After constructing a Balsa USA Fokker D7 1/4-scale biplane and doing the ultimate establishing on the engine previous to taking to the sector, I found a violent vibration drawback with not one however TWO model new engines!
Good Outcomes on the Take a look at Stand
I first ran my authentic engine alternative for the D7, a DLE-35RA, on my check stand for only a little bit of break-in and to set the needle valves. The engine ran nicely in any respect speeds and I used to be happy with the outcomes of the check runs.
Determine 1. DLE-35RA on the check stand blowing and going! Word the engine is run inverted with the manufacturing facility DLE muffler. I ran it inverted since on the time I had no different muffler for operating upright.
Issues Seem Out of the Blue
Because the manufacturing facility muffler was not appropriate with the D7 for mounting the engine upright, I purchased a J-TEC muffler specifically made for the upright mounted engine, bolted it on, and put in the engine on the firewall.
With the D7 construct completed and the DLE-35RA mounted and able to run, it began and idled usually, and all appeared wonderful till I moved the throttle as much as see how she ran within the D7. As I revved it up, round 3,000 – 4,500 RPM, a most violent vibration shook the engine and mannequin virtually to dying (discuss with Determine 2). That RPM vary is roughly the “transition level” the place the gas meters between the LOW and HIGH velocity needle valves). This anomaly was very exhausting to digest as I had only recently run the engine on the check stand with nice outcomes. At this level, I had thought-about operating the engine a formality; a dress-rehearsal-test of all techniques earlier than the maiden flight on the D7 – however nope, no maiden flight taking place with this case! After adjusting needle valves, rebalancing a number of check props, and searching every little thing over a number of instances to no avail, I noticed a brand new, surprising, and exhausting to elucidate, drawback had come to hang-out me.
Determine 2. DLE engine with EXTREME engine vibration on the 3,000 – 4,500 RPM Vary! When you had been flying with this case in a full scale plane, it could have shaken your fillings out, adopted shortly thereafter by the engine parting firm with you and your plane. With about the identical outcomes for an RC mannequin. Ha.
My first thought was there could also be an issue with the exhaust extensions I made out of normal automotive heater hose. As a result of relationship between the J-TEC muffler and the standoffs, I used to be compelled to search for a tubing that may very well be simply curved round and previous the standoffs. Consult with the Determine 3 to grasp the scenario. Maybe the hoses had been lots much less easy internally than a metallic tube and stifling the exhaust move, thus choking the engine? Let me digress a second and say that utilizing the heater hoses could sound like a hokey plan, however a fellow RC’er advised me he had used them with out coming aside for a whole flying season earlier than changing them. That sounded uncommon to me, however as I had no different decisions, gave them a strive. I might discover no vibration drawback instantly associated to the heater hoses as I ran the engine with out them and nonetheless had the identical vibrations on the identical RPM vary. I now knew this engine have to be haunted. Hmmmm. Unusual certainly.
Determine 3. DLE-35RA in place. The J-TEC Muffler required some uncommon exhaust extensions – 3/4” Heater Hose! However they weren’t the perpetrator.
Spare DLE-30 To the Rescue!
To not be derailed, I simply occurred to have readily available a “spare” DLE-30 facet exhaust engine, new in field. Certainly this model new engine was going to be freed from haunts and unusual vibrations. As a bonus, this engine-swap regarded promising and pretty straight-forward because the mounting bolt sample was very practically the identical because the DLE-35RA. The DLE-30 backside holes had been the identical and the highest holes solely barely narrower. And the DLE-30 manufacturing facility muffler being a side-mount match the fuselage nicely with no heater-hose extensions required. However I wanted to vary the exhaust tube route from top-exit to bottom-exit, which required a little bit of residence machine work and TIG welding by the native weld store. With the alternative engine and modified muffler put in, it was time to see how she ran. Sadly, virtually the identical vibration drawback offered itself, however not fairly as violent because the DLE-35RA. Two haunted engines! This was GETTING VERY STRANGE! What was happening right here?
I known as in assist from a fellow RC membership member who likes to get entangled with unusual occurrences like this. I ran the DLE-30 within the driveway for him to witness. He was baffled and after spending at the least an hour tinkering with the engine, he stated he was outta concepts in the intervening time. For the reason that DLE-30 was nonetheless mounted within the D7, I gave him the DLE-35RA to take residence with him to play with. He checked the ignition unit and timing and even ran the engine at his store. He didn’t have something good or dangerous to report with the 35RA, type of a wash. So, his report ended any speedy plans for a D7 maiden flight. I made a decision to focus on a number of different fashions I used to be constructing.
Maybe it’s the previous Fuel Tank?
Time handed and I used to be completed with the opposite initiatives and again to the 1/4 D7. Decided NOT to let this engine-vibration-thing break all the construct and rob me of a maiden flight, I made a decision to crawl WAY out on the limb of potentialities! Ha. Thus, at this stage of the sport, I didn’t rule out any risk on what the issue may very well be. May the issue be the way in which gasoline was coming to the carb by means of the rube goldberg gas line setup of an old-school gasoline tank? This was an extended shot, however went with the concept anyway. So, pulled the old-school tank out and changed with a model new 16 ounce Valley View 3 line gasoline tank. After an intensive check run, the brand new tank functioned correctly however had no impact on the vibration drawback. However it’s a stupendous tank so I stored it anyway.
Determine 4. Valley View 16 ounce, 3 line gasoline tank. Nice gasoline tank, however didn’t repair the issue.
One Extra Strive
If it wasn’t the engine itself, muffler, or gasoline tank associated, it MUST be as a result of one thing within the fuselage construction was weak. I had thought-about this a number of instances, however all the time dismissed it as a trigger, “Naw, simply couldn’t be!” In any case, I constructed the firewall to the plans and used the standoffs that got here with the DLE engines. However now, the one factor left was to circle-back and take a look at the firewall and/or standoffs because the culprits.
