Thrust tubes ?

[Gixerbenten]

I won a GRAUPNER F15 on ebay for £15 with a u/s esc. Picked it up today, put in a new esc and went for my maiden. It flew great, high/low speed passes, did everything I thought it should. After getting home and having a propper look at it, know I should have done this before flying but little boy on Xmas morn, noticed theres no thrust tubes in it. Is it norm for this model and if not, what can I do about it ? Dont fancy cutting it in half and as my 1st edf it seems plenty fast enough but vertical is not that good. Any advice, tips ???? Cheers

[Dizz]

Some do, some don't. Is the exhaust duct fairly smooth anyway? Not many EDFs have a sparkling vertical performance unless specifically built for a high thrust; typically static thrust:weight ratios are around 0.65, but of course there are many variations. To get higher thrust (at the expense of top speed) and keeping the same motor/fan combos you would need to increase the eflux area, but it would be almost impossible to predict exactly what increase you would get.

As it flew OK and the effort required to bump up the thrust I would be inclined to leave it.

Pete

[Gixerbenten]

Ok, cheers Pete. I thought they all had them and yes it is smooth. Live and learn. Think Ive got the EDF bug now tho

[stuey]

I have recently finished converting my Graupner F-15 to a 69mm minifan and 4s setup. The thrust tubes have a splitter just behind the fan which divides the air into two. If I remember rightly the outlets are 40mm diameter. The ideal diameter of the thrust tube/tubes depends the the size of the fan, and if you are looking to maximize the thrust or the air exit speed. It is normally a compromise between trying to get more thrust for better vertical, or more top speed.

I am guessing you are using the kit 64mm fan and a 3s lipo? I flew a slightly different version on 3s and an alloy 64mm fan, and it was disappointing. Even on 4s it was not what I was hoping for. That model was jinxed and eventually scrapped. My new version has approx 1:1 thrust ratio, and a much better top speed. The F-15 is known for being a draggy airframe, and the intakes are quite a bit bigger than required so it will never be that fast. The mods I have made were quite involved, as the battery now goes in through the cockpit and was moved back as much as I could to get a better cg. I also removed some of the tailplane incidence to try and reduce its tendancy to climb at full power, and moved the cg back approx 10mm from the manual. I am very pleased with how it goes now

[Simon Chaddock]

Gixerbenten

With any EDF the duct is always a compromise. The highest thrust is delivered when the EDF is on its own. From then on any further ducting saps the thrust by adding surface drag to the air stream which reduces the total flow rate.

The thrust is proportional to the square of the exit velocity so the best way to increase thrust is simply to make the fan work harder (turn faster) with more cells but this is always a process of diminishing returns.

Restricting the outlet with a 'nozzle' does increase the exit speed but in doing so increases the effective drag on the air stream which reduces the flow rate thus eating into any extra thrust obtained.

It all comes down to finding the best solution from a lot of variables!

[Bill_B]

Posted by Simon Chaddock on 05/11/2013 13:20:42:

Restricting the outlet with a 'nozzle' does increase the exit speed but in doing so increases the effective drag on the air stream which reduces the flow rate thus eating into any extra thrust obtained.

Perhaps not necessarily true in all cases: I have two 70mm Wemotec EDFs and the performance has been modestly increased by fitting their recommended thrust tubes. In my experience German manufacturers usually design their stuff well and are not known for making false claims.