Jump to content

Balsacraft Bristol Blenheim Motors


David Davis
 Share

Recommended Posts

I have swapped a foamy Phoenix 2000 for a Balsacraft Bristol Blenheim kit previously owned by the treasurer of my club. Foamy electric gliders are not my thing and he does not like to build so it was a fair swap! I hope to build the model over the winter. I have not yet even checked the contents of the kit box but I'll replace the original brushed motors and ESC with brushless motors, a LiPo battery and a modern speed controller.  Originally the model used a 7 cell 1700 NiCad battery and it weighed 4 Lbs 2 ozs (1.87kgs.) It has a 60" (152 cms)  wingspan.

 

Electric flight is not my strong suite so what equipment would the cognoscenti recommend? I know I'm looking at two electric motors and two propellers but will I need one or two speed controllers? Will I need two LiPos or one? What size LiPo(s) would be suitable for the model.

Link to comment
Share on other sites

You need separate ESCs for each brushless motor. I think most people would have one battery for a twin, but two would also be fine.

 

Having passed your Phoenix 2000 on, have you still got other electric models? If you have, what batteries have you got? Having different size batteries for each model is a pain, and expensive. If it comes out around 4lbs then something like 4s3000mAh or 3s4000mAh or 2×3s2200 would have the energy you require.

Then what prop diameter do you want? 

 

Edit: 7cell 1700 NiCd is very small, it must have been two of those for a 4lb model, surely?

Edited by Dad_flyer
Link to comment
Share on other sites

Thank you Dad and Alan.

 

I have just checked the kit box and there are no electrics whatsoever so I'll need two brushless motors, two speed controllers and two props. As to prop size I'll be guided by the experts. I'm quite prepared to buy some 2S LiPos if the consensus is that they're preferable.

 

As far as LiPos are concerned, in 3S I have four 3000mAh Lipos and two 4400mAh Lipos. I also have four or five 2200 mAh LiPos which I use in a foamy trainer while instructing beginners.

 

In 4S I have a 3000mAh LiPo and two 4800 LiPos. These haven't been used for at least ten years.

Link to comment
Share on other sites

David your prop diameter is of course limited to the distance the centreline of the nacelle and the fuselage side. I think you will struggle to fit anything bigger than 8" diameter props IIRC.

 

My reason for using 2S batteries is simply to do with power to weight ratio. Having flown the model with the original 7 cell nicad pack it certainly had plenty of power but was heavy and was easy to flick into a spin on a bad launch or if letting it get too slow especially when turning tightly.

 

I reasoned that around the same power but less weight would make the model less critical to fly and so it proved. I was of course able to balance the model using the small and light 2S pack by retaining the heavy 600 brushed motors.

  • Like 1
Link to comment
Share on other sites

Hi all, the best about using the 600 brushed is the weight where it is needed and only one speed controller is required. I lost mine in a turn when the battery was low, it done as Alan said, flick into a spin from which it did not recover.

I don't know about modern brushed speed controllers but when I last used them lipo's were just coming out and the power cut off for lipo's was too low so I did not use them until I went brushless. I still have part of a Blenheim kit in the loft and a drawing for the missing parts. A good winter project? 

  • Like 1
Link to comment
Share on other sites

3 hours ago, Dad_flyer said:

You need separate ESCs for each brushless motor. I think most people would have one battery for a twin, but two would also be fine.

 

Having passed your Phoenix 2000 on, have you still got other electric models? If you have, what batteries have you got? Having different size batteries for each model is a pain, and expensive. If it comes out around 4lbs then something like 4s3000mAh or 3s4000mAh or 2×3s2200 would have the energy you require.

Then what prop diameter do you want? 

 

Edit: 7cell 1700 NiCd is very small, it must have been two of those for a 4lb model, surely?

When the BC Blenheim debuted 1700mah Sub-C cells were still the most common batteries and 7 cells the most common hop-up from the 6 cell buggy packs. The 2400mah Sanyo Sub-Cs hit the market in the same general time frame and replaced the 1700mah cells. The Blenheim was designed around a single 7 cell pack, sitting in the nose, with a removable nose section.

I've retained the Sp600s in mine -as pointed out, their weight is in the right place and the model flies well - mine was originally using an 8 cell 2400mah pack and I haven't flown her for ages,. When I do it will be on 3s1p3000mah lipo, which is certainly pushing the Sp600's to the limit.

Link to comment
Share on other sites

Please forgive my lack of knowledge on electronic matters gentlemen, but as I see it, if I were to fit two Speed 600 motors I will need two of the older type of speed controller with just two output cables; as you know, speed controllers for brushless motors have three. I have found somewhere which sells Sp 600 motors but all of their speed controllers have three output cables. Please advise of a seller which stocks the older type of speed controller suitable for a SP 600.

 

If I were to fit brushless motors, surely I could make up for their lack of weight by fitting a larger LiPo to the model. Please advise which brushless motor would be a suitable substitute for a Speed 600.

 

I assume that a 30-40 amp speed controller would be suitable for each of the motors.

 

 

Link to comment
Share on other sites

Hi David,

with brushed motors one controller will do for two motors, the reason two are used for brushless is I am told there is a possibility that one may start in the opposite direction even after set up. The battery box was designed for sub c cells so the battery size is is worth looking at as the box is part of the fuselage construction.

Link to comment
Share on other sites

45 minutes ago, David Davis said:

Please forgive my lack of knowledge on electronic matters gentlemen, but as I see it, if I were to fit two Speed 600 motors I will need two of the older type of speed controller with just two output cables; as you know, speed controllers for brushless motors have three. I have found somewhere which sells Sp 600 motors but all of their speed controllers have three output cables. Please advise of a seller which stocks the older type of speed controller suitable for a SP 600.

 

If I were to fit brushless motors, surely I could make up for their lack of weight by fitting a larger LiPo to the model. Please advise which brushless motor would be a suitable substitute for a Speed 600.

 

I assume that a 30-40 amp speed controller would be suitable for each of the motors.

 

 

 

David

 

You were generous enough to send me your unused plan for an IL-2 earlier this year. If you decide to go ahead with using brushed motors, then please PM me as I have an unused 40A brushed EC that I will gladly send you to use.

 

As Eric rightly says, the battery box on this model is fairly integral to the fuselage build, so it is worth considering if this part limits your battery size first.

Link to comment
Share on other sites

52 minutes ago, David Davis said:

Please forgive my lack of knowledge on electronic matters gentlemen, but as I see it, if I were to fit two Speed 600 motors I will need two of the older type of speed controller with just two output cables; as you know, speed controllers for brushless motors have three. I have found somewhere which sells Sp 600 motors but all of their speed controllers have three output cables. Please advise of a seller which stocks the older type of speed controller suitable for a SP 600.

 

If I were to fit brushless motors, surely I could make up for their lack of weight by fitting a larger LiPo to the model. Please advise which brushless motor would be a suitable substitute for a Speed 600.

 

I assume that a 30-40 amp speed controller would be suitable for each of the motors.

 

 

I would go brushless, its getting harder to source ESCs for large brushed motors and brushless motors have a much better power to weight ratio. Assuming a total mass of approx 5 lbs and  a requirement of 120 watts/lb gives a total of 600 watts or 300 watts per motor. 

The Graupner speed 600 has a diameter of 36mm so a pair of 3535 outrunners would be about right, you would have to select the correct Kv value for the size of prop required.

3535 motors

Lets assume you go for the 3535-1090. Putting the figures into eCalc and with a 10x5 3 blade prop each motor draws a max of approx 25 amps producing a static thrust of about 1300g each. With 2 motors running on a single 3s 4400 mAh battery you should get around 5 minutes flying, two 3000 mAh 3s batteries should give around 7 minutes flying.

Edited by Shaun Walsh
Link to comment
Share on other sites

12 minutes ago, Shaun Walsh said:

I would go brushless, its getting harder to source ESCs for large brushed motors and brushless motors have a much better power to weight ratio. Assuming a total mass of approx 5 lbs and  a requirement of 120 watts/lb gives a total of 600 watts or 300 watts per motor. 

The Graupner speed 600 has a diameter of 36mm so a pair of 3535 outrunners would be about right, you would have to select the correct Kv value for the size of prop required.

3535 motors

Lets assume you go for the 3535-1090. Putting the figures into eCalc and with a 10x5 3 blade prop each motor draws a max of approx 25 amps producing a static thrust of about 1300g each. With 2 motors running on a single 3s 4400 mAh battery you should get around 5 minutes flying, two 3000 mAh 3s batteries should give around 7 minutes flying.

120W/lb! Try 60 or 70W/lb.

 

Also, new brushed ESCs seem to be available cheaply:

 

https://hobbyking.com/en_us/turnigy-30a-brushed-esc.html?queryID=0e08fe6dab423cf74993aeb5d1b4724c&objectID=18913&indexName=hbk_live_products_analytics

 

I have to say I'd be more worried about getting a pair of Speed 600s these days as I haven't tried buying brushed motors for a long time. My existing ones work fine.

Link to comment
Share on other sites

This is all very useful information gentlemen.

 

Having sorted through my souvenirs I've found an old Robbe RSC 750 speed controller which could be alright if I wanted to go down the brushed motor route. I have had it for decades and may have used it in a vintage model but I really can't remember.

 

If it works, using Sp600s could be a cheap do. They're only 15.20€ each from a French supplier.

 

 

Link to comment
Share on other sites

2 minutes ago, Alan Gorham_ said:

120W/lb! Try 60 or 70W/lb.

 

Also, new brushed ESCs seem to be available cheaply:

 

https://hobbyking.com/en_us/turnigy-30a-brushed-esc.html?queryID=0e08fe6dab423cf74993aeb5d1b4724c&objectID=18913&indexName=hbk_live_products_analytics

 

I have to say I'd be more worried about getting a pair of Speed 600s these days as I haven't tried buying brushed motors for a long time. My existing ones work fine.

I do like to be generous on the power/weight ratio🤣 on the basis that it's easy to throttle back but if you ever need more power and it's not there then you can have problems. But you are right, 70W/lb would give you a flyable model. I looked around for suitable brushed ESCs and the Hobbyking ones you found were the only suitable ones I found too. And you're right about the availability of Speed 600 motors too!

Link to comment
Share on other sites

I have found these motors from a French supplier. (I live in France.) While they are not specifically listed as Graupner motors they are rated at up to 12 Volts so they should be alright witha 3S LiPo. I plan to use them with my old Robbe RSC 750 speed controller. If it works, happy days. If it doesn't I haven't lost much. https://www.weymuller.fr/article/power_600_21.html

  • Like 1
Link to comment
Share on other sites

IMO, for our models, no. My first twin was a Dueces Wild which I bought off a flying buddy, he had both props rotating the same way. I flew it then changed to counter rotating, made no discernible difference. As a consequence my subsequent twins do not have counter rotating props (somewhat difficult to achieve on the IC ones anyway!)

Edited by Ron Gray
Link to comment
Share on other sites

A fully charged 3s battery will deliver 12.6v, not sure how happy the motor would be given that the design voltage is 8.4V, probably OK with good ventilation but 2s Lipo would be safer, but then I'm no expert on brushed motors.

Also those motors weigh 150g more than the alternative brushless motors so an extra 300g that you have to haul around.

Link to comment
Share on other sites

One of the biggest mistakes that those who dipped their toes into electric flight with cheap can motors and NiCds made, was taking the nominal voltage printed on the motor as an indication of the battery pack needed. So for the bog-standard 8.4v motor they would use a 7 cell NiCd pack and then be disappointed that the model wouldn't fly. It was important on those days to overdrive the brushed motors, at the expense of motor longevity and heat generated, in order to get a useful amount of thrust.  When I started electric flight there were even recommendations being made to use a 6 cell pack with a 7.2V motor. We certainly learned very quickly to swap out our 8.4v Sp 400s for 6v versions and a minimum of 8 cells - useful thrust but short motor life.

Link to comment
Share on other sites

1 hour ago, leccyflyer said:

One of the biggest mistakes that those who dipped their toes into electric flight with cheap can motors and NiCds made, was taking the nominal voltage printed on the motor as an indication of the battery pack needed. So for the bog-standard 8.4v motor they would use a 7 cell NiCd pack and then be disappointed that the model wouldn't fly. It was important on those days to overdrive the brushed motors, at the expense of motor longevity and heat generated, in order to get a useful amount of thrust.  When I started electric flight there were even recommendations being made to use a 6 cell pack with a 7.2V motor. We certainly learned very quickly to swap out our 8.4v Sp 400s for 6v versions and a minimum of 8 cells - useful thrust but short motor life.

 

In those early electric flying times, I flew regular with the available can motors, but my flying buddy flew competion, and though my motors lasted years until brushless came in, my buddy told me then that he was lucky to get 40 competition flights from a brushed motor, as they pushed every ounce of speed from them, with larger power packs.

  • Like 1
Link to comment
Share on other sites

9 minutes ago, David Davis said:

Would I be better off going for two brushless motors and two speed contollers? That set up would be lighter and expensive but the motors would last longer.

 

If I need nose weight perhaps I could move the LiPo further forawrd.

That would be my preferred route, put a large lipo in the nose.

Link to comment
Share on other sites

David - it's entirely your choice, but do bear in mind that the model was designed around a heavy NiCd fitted in the nose and two heavy sp600 motors in front of the CG.

IMG_0114.thumb.jpg.c9db2bfc06de16ab76577dd6f3559f17.jpg

 

Brushless would give you more power and is perhaps better suited to a lipo - without modifying the internal crutch you should be able to fit a 3s1p4200mah battery in there, that's what the pictures show - 

IMG_0112.thumb.jpg.639140152c592f1a22a4a2a778a132f2.jpg

 

but anything larger than that you would need to modify the battery tray, which is an intrinsic part of the internal crutch of the fuselage. You'll also need to weigh up your motor choice to suit the limited prop diameter that you can fit -you can stretch to 9", maybe 10", but it would be very close to the nose -see pictures - no room for  11" or 12" props on this Blenheim. 

553445869_blenheimprop.thumb.JPG.b154a9821454ffdda9566abdffd05abc.JPG

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

 Share

×
×
  • Create New...