Review By: Stephen Bell
Thunder Tiger has just released a completely new helicopter engine to replace the TT50. This is not as we often see a punched out modified power plant aimed at extending product life. The popular 50-sized helicopter exists due to low operating costs both for fuel and parts plus has the attraction of being very easy to transport. The power to weight ratio is slightly less than the 90 format in most cases, this however is about to change. Recently the strongest engines available were the OS hyper and pumped YS51-ST. Volumetric efficiency of these two very good engines is about as good as it gets, meaning any further attempt to increase this parameter would likely have an affect on throttling smoothness.
If we are unable to cram more mixture charge into the cylinder or use a more energy intense fuel we are left with few options. Internal combustion engines produce power due to the gas pressure created during the burn. So if we cannot burn more fuel and add internal pressure with every power stroke we are left with increasing the number of power strokes over a fixed period of time. Increasing engine-operating rpm is the easiest method possible to create more power from an existing and already efficient, fixed displacement power plant. Added rpm as it relates to a helicopter engine requires close attention to engine balancing, porting, carburetion and compression in order to remain reliable with good handling. Because the engine by application is a constant rpm device designers are highly focused upon a more narrow speed range smoothness, hence we usually pick a gear ratio to suite a desired rotor rpm. Today’s 3-D flyers are continuously adding rotor rpm so this is definitely another favorable dynamic used in selecting an engine rpm ceiling with existing gear ratios. Collective acceleration is augmented at higher rotor rpm. Cyclic rate is a direct function of rpm, so play value increases with greater rotor speeds. We are fortunate to have available strong 50 sized blades and rotor heads to handle the increased stress.
Engineers refer to crankshaft throw as the stroke; an engine with a bigger stroke will have a larger leverage from the connecting rod to the shaft center along with a longer piston travel. This will increase output torque when applied correctly. Concerns with compression ratios, piston speed and porting must be addressed. The nice thing about small glow engines is that they are so strong scale wise that they are able to scream without destroying themselves.
A major issue with respect to large displacement glow engine shoehorned into a smaller aircraft capable of handling the gear stress is that too much crankshaft torque eventually causes poor running with lean spots. This is most prevalent in an unloaded condition. This lean unstable oscillation has been crudely termed by modelers as the “wa-waas”. Heck, it works for me. Adding rpm instead of massively increasing crankshaft torque will increase peak horsepower but with reduced engine instability issues. The weight and fuel consumption of a 50 spec engine and exhaust will be much lighter than that of the larger displacement. When the engine fan turns faster increased cooling air does not necessarily add perfectly to the equation, so with this in mind better cylinder head design and consistent mixture control are required. We all know increased horsepower creates more excess heat that we need to get rid of.
I’ve taken the liberty to disassemble the Redline and the older TT50 so the reader might fully appreciate the complete redesign. The result is a careful balance of several design parameters and some very unique thinking.
Physical construction:
The first interesting change that jumps out is the carburetor. Internally the venturi is substantially larger. It actually has an internal diameter increase of .060″ at the throttle barrel. The new method of attachment is something I hope is followed by other engines of the future. Attachment is by means of a two-bolt flange eliminating the wedge and distortion stress applied to both the crankcase and carb body associated with most production glow engines. Besides being more secure and under less stress the carb is better isolated from heat transfer. This is because a plastic insulator fits between the crankcase and the carburetor. Better hot starting results from fuel that does not readily evaporate from a warm carb. While heat transfer may affect operating performance in theory, my real world feeling is this is a minor issue with most glow engines produced today. The insulator is double flanged, male for the crankcase recess and female for the carb raised sealing ring. This process guarantees long term, reliable, leak free operation. The carb has two needles and a rather solid control arm. A nice feature of the arm is it has recesses to friction fit the nut holding the bolt for a rod end ball. Also included is an anti rotation washer that fits between the plastic arm and the throttle barrel which makes adjustments secure disallowing arm slippage as the bolt is tightened down. If for some reason a modeler wanted a push/pull throttle linkage this is certainly possible with the two-spoke arm. The numerous little things all add up in my book demonstrating just how methodical the TT engineers have been.
