logo
Thread Tools
Old 08-11-2019, 04:26 PM
grosbeak is offline
Find More Posts by grosbeak
Illegitimi non carborundum
grosbeak's Avatar
Canada, ON, Ottawa
Joined Oct 2012
2,080 Posts
Question
How to adapt cowl for forward engine mount

My Pulse 125 with RCGF 32 used to have the recommended firewall-to-backplate distance and the cowl was a perfect fit.



But in order to do that, I had to carve out a lot of the firewall.



During repairs after a recent crash I found the firewall was failing, and I would prefer to mount the engine further forward this time. Clearing the firewall means moving the engine forward one inch. Now, of course, it looks like this...



I am looking for ideas on how to add an inch to the back of the cowl to bridge that gap. Suggestions are welcome.
grosbeak is offline Find More Posts by grosbeak
Reply With Quote Quick reply to this message
Sign up now
to remove ads between posts
Old 08-11-2019, 09:25 PM
Hinckley Bill is offline
Find More Posts by Hinckley Bill
Registered User
Hinckley Bill's Avatar
Hinckley Illinois
Joined Jan 2006
2,152 Posts
Curious......the back of the firewall looks really, really, chewed up where the motor mount bolts come through. Was that your doing in order to install the motor?????
Hinckley Bill is offline Find More Posts by Hinckley Bill
Reply With Quote Quick reply to this message
Old 08-12-2019, 05:51 AM
grosbeak is offline
Find More Posts by grosbeak
Illegitimi non carborundum
grosbeak's Avatar
Canada, ON, Ottawa
Joined Oct 2012
2,080 Posts
Quote:
Originally Posted by Hinckley Bill View Post
Curious......the back of the firewall looks really, really, chewed up where the motor mount bolts come through. Was that your doing in order to install the motor?????

No, that was from the motors that were previously mounted to that airplane and had their mounting holes plugged with dowels. I have learned a lesson in dowels and glue starvation...
grosbeak is offline Find More Posts by grosbeak
Reply With Quote Quick reply to this message
Old 08-12-2019, 05:57 AM
ponyboy082662 is online now
Find More Posts by ponyboy082662
wrongwayfeldman
ponyboy082662's Avatar
United States, AL, Florence
Joined Mar 2010
2,291 Posts
You'd be better off just beefing up the frame and firewall and leaving the cowl alone.....imho
ponyboy082662 is online now Find More Posts by ponyboy082662
Reply With Quote Quick reply to this message
Old 08-12-2019, 07:39 AM
Hinckley Bill is offline
Find More Posts by Hinckley Bill
Registered User
Hinckley Bill's Avatar
Hinckley Illinois
Joined Jan 2006
2,152 Posts
Quote:
Originally Posted by ponyboy082662 View Post
You'd be better off just beefing up the frame and firewall and leaving the cowl alone.....imho
Based upon the current condition of the firewall I couldn't agree more ....
Hinckley Bill is offline Find More Posts by Hinckley Bill
Reply With Quote Quick reply to this message
Old 08-12-2019, 08:33 AM
acerc is offline
Find More Posts by acerc
Registered User
acerc's Avatar
United States, FL, Center Hill
Joined Jul 2014
2,007 Posts
With the shape of that firewall I believe it has been compromised. I would epoxy another 1/4" over it, probably two, with some 1/8" dowels around the perimeter then remount the engine cleanly. At that point one could fill the remainder of the desired perimeter gap with balsa, or hardwood, and add some matching covering.
acerc is offline Find More Posts by acerc
Reply With Quote Quick reply to this message
Old 08-13-2019, 10:37 AM
KMatch is offline
Find More Posts by KMatch
Registered User
KMatch's Avatar
United States, TX, Bay City
Joined Nov 2016
532 Posts
IF you insist, you're doing it the hard way. It would be easier to splice in and lengthen the fuse to meet the cowl. Balsa and covering are easier to patch than to extend the fiberglass/plastic in the cowl.
KMatch is offline Find More Posts by KMatch
Reply With Quote Quick reply to this message
Old 08-13-2019, 02:09 PM
ml82232 is offline
Find More Posts by ml82232
Registered User
ml82232's Avatar
United States, NE, Wahoo
Joined Oct 2013
155 Posts
Just my 2 Cents
Be cautious stretching out very far
Most airframes are designed around a given percentage of length / Prop > CG > Tail
I tried doing what your considering on a plane once to get the CG forward without adding weight
Flew like doggie doo /afterI triple checked all the thrust angles moved the engine back and added weight and was fine
Before I changed it back I measured out about 10 different planes and the percentages came in really close to the same on all of them /low wing /high wing/ trainer and aerobatic (don't have any war birds so couldn't check them)
I think the aerodynamics are affected by how far the prop is forward of the wing
I'm not an engineer so can't testify in court to it / just had the experience happen to me
ml82232 is offline Find More Posts by ml82232
Reply With Quote Quick reply to this message
Old 08-13-2019, 07:43 PM
Bunky F. Knuckle is offline
Find More Posts by Bunky F. Knuckle
Just your Average Joe.
Bunky F. Knuckle's Avatar
Joined Jan 2006
8,330 Posts
Riddle me this, Batman..... Why is the engine pushed out 1"??? Why not cut the existing firewall out, and install a new one, made of 1/4" aircraft ply?? Pushing the engine out, that will drastically change your CG. CG affects flying characteristics

