We have found through the last 50+ years of experience that this has been overlooked by engine builders, propeller manufacturers and airframe builders. What your engine is resting on and bolted to is the number one source of vibration thru the cabin and airframe.
Our experience shows that airframes are getting weaker mainly due to vibrations from engines, taxiing, stop/starts. Throw on more hp, weight, larger propeller, and combine that with mission specific modifications to land on shorter runways, snow, water, and unprepared strips and have a potential for making the problem worse.
History and new technology has made us all smarter. Let me explain:
Cessna designed the 180 for a 225 hp O-470-A/-J engine. The design of the engine mount was revised TWICE within the first year of production. First at SN 30527, and then again at SN 30640. In June 1954, a bolt on kit in a attempt to strengthen the mount on older aircraft. By September 1955, a standard repair was added to all the mounts to again strengthen the mount. Ultimately, Cessna abandoned the original design in February 1956 and issued SL180-1 allowing owners of the O-470-A/-J powered aircraft to install the new 0751001-1 O-470-K engine mount. By 1956 there were 4 versions of the original 0751000 engine mount. In 1956, Cessna claims that the engine mount design change from the original engine mount to the 0751001-1 engine mount reduced approximately 95% of the engine vibration.
The Aircraft on the Right has a 3rd generation 0751000 Engine mount. It lacks the last set of reinforcements added prior to the introduction of the O-470-K.
In 1956, Cessna developed the 182 by tipping the aircraft forward onto Tricycle Gear. By this point, there were already 5 re-designs of the engine mount. This did not stop improvements to the engine mount. In July 1955 Cessna started offering aircraft with float fittings and by 1959 reinforcements were added to float plane mounts to prevent cracking of the engine mount pads. After that, the 180 float mount was strengthened first by changing the tube sizes, then by adding vertical tubes on the firewall. In total there were from at least what we can tell, 8 major re-designs of the 180 mount with 19 minor changes during the years(Things like Improving Paint, finish etc). The 182 did not fare much better, with 2 additional re-designs for the O-470 powered variant by 1990. One of which was to harmonize the installation with the Cessna 180.
Without even changing the engine away from a O-470 we know that:
A lot of owners will add 3 blade propellers to their airplanes and experience added vibrations. In 1979 Cessna issued a service letter for the C-185 and IO-520 that recommended a “3 blade mount” for their McCauley prop options. The problem was identified way back then. While this new mount did improve the Cessna 185, it was not taken any further. Cessna replaced large isolators to the AFT positions only. The small rubbers still remained in the front positions.
Our mount uses these large rubbers (pictured) in all 4 positions!
This equates to … less fatique, less avionics wear, more passenger comfort, less airframe noise. It has been said that our system is turbine smooth. Most test drivers cannot believe the difference. Our experience is that the engine runs EVEN SMOOTHER with a 3 bladed prop installed vs a 2 blade. This is due to the extra weight on the mounts which they were designed for. You owe it to yourself to try one out. With our mount on your airplane, run the rpm right up to red line, and slowly back it down to 1500 rpm – you won’t experiencing any shaking. Better yet, try this exercise out first on your existing mount ... our mount is cheaper than the competition that uses the old system of rubbers, and way cheaper than Cessna factory prices. The Lord rubber isolator mounts are expensive, but they are designed to go the distance and definitely won’t require any changing between overhauls.
Since aircraft engines are air cooled, they require larger tolerances for piston to cylinder clearance so they don’t seize up during cooling and heating cycles. This is totally unlike car engines that are controlled constantly thru water jackets. Most of the O-470 engines are vibrators at best and the slower turning O-470-U engine is even worse of the group. What the engine is resting on and bolted to is the usually the number one source of excess airframe vibration thru the cabin and one of the most effective spots to eliminate the vibration. The problem is that most pilots do not even know there is a vibration problem, and they think it's just supposed to be like that. It can get much worse if you were to do an engine upgrade to more horsepower.
Cessna Introduced the 185 in January 1961. The aircraft was equipped with a IO-470-F engine producing 260hp. This engine weighs 426lb empty so Cessna strengthened and redesigned the 180 mount to better support the engine isolators. The isolators are now supported on both the inside and outside of the isolator. This design element was used on other aircraft like the 206, 210, 310. The result was the best mount Cessna designed using the smaller isolators. The problem is that these isolators themselves started to sag and seemed to require replacing every 300-500 hours. These sagging engine mounts can cause additional wear and tear on engine accessories that suddenly find themselves closer or further away from the cowling. Engine baffling on one end of the engine will now be pushed against the cowling accelerating wear and on the opposite side be further away preventing a good seal.
Additional wear is not the only problem you encounter. When engine mounts sag, the thrust line of the engine changes degrading performance on ground and in flight. This is more of a problem on the 180 and 182 than the 185. The degraded performance is compounded when excess trim is applied to counter the effect. Not only are you flying slower, burning more fuel to fly the same speed, but you are also wearing out your flight controls over time. Having said that, the isolators are definitely too small for this engine - and these are the O-470 series.