The throttle arm has a long standoff and a nut holding slot on the backside so that a wrench is not required when securing a ball. It works quite well in practice. Throttle arm designed with two spokes for wider applications like push/pull. Note the locking washer to better aid throttle setup.
Engine and carburetor flange design employing a male and female gasket seat.
Metering side of carb has a wide slot low speed and typical high speed needle adjustment design. Everything clears the mainframes nicely with no plastic grinding required.
The crankcase is a new casting being slightly taller with more internal volume to accommodate a larger crankshaft stroke. Areas like for example where the big end of the connecting sweeps has only the external dimension increased locally to keep weight down and strength up. The front area between the two bearings has additional webbing to again add strength at minimal weight cost. The engine weight is a meager 380 grams. The material in the carb mounting area is generous to prevent warpage and guarantee easy sealing. Looking closely at the rotary valve area inside the crankcase we see two tabs used for more rapid and accurate valve duration, start and finish. Theoretical valve timing and functional application become closer. The bore for the back cover is slightly larger and employs reusable O-ring sealing instead of a paper gasket. As with any decent engine the cylinder sleeve is indexed to the crankcase with a roll pin assuring accurate and repeatable assembly position. Comparing the old to the new crankcase the raised exhaust port can be easily viewed. Muffler attachment wisely uses through bolts to remove any chance of stripping soft aluminum threads and thus allows the application of maximum bolt tension.
Tabs can be seen cast in the rotary valve cavity to better control port opening and closing accuracy in a solid or positive manner. The older TT50 crankcase is placed next to the new Redline engine to demonstrate a totally fresh new design. The higher exhaust port is obvious. Back plate cover is “O” ring sealed removing the issue of torn gaskets and poor sealing. The cover design is larger and takes into account the larger stroke and crankcase volume of the new engine.
The supplied bearings are NTN with a riveted ball retainer. Bearings with folded retainers are not the best since they have been know to come unwound from vibration fatigue and seize an engine. An engine unlike this one with an integrated crankshaft washer area covering the rear bearing from view usually will not allow a fractured folded type retainer to chew the piston and sleeve. The failed retainer often jams between the bearing and crankshaft causing seizure. While the crankshaft washer area may contain large debris, it also reduces lubrication to the aft bearing. Like this engine upgraded engine designs have been known to remove the washer area of a crankshaft.
The increased stroke can be clearly seen with the new crankshaft design over the older TT50 unit.The crankshaft is precision balanced to the reciprocating mass of the piston and connecting rod. Seen next to the old crankshaft it is apparent more manufacturing steps and consideration went into the new engine.
The crankshaft has a larger stroke than the TT50 and is carefully balanced for high rpm through effective counter weight machining. The finish and tolerance is exceptional. The internal passage is the same as the TT50 while the rotary valve is set differently. Industry standard 50 sized dimensions and threads are retained for easy fan mounting, airframe installation, and bearing selection.
The Redline piston crown is clearly marked with an arrow to make reassemble “Murphy Proof”. The cylinder radial position is indexed to the crankcase with a roll pin for a similar purpose. The TT50 and TT 53 redline piston and connection rods are obviously quite different. The Redline connecting rod and piston are both longer to match up with the new stroked design. Typically the rod ends are bushed with lubrication holes.
The ringed piston has the same outside diameter as the TT50 but is taller; the slight increase in engine displacement is due to the change in piston travel. The longer connecting rod is bushed at both ends and employs lubrication holes. The Redline cylinder has different port timing to accommodate the higher design rpm and larger stroke.
The cylinder head fits around the top of the crankcase with a dropped fin design to increase cooling fin area and at the same time keep the engine height low. An aluminum gasket seals the combustion chamber and sets the correct compression ratio for 30% nitro methane glow fuel. External fit and finish of the cylinder head is top notch and slots are cut in the top two fin spacing sections to allow easier electrical wiring for a remote glow plug connection.
I installed the engine as a replacement to a YS50ST in a Raptor 50. Prior to this I had run the TT50 for several seasons performing normal bearing maintenance only and the engine still runs very well to this day. After installing the YS50 I found it necessary to add collective pitch to take advantage of the increased power and rpm. Since the modification for extra pitch allows a 32º