By cutting out the existing firewall, and installing new, you will retain you once great spinner/cowl gap.
Bunky F. Knuckle is offline Find More Posts by Bunky F. Knuckle
Reply With Quote Quick reply to this message
Old 08-13-2019, 08:46 PM
ponyboy082662 is online now
Find More Posts by ponyboy082662
wrongwayfeldman
ponyboy082662's Avatar
United States, AL, Florence
Joined Mar 2010
2,291 Posts
Am I mistaken or didn't he say this was a 120 size plane...meaning a 1.20 glow engine. If this is right , that frame and firewall was never ment to handle the extra weight/Vibration of a gas engine. Also if this is the frame I think it is...how could the extra weight be needed up front for C/G with an already heavier than designed for engine ?
ponyboy082662 is online now Find More Posts by ponyboy082662
Reply With Quote Quick reply to this message
Old 08-14-2019, 05:20 AM
grosbeak is offline
Find More Posts by grosbeak
Illegitimi non carborundum
grosbeak's Avatar
Canada, ON, Ottawa
Joined Oct 2012
2,080 Posts
Folks, thank you for your input. A little history on the Pulse 125, which is the first airplane I built since learning to fly on an RTF glow trainer the year before.

When I built it in 2012 I installed a DLE 20. Even so, it required a bunch of lead in the nose to balance on the CG.



After a year with the DLE 20 I found it short on power and I installed the RCGF 32, which allowed me to remove the lead. The airframe and this engine have been flown together very well since 2013, though the second engine did require the big hole in the firewall.

After a recent crash (loss of engine power) I re-examined the firewall and found it on the point of failure. I cut it free and made a new firewall...



... and wanted to avoid making a large hole. So I put longer standoffs on the engine, thinking it would probably be WAY too nose heavy. But I was curious, so I temporarily attached the engine and firewall to the airframe.

During the crash repairs I decided to replace the two NiMh batteries with a single A123 and an IBEC which is the setup I have used successfully on all my other planes. The ignition NiMH was a 4-cell C pack as far forward as I could make it and almost twice as heavy as the replacement A123. So with the engine further forward and the A123 set in place I was still a little tail heavy - about 15mm behind the CG.

This is the reason I want to extend the cowl instead of moving the engine further back. And I have made progress doing so - I went to my LHS to look for thin balsa and found something even better... 1/64" plywood.

With CA I glued a strip to the inside of the cowl with about 1/2" sticking out.



Onto that strip and level with the cowl I glued another strip wide enough the extend the cowl back to where I wanted it.



After taping and bracing the cowl in place I marked the screw holes for attachment to the airframe, then glassed the inside... that should be cured today so I can move forward with final fitting and covering.

grosbeak is offline Find More Posts by grosbeak
Last edited by grosbeak; 08-14-2019 at 05:57 AM.
Reply With Quote Quick reply to this message


Quick Reply
Message:


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools