By continuing to use this site, you agree to our use of cookies. Find out more
Forum sponsored by:
Forum sponsored by CML

New EDF Mini Jets- Jet Provost & Folland Gnat

All Topics | Latest Posts

Search for:  in Thread Title in  
Barry Buxton21/12/2019 19:23:03
13 forum posts
5 photos

Thanks Tim,

without the access hatch I get about another 60g of thrust but I’m not convinced that’s going to be enough but I will add more access holes, not too sure how to shave 6oz off it though to get to your weight, I could lose 2 oz moving to a 3S setup but sacrificing thrust. I tried to see if I could upload a photo from my iPad but not too sure how to do that, do I have to create an album first then share from that ?

Barry Buxton21/12/2019 19:34:33
13 forum posts
5 photos

Found out how to add photos if these helpcdec57c1-c890-4c58-9ba2-4fcf49c9fa94.jpeg9d409c2f-b7cc-42b7-9a04-77422886ac0f.jpeg038d2a43-cfc0-4952-8e1d-5df56e36fe97.jpeg

Tim Ballinger21/12/2019 19:45:48
avatar
750 forum posts
277 photos

See you sorted the photo album out.

So thats an extra 10 % thrust based on the 3s fan but still sounds well down on the 4s spec.
Grasping at straws are you sure you have the throttle calibrated for the ESC so that you achieve full throttle.

Only way to shed weight at this stage is to cut lightening holes as is shown for the Provost a few pages earlier.

What did you cover it with ?

Tim

Barry Buxton21/12/2019 19:58:03
13 forum posts
5 photos

Yes, I calibrated the throttle and measured 36A / 500W at full throttle. I painted the plane after giving it a coat of eze-kote, grey primer rubbed quite well down after applying and 3 tins of humbrol enamel thinned out for spraying. Must admit I am thinking that the paint has added more weight than covering. I’ll put some extra vent holes in it and when / if the weather improves I’ll give it another go, would like to sort this out as I’ll be getting the Phantom kit for Christmas.

As a matter of interest what is the best way to measure the static thrust, I basically hung it from a set of luggage scales and powered her up

Tim Ballinger21/12/2019 20:51:57
avatar
750 forum posts
277 photos

Fingers crossed for the next attempt.
as for thrust measurements, I guess you will find not many folks bother. I have used a spring balance in the past.

Perhaps deserves a thread in its own right . I am certainly no expert.!

Tim

Steve Nash 121/12/2019 21:11:57
avatar
45 forum posts
11 photos

Barry,

lovely looking model you've made there, I've just started building mine.

I measured the thrust on my Provost using a digital hanging scales. I suspend the plane from the tail using a string loop around the tail and zero the scales. Then just open up the throttle and see what you get. I got 530g thrust on the Provost with a freshly charged battery that had been kept warm in my pocket for a while, otherwise the lipo's don't perform very well this time of the year when they are cold. That 530g is on a 3s fan using an Overlander 3s 2200 battery. After a while the thrust dropped down gradually to around 515g. I would have thought that your 4s fan should perform better than that.

You say that you would only save a couple of ounces by swapping to 3s, but on an aircraft that should weigh around 20-22 oz that is 10% of the weight, so it will make a difference. On a 40 size IC plane a couple of ounces wouldn't even show a difference, but on models like this weight is the enemy.

Steve

Edited By Steve Nash 1 on 21/12/2019 21:13:02

i12fly21/12/2019 23:40:51
avatar
610 forum posts
21 photos

Just an idea......

As air intake seems to be a limiting factor, maybe if you use a 4S set up you need to open the cheat hole area a 'corresponding amount' to get the extra air throughput?

This could affect some of the model types but not others of course, depending on how the air is balanced of cheat versus intakes.

In reality axial fans (which effectively EDFs are), move air and generate relatively little pressure, so much so that they soon stall when restricted on the inlet or outlet. On the other hand centrifical fans (not practical for EDF) generate a more significant pressure, increasing right up to the point where the outlet is blocked completely.

Steve Nash 122/12/2019 00:23:16
avatar
45 forum posts
11 photos

Another idea is to check the timing of the esc, as I read that this affects the performance of the motor. My esc was from another plane which had an outrunner swinging a prop. In that plane I had the esc set to low timing. In the Provost I have set it to medium timing due to the high rpm required.

Steve

Barry Buxton22/12/2019 10:40:28
13 forum posts
5 photos

Thanks for the ideas guys, I’ll try retiming the esc later and add extra breather holes, failing that I’ll order a 3S fan. It as it’s a nice day here in Stoke on Trent I’m going to take the Riot out for a fly today and get back on this after Christmas. Have a good Christmas all

alan barnstable22/12/2019 11:44:54
16 forum posts
2 photos

I've been doing some testing with my Provost whilst it's raining. (note i've only had 2 successful launches out of 8 attempts).

Total weight including 2650 3 cell battery and 140g lead weight in nose is 836g (29.5oz).

Thrust is 404g with standard cutouts in bottom of fuselage and side ducts.

Thrust is 514g with top hatch removed.

I'm going to carry out the following steps:

1/ remove weight from rear of plane, and remove some of the ballast weight

2/ enlarge the hole in bottom of fuselage

3/ try flying it without top hatch

Tim Ballinger22/12/2019 13:59:04
avatar
750 forum posts
277 photos

In light of the queries regarding thrust values I offer my own results for the Gnat using a 50mm FMS fans and 2200 Lipo.

Test set up is crude but seems repeatable +/- 20g and is shown below

fan_test - 1.jpeg

As you can see the model is uncovered and I asked the pilots to step out for the tests on H&S grounds.

