Brunters, hadn't been painted at that point and she looked gorgeous in wood. If I remember correctly the comet has 2 turbines, 1 in each wing... she even gave a little "puff" on startup. A beautiful a/c

[britishtourer]

It's funny too that she's the only flying comet...

Other than those that are Nimrods.

[TobyV]

Are these not gas-turbine powered? (usually built from turbochargers) In which case, obviously they dont sound "just like the real thing" (mainly due to the small size and much higher rpm of the spool, but they dont sound unpleasant

1) No current R/C jet engines are built from turbochargers. They are all factory-built units which are specially designed for R/C applications. I don't know of any country's R/C governing body that currently allows "homebuilt" turbines to be installed on R/C aircraft because of the safety and liability issues.

The vast majority of all model aircraft gas turbines use compressors sourced from automotive turbochargers and many are on sale in the UK. See http://www.gtba.co.uk . "Major" manufacturers include Phoenix/Microjet (http://www.microjeteng.com/) and AMT

2) No, not all "jets" in the R/C world are turbine powered. Many are powered by a 2-stroke engine turning a ducted fan. This is because the DF is much less expensive than a turbine (~$300 versus ~$1000) for the same thrust and fuel consumption. Turbine price is continuing to fall, so I think we'll see the number of DF's dwindle as they are converted to turbine, but it'll be a few more years. In 2001, a turbine was $2500+, so to drop $1500 in cost in just 5 years is a good indication of how well they're selling and how good the technology is getting.

This is true, but given the size of these models, I think a small gas turbine is not an unlikely engine to be powering them

3) The current generation of turbines actually operate at similar speeds to larger turbojets and low-bypass turbofans and sound like miniature versions of the big ones (in decibels and total sound produced).

Chris, this is just plain untrue. All turbomachinery components have a "design speed" and can operate within a certain band either side of it (although typically it tends to be more towards higher end of the permitted range. Basically the rotational speed can be obtained simply (estimated) from the velocity triangle and knowing the diameter of the component. There is of course a difference between axial flow and centrifugal flow devices, centrifugal flow by nature needing to rotate faster, but to give you some ideas, heres some typical operational rpms:

RR trent Front fan ~ 3000rpm (axial, diameter 2.5-3m)
RR Spey low pressure compressor ~11000rpm (axial, diameter ~0.5-0.75m)
RR Dart compressor ~15000rpm (centrifugal, diameter ~0.75m)
Small turbocharger or model gas turbine compressor 60000-150000rpm (centrifugal, dia 0.05-0.1m)

These figure may not be "bang on", but they are not unreasonable.

[henk hugo]

i'm with Toby on this as he has a BEng in Mechanical Engineering and an MSc in Turbomachinery and has spent about 2 years studying compressor and turbine design...

[Chris Trott]

The vast majority of all model aircraft gas turbines use compressors sourced from automotive turbochargers and many are on sale in the UK. See http://www.gtba.co.uk . "Major" manufacturers include Phoenix/Microjet (http://www.microjeteng.com/) and AMT

I'm sorry, but you said "usually". Of all of the current factory-built jet turbines, none use automotive turbocharger parts. All parts of these units are custom-built for the application. Primarily, in the few centrifugical engines that are still manufactured, the model turbine is much more compact with a much flatter turbine disk compared to the automotive turbocharger (reference - http://en.wikipedia.org/wiki/Image:Turbocharger.jpg). However, most of the new engines are axial turbines instead of centrifical (reference - http://www.airtoi.com/images/turbine1.gif).

This is true, but given the size of these models, I think a small gas turbine is not an unlikely engine to be powering them

True to a point, however I've seen several aircraft that would normally be "de facto" turbine powered that were actually ducted fan powered.

Chris, this is just plain untrue.

I stand corrected on the speeds. However, while the RPMs are different, if you listen to the startup on the GTBA, you'll find that the sound is very similar to the Avon engine. Also, according to the AMA (which bases most of its turbine regs off the GTBA) max allowable RPMs for an R/C turbine is 150,000, however most seem to produce max power around 130,000 RPM. I also note that most of the modern turbochargers produce their power at 140,000 - 160,000, significantly higher than what you'd want for all but the smallest of turbines (we're talking the 10-15LBS thrust range, not the 45 pounders).

[thehappyotter]

They are absolutley amazing.

I'd love to have a go, don't think that would end too well though...

[TobyV]

The vast majority of all model aircraft gas turbines use compressors sourced from automotive turbochargers and many are on sale in the UK. See http://www.gtba.co.uk . "Major" manufacturers include Phoenix/Microjet (http://www.microjeteng.com/) and AMT

I'm sorry, but you said "usually". Of all of the current factory-built jet turbines, none use automotive turbocharger parts. All parts of these units are custom-built for the application. Primarily, in the few centrifugical engines that are still manufactured, the model turbine is much more compact with a much flatter turbine disk compared to the automotive turbocharger (reference - http://en.wikipedia.org/wiki/Image:Turbocharger.jpg). However, most of the new engines are axial turbines instead of centrifical (reference - http://www.airtoi.com/images/turbine1.gif).

For most small companies and private indiduals involved in this field, fabricating axial components is tricky and expensive. Also the typical pressure ratio across an axial compressor stage is 1.1-1.4:1 compared with 1.5-2.8:1 for a centrifugal device which is both "off the shelf" and very robust. Technology-wise, most available on the modelling market are similar in concept to the early engines of the Whittle era. True there are more axial flow ones emerging, I have been shown a turbojet, similar to a miniature Avon and also you'll see theres a turbofan on the GTBA website. The axial flow ones do sound more like full size jets unless you are used to the sound of the centrifugal flow engines, like the engine in say a Vampire, which I'm guessing you dont get too many things like that your side of the pond.

You are right about them pretty much all having an axial flow turbine. You can actually have a greater pressure ratio per stage across an axial turbine than an axial compressor (there is a good mathematical reason for this that presently evades me!) and you can thus avoid all the disadvantages in terms of weight and flow of the centrifugal turbine.