If this avenue of thought was legitimate, then I have to beef-up and add rigidity to the present firewall, which was made with the precut 1/4” plywood that got here with the equipment. Now, even this repair offered an issue as I couldn’t merely add a brand new layer of plywood to the ahead facet of the firewall, as this is able to transfer the engine ahead and alter the match and interaction between the engine/muffler and higher/decrease cowls, and switch this repair right into a HUGE (unimaginable) rework. Needed to hold the answer easy.
Thus, the one sensible answer was so as to add the extra plywood to the AFT of the present firewall. The few gadgets dwelling AFT of the firewall comparable to batteries, gasoline tank, and ignition unit, may very well be moved again and out-of-the-way simply sufficient to work, however not sufficient aft to adversely have an effect on the center-of-gravity. So, that was my plan of assault. My preliminary thought was to make use of 1/4” plywood to make a full 1/2” thick firewall. However, simply so occurred that I had a perfect piece of three/8” plane grade plywood, so now I had the makings for a 5/8” firewall – even higher! After chopping to the precise dimension together with drilling a number of holes to match the present firewall, I lathered-up the firewall and new plywood with 30 minute ZAP Z-Poxy and laminated the canine out of them! Clamped in 7 locations and let dry in a single day.
Determine 5. I glued the brand new 3/8” plywood addition behind the present 1/4” firewall with 30 minute Z-Poxy and clamped her down.
To proceed with the concept of better general rigidity for the engine, I wanted to create beefier standoffs. I went to McMaster-Carr on-line to seek out some new uncooked materials. I discovered the right answer – 3/4” 6061 spherical aluminum bar inventory and had them ship 24 inches to me. As soon as in hand, I took the aluminum inventory to my machinist neighbor and he lathe-cut them to my specified size (Determine 6). He additionally drilled 1/8” pilot holes at my request whereas within the lathe to ensure the holes had been straight and true. I needed to barely modify two of the standoffs after I obtained them residence to clear a number of spots that barely interfered with the engine and muffler, however no huge effort required.
Determine 6. New DLE-30 Standoffs comprised of 3/4” 6061 Spherical Aluminum Bar Inventory.
Determine 7. DLE-30 with Beefier Firewall and Standoffs in place and able to run the engine! Standoffs are a little bit of overkill, however so what? They aren’t going to flex ANY! Ha. I used 3.5 inch, 5mm cap screws, all the way in which via the standoffs to the Tee Nuts.
Victory at Final!
From IDLE to 6750 RPM not even a burble! Wow! With 20×8 Zinger Prop. Crimson Line oil combined 50:1 (3 ounces per gallon). Lastly prepared for the Maiden Voyage. What a aid!
Determine 8. With higher and decrease cowls in place, and safely tied down, DLE-30 engine operating at about 4500 RPM and ROCK STEADY! Word how regular the prop hub is (prop hub shouldn’t be a D7 scale merchandise, however used for ease of repeated beginning at residence utilizing Sullivan Starter).
A Doable Rationalization of the Scenario
The LOW/HIGH needle valve transition vary I discussed above is generally WAY smaller (RPM clever) than what it was with the vibration issue attributable to the weak firewall. Generally earlier than the needle valves are set to perfection, a second of tough operating happens at this level, normally indicating a too-rich setting. Regardless, the engine(s) ought to have revved proper by means of that spot with no drawback, even with some fine-tuning nonetheless being required. However as soon as the slight vibration or burble started, the weak firewall started to vibrate and that vibration was feeding again to the engine via the flimsy standoffs! Type of a self-perpetuating monster that grew exponentially! Simply check out Determine 2 once more to think about how violent the vibration was. And worse, someplace within the 3000 – 4500 RPM vary was seemingly the place she could be cruising as soon as within the air. With continued operating at that vibration stage, it could have merely been a matter of which was going to fail first – the engine or the mannequin. Even whereas on the run desk, one flying wire on the horizontal tail had snapped from vibration-fatigue on the crimp (I ended up utilizing 2-56 rods to switch the flying wires).
I need to add that within the curiosity of effort and time, I skipped a step within the true scientific course of by assuming that the standoffs had been defective together with the firewall. It could have turned out that simply beefing up the firewall with out new standoffs would have solved the issue. However, having spent over one 12 months being annoyed by all the issues described above, I wished to shorten the time to a profitable answer by fixing each gadgets without delay and shifting to a maiden flight sooner. So, I constructed the brand new standoffs. However the previous standoffs did look a bit wimpy. Ha.
And regarding the firewall design itself. The DVII equipment solely supported the firewall on three sides (left, proper, and backside), in contrast to most different plane which have 360 levels of firewall help. By leaving the highest of the firewall with out help, it leaves open the potential of flexing below load with only a 1/4” firewall.
And, it’s possible you’ll be asking your self, after discovering and fixing the supply of the issue, why not return the DLE-35RA again to the mannequin? Ha. Nicely, that’s actually attainable to do, and I’ll try this. However after TONS of labor and COUNTLESS hours to get thus far, I’m occupied with first getting a maiden flight with the DLE-30.
Ethical of Story
Your firewalls CAN be too flimsy, alongside together with your standoffs, although they had been constructed and put in to the equipment maker’s plans and engine maker’s specs. Chances are you’ll discover that know-how has improved for the reason that equipment you might be constructing was developed. Previous-school magneto-engines should not as robust operating as the present era gassers (in my humble, private, opinion). Thus, for instance, operating the 30 and 35cc DLE engines could also be extra highly effective than you understand and wish extra help than what the equipment requires.
TEXT & PHOTOS BY LANE CRABTREE