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DB Sopwith Pup flying weight

is it too fat?

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Ken Lighten19/01/2020 13:17:07
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256 forum posts
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Hi all,

my DB 1/4 scale Pup is ready to fly, I've checked the balance point using a vanessa rig and also using scales and calculation and both agree the same position, my concern is that to achieve this required an inordinate amount of lead to bring the point to within 1/2" of the recommended point, meaning the total weight is 24lb, I'm not happy at all with that but wondered if I'm being overly concerned. I do intend to possibly redo the tail plane and fin building it with a lot lighter more scale like structure which will in turn reduce the nose weight needed, but, as I say, am I being too cautious?

Ken

PatMc19/01/2020 14:32:17
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That's about 5lb over the scale weight of a full size Pup carrying max load. Seems a bit lardy to me.

Edited By PatMc on 19/01/2020 14:33:34

Jon - Laser Engines19/01/2020 14:50:01
5070 forum posts
217 photos

i agree with pat. My 1/4 stampe is 16lbs and while its got a longer nose its not got any lead in it at all. DB also seem to suggest 15lbs as a flying weight.

How much of your weight is lead?

Ken Lighten19/01/2020 15:26:19
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256 forum posts
42 photos

a lot Jon!, I was aware of the weight shown by DB but wondered, if like a lot of these things, there was leeway. I shall make the new tail feathers and see what savings can be made, I think the original is quite over engineered considering there is bracing as per the full size, will try using carbon fibre spars and laminated outlines to reduce the woodwork currently employed, I have added a bit of detail (stitching etc) and painted it but didn't think I'd gone overboard with it

Ken

AndyD19/01/2020 15:46:25
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717 forum posts
503 photos

don't know about the DB pup but my 1/4 scale pup built from a 80s plan comes in at 22 pound but is still a floater.https://youtu.be/Za5s1-QVesU

bert baker19/01/2020 16:01:42
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1510 forum posts
306 photos

Got the older one piece wings DB Pup 

Laser 180 up front

Just weighed it at 8.6kg or 18.96 lbs

Edited By bert baker on 19/01/2020 16:03:07

Edited By bert baker on 19/01/2020 16:23:32

Jon - Laser Engines19/01/2020 16:54:00
5070 forum posts
217 photos

You say you calculated the c/g, are you sure its in the right place?

Also, if its getting porky dont use the ball bearing rigging gripper things. Loads of people have told me they fail so its not worth the risk.

George P.19/01/2020 18:25:16
54 forum posts
20 photos
Posted by Jon - Laser Engines on 19/01/2020 16:54:00:

Also, if its getting porky dont use the ball bearing rigging gripper things. Loads of people have told me they fail so its not worth the risk.

Now that is a bit of a worry but glad you mention it ! I have used the grippers and was hoping they would be reliable. Anyone out there who can vouch for the system? Or should I be thinking of some kind of contingency plan before the maiden in Spring?

Ken - My Pup has come out at around 17 lbs. I have final rigging check/ cg check to do but hoping that will be it. I marked the cg location as per the plan and it appears to balance at that point and at that weight.

Andrew Calcutt19/01/2020 18:27:09
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35 forum posts
1 photos

Perhaps try some different scales,can't see how it weighs that much.

bert baker19/01/2020 18:37:59
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1510 forum posts
306 photos

In my opinion

The ones that really count are the flying wires,ie from bottom of fuz and connected to the top wing at the struts.

on my DB Pup I only have flying wires doubled up on the front only,

They are sprung loaded,

All very basic installation,

I’m not the gentlest on the sticks so can say I’ve stressed tested it and found it to be ok

However if you have the split wing panel one full rigging may be a must,

Im very sceptical of any quick realease system

John Lucas 419/01/2020 19:46:41
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52 forum posts
8 photos

Ken

I have a DB Pup which came out at around 15LB I was also concerned about the weight. After the first flight I decided to move the c/g 5mm rearward from what the plan indicated because of the landing speed. The flight with the new c/g position went fine but I moved the c/g another 5mm and it flew much better and of course I was able to remove some of the church roof. My Pup is covered in Solartex and is powered by a Saito FG R33. The wings are held by the gripper method although I have had no problem but I am wary of this type of fitting. If you decide to alter the c/g position move it 5mm at a time after the initial flight .

John

John Lucas 419/01/2020 19:49:48
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52 forum posts
8 photos

 

 

Edited By John Lucas 4 on 19/01/2020 19:51:39

Jon - Laser Engines19/01/2020 20:15:02
5070 forum posts
217 photos
Posted by George P. on 19/01/2020 18:25:16:
Posted by Jon - Laser Engines on 19/01/2020 16:54:00:

Also, if its getting porky dont use the ball bearing rigging gripper things. Loads of people have told me they fail so its not worth the risk.

Now that is a bit of a worry but glad you mention it ! I have used the grippers and was hoping they would be reliable. Anyone out there who can vouch for the system? Or should I be thinking of some kind of contingency plan before the maiden in Spring?

I cant speak from personal experience on this but a number of my customers have commented on it. Often when ordering spares for their now damaged engine.

One chap was using those grippers on a 1/4 nieuport and i mentioned it to him when he ordered the engine. He was already committed to using them but was able to get inside and installed a collet so that if they did fail there was only a small amount of slack in the system and this would hopefully keep the wings on. He called me some time later to say that he experienced failures in the rigging on about 25% of his flights and the wheel collets he installed prevented disaster. many others have said similar things

Now i cannot vouch for the quality of the installations and i dont want to malign DB when i have not tested it myself, but given the risk factor i would say its better safe than matchsticks.

Ken Lighten20/01/2020 10:30:03
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256 forum posts
42 photos

I shall do a lot more double checking of weight and balance position, I also was concerned about the rigging attachments, I have crimped a loop behind the gripper (I think as per the plan, too cold to go and check at the mo!) to prevent complete withdrawal of the cable. I still think the tail is over engineered in respect of the amount of wood and corresponding amount of glue required to assemble it so will probably, after double/triple checking my measurements etc, still replace it with a lighter version

Ken

Mike T20/01/2020 12:41:04
435 forum posts
29 photos

My Pup is about 15/16lb and built from a very old (pre-Stocker) kit. I made sure I got all the weight up front. The elevator and rudder servos are as far forward in the fus. as possible and the throttle servo lives in the (near-scale length) cowl, along with the servo battery (5-cell Sub-C NiMH), ignition pack and Rx pack (both 4-cell AA NiMH). Even the switches/charging jacks are behind the cowling side panels.

I also built the tail surfaces to scale outlines by trimming the kit components. It's all covered with Sig Koverall, to reduce the acres of redundant adhesive that are carried around with Solartex. (Moot point, as it still had to be doped/painted!)

Despite all that, it needed a substantial lump of brass bar to be strapped under the battery mount, to get the CG to an acceptable point. It's still a little pitch-sensitive, but the sort of thing a gyro (or gyro-receiver) could easily sort out.

pict0023.jpg

BTW - although the rigging adds tension and stiffens everything up a bit, the wings (on the original kits at least) were all intended to be self-supporting. My rigging is all single strand and fixed with standard clevises.

Edited By Mike T on 20/01/2020 12:42:16

bert baker20/01/2020 17:19:50
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1510 forum posts
306 photos

I must have gone wrong no leadb40962fd-ce2a-4d9a-af94-0d33506ada32.jpeg

Chris Berry20/01/2020 18:23:36
239 forum posts
1 photos

My DB 1/4 pup weighs 16lbs with a Laser 200v. 4 flights with it today.

Alan Hilton20/01/2020 19:03:28
114 forum posts

My one piece wing pup has a laser 200 and weighs about 18 pounds .I used car wheel balancing weighs stuck to the front inside of the cowl to achieve the recomended c of g .Getting the weight as far forward as possible is the way to go

Alan

George P.22/01/2020 11:24:33
54 forum posts
20 photos

It's been very useful reading this thread for me. I am heeding Jon's warning and I am thinking about options for retro fitting some doubling up on the flying wires. Good to hear of one that is flying with the grippers with no problem. I may fit something that I can detach again later if I feel the grippers do the job.

Ken- the buy new scales suggestion is not a bad one. I recently weighed a Seagull Chipmunk ARTF and the scales read 10 3/4 lbs. Later they weighed 11 lbs when I had added no weight and then something slightly different next time. I purchased some new scales and checked the calibration with known weights and then weighed the Chipmunk to find it was 10 1/2 lbs every time. You never know, may wipe a pound off the total?

Regarding the weight, I was feeling concerned that the 17lbs ish was too heavy but the weights I have read here makes me feel reassured. Going electric and have a set up that provides approx. 80 watts per pound so am hoping that should be enough.

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