Cessna upsized the isolators on every aircraft equipped with a IO-520 Series engine from the start of production EXCEPT the 185. It was not until the 3 blade D3A34C403 McCauley propeller was introduced in 1978 that the 185 received a substantial reinforcement to the engine mount. This introduction was specifically introduced via SL 1977-13 and specifically mentions that the propeller is approved only on aircraft with engine support structure that has been reinforced.
This new 3-Blade structure added 5.1lbs to the standard engine mount and 2.2lbs to the float mount. It introduced the largest isolators available in the aft position but re-used the same original, sag-prone isolators in the front position. The IO-520-D Center of Gravity is 11.29" from the aft accessory Case. When you install the propeller on the front of the engine this center shifts forward. Essentially the installed center of weight is ~19.5" From the aft accessory case placing it over the forward Isolators. As the propeller spins, the aft isolators dampen the vibration and the forward ones hold the engine. The Cessna 188 Turbo 188 With the TSIO-520-T engine tips the scales at nearly 492.5lb. Cessna took the aft Isolators from the 185 3-blade mount, make them solid to support the extra weight, and used them in the forward position. Ever notice and wonder why the new Cirrus aircraft use 2 smaller isolators in the front?
Finally, after 11 re-designs the engine mount is at the starting point where Seaplanes West began. This engine mount while not perfect was the basis of the vibration isolator selection. The challenge now was to optimize the structure and adapt that benefit to all the older airframes. Although similar to the 188 mount the engine mount ended up being a clean sheet design using modern day computer analysis to determine the optimal design.
The re-designed mount was initially to address the issues with the standard engines, mostly Continental O-470 thru to IO-520. However, as you know there are numerous engine STC's that bring more power and performance to the Cessna 180, 182, and 185 series aircraft. There are also numerous STC's for propellers, 2 blade and 3 blade designs. Many of these STC's are made and approved by different companies over many years using different aircraft during certification. Some STC's require you to cut and modify existing light weight engine mounts like the one to the left. Some require adding additional reinforcement tubes, gussets, or doublers. None have addressed the extra vibration problems associated with adding extra HP to the existing Cessna mounts. When you add more horsepower plus a heavier propeller to the existing firewall, everything needs to work in harmony to have a balanced system. Our engine mount and isolators are designed to finish off and balance that system.
Many of these STC's were certified using some of the best minds, knowledge and good quality type conforming test aircraft. The problem in the field that we come across is that many of the aircraft today have had the mounts modified, repaired improperly or have other modifications that add weight or unforeseen loads onto the trusses. The Seaplanes West Mounts were certified with some of the highest loads in mind from the start and actually work even better with the added weight of a 3 bladed propeller installed. The latest CAD modeling, computer analysis, and manufacturing practices are used in the design.
All of our engine mounts are designed using 21st century state of the art drafting software and optimized for minimal stresses and vibration transfer to the airframe.
All our engine mounts are welded in fixtures manufactured using CAD designed and CNC manufactured welding fixtures. This greatly improves the nominal fit of all the mounts and controls tighter tolerances before assembly. All our Seaplanes West series engine mounts are manufactured from Aerospace Grade 4130 chromium-molybdenuminum (Chromoly) materials using advanced CNC technology.
We use Seamless Aerospace Grade(AMS6360) over commercial grade Mechanical Tubing(AMS6371). This allows us to have a more consistent construction for the highest strength to lightest weight ratio possible. The CNC manufacturing of all the tubes allows the tube intersections to be tightly controlled to ensure the tightest fit between fillet joints possible. These mounts are welded using fully qualified AWS D17.1 Multi-Position Certified fillet welders with many years of experience. After welding, all engine mounts are fully stress relieved to ensure there are no residual stresses from welding.
Prior to painting, all engine mount trusses go through not only a visual weld examination by a Weld Inspector but also get fully Magnetic Particle Inspected to ensure there are no cracks. All Tubular members are treated internally for corrosion resistance and weld sealed. All engine mounts are then primed with a Aerospace Designed, Mil-Qualified primer(MIL-PRF-23377 Type 1, Class 2) and top coated with a flexible Mil-Qualified Polyurethane Top Coat.
Each mount goes through a series of inspection checkpoints that proves quality and reliability is assured. Prior to shipping, each engine mount is re-checked for fitness and issued a serial number. Our Transport Canada approved and fully licensed structural manufacturing status within house manufacturing and Non-Destructive testing allows us to have full traceability on all materials and workmanship.
Bottom Line...This equates to … less fatigue, less avionics wear, more passenger comfort, less airframe noise. It has been said that our system is turbine smooth. Most test drivers cannot believe the difference. Our experience is that the engine runs EVEN SMOOTHER with a 3 bladed prop installed vs a 2 blade. This is due to the extra weight on the mounts which they were designed for. You owe it to yourself to try one out. With our mount on your airplane, run the rpm right up to red line, and slowly back it down to 1500 rpm – you won’t experiencing any shaking. Better yet, try this exercise out first on your existing mount. The Lord rubber isolator mounts are expensive, but they are designed to go the distance.