Next the results

fms fan data.jpg

What I conclude from this is

1. Although removing the hatch increases the airflow/thrust it is not as effective as providing a clean unobstructed intake right in front of the fan. Hence relatively smaller increases in 'cheat' intake size are very worthwhile.

2. In these static tests the original 'cheat ' intake size is perhaps too small as it does not deliver max thrust. I say perhaps because once in flight with a forced airflow it probably does achieve full thrust. However with folks having trouble achieving flying speed ( ignoring the issues of being above design weight) from a hand launch perhaps max static thrust is a good design point.

3. I achieved 600g thrust with a 60% increase in 'cheat' intake size ( 48mmx48mm). This is pretty close to the spec value of 620g so I will leave it at the new size.

4. My current values of 42A are in excess of the 32A in the spec. Not sure why but are within the ESC spec so I am still happy. I was actually using the Neuron S ESC for these measurements and have not cross checked the values on a another sensor yet but I have no reason to doubt the accuracy of the values.

Tim

Steve Nash 122/12/2019 15:51:09
avatar
45 forum posts
11 photos

Tim, very interesting results. The best approach with these models I think is to keep them light and make the cheat holes bigger like you gave done. Its great that you have achieved almost 100% of the thrust quoted in the specs for the fan.
Steve

Steve Nash 123/12/2019 10:42:15
avatar
45 forum posts
11 photos

The Provost needs a good launch to get airborne, so what I have done is to glue a small piece of coarse sandpaper each side of the underneath just where my fingers grip it. This gives a good grip, and the sandpaper can be painted a matching colour if required. I hold it as shown with one finger just inside the cheat hole to give good leverage when I throw it. I get a really good chuck doing it like this.

7c11b005-7101-4076-8053-0ee46b8aa140.jpeg
43a6a248-58e9-4a09-be27-2eae3fc083f8.jpeg

Edited By Steve Nash 1 on 23/12/2019 10:44:06

Tim Ballinger23/12/2019 12:40:56
avatar
750 forum posts
277 photos

Good tip Steve, I expect a bit of breeze helps as well.

I’m still fitting out the office but covering is not far away.

0cacf44d-bb9a-4036-b55a-e7b70bd79e66.jpeg

Barry Buxton23/12/2019 14:41:31
13 forum posts
5 photos

Following on from Tim’s post, I enlarged the breather hole to ~48x48mm and this has increased thrust slightly to about 520g with the battery hatch closed so a good mod to make, opening the battery hatch isn’t having any major effect, maybe adding ~5-10g extra thrust, I’m beginning to wonder if there’s something wrong with the fan unit so I’ve ordered another one up for trying out, in the meantime I’ll look at how to reduce the weight, I’ll start with a 4S 1800 as that will take 30g off and then investigate making lightening holes

Tim Ballinger23/12/2019 15:49:39
avatar
750 forum posts
277 photos

Barry,

I’m just guessing here but the extra thrust of the 4S version can only come from increased airflow relative to the 3S , either bigger flow area or higher velocity. In the static case where the airflow starts from rest it probably needs even more intake area to achieve the spec thrust. Why does it not even match the 3S thrust for the same size? Anybody’s guess but the fan aerodynamics are probably quite different and possibly operating with heavy losses at non design flows. Not sure there is room to get a bigger intake in the model so unless you can achieve flying speed and get a higher entry speed the 4S could be a noose round it’s neck. If you can get it flying by loosing weight of course you might end up with a rocket!

Incidentally I did check my current values with a different sensor and my 3S fan is definitely pulling 42A with a newly charged battery which is what my tests were done with.

And as noted at the start of this para I am just guessing as to why your thrust values are so low. Very happy if you find a simpler solution like a defective motor.

Tim

Tim Ballinger24/12/2019 16:54:24
avatar
750 forum posts
277 photos

Oh and a belated thanks to Dave Chatterton for his idea of joining the gnat wings in situ. Had the anhedral and brace set up on the bench and as the brace was a tight fit I tried manoeuvring the wings into position in one piece. Possible but I felt in danger of breaking the thin parts of the fuselage side so adopted for Daves approach of pushing each side in separately. Rather than epoxy and since I pretty much used cyano exclusively to build this model, wicking thin cyano into the root join and around the brace provided a pretty instant and easy assembly.

Again thanks fo4 suggesting the alternative.

Thanks and Merry Xmas

Tim Ballinger28/12/2019 18:49:16
avatar
750 forum posts
277 photos

Covering finished just decals to go. Currently 22.6 Oz.

0cf0cb40-6508-485f-bdd3-0049fba6eabc.jpeg

10a2a42b-005e-4111-bc89-91f6ac693bf1.jpeg

martin collins 128/12/2019 19:12:35
avatar
316 forum posts
143 photos

Very nice, looking forward to the flight report, starting mine next week..............

Steve Nash 128/12/2019 20:31:15
avatar
45 forum posts
11 photos

Looking good, Tim.

been cutting out the formers and fuselage sides for my Gnat. Took quite a while, maybe I should have paid out for the Laser cut set from Tony. I brought the canopy though.

All Topics | Latest Posts

Please login to post a reply.

Magazine Locator

Want the latest issue of RCM&E? Use our magazine locator link to find your nearest stockist!

Find RCM&E! 

Latest Forum Posts
Support Our Partners
Wings & Wheels 2019
CML
Sussex Model Centre
electricwingman 2017
Cambridge Gliding Club
CADMA
Slec
Advertise With Us
Sarik
Latest "For Sale" Ads
Has home isolation prompted you to start trad' building?
Q: The effects of Coronavirus

 Yes - for the first time
 Yes - but Ive bashed balsa before
 No - Ive existing projects on the bench
 No - Im strictly an ARTF person

Latest Reviews
Digital Back Issues

RCM&E Digital Back Issues

Contact us

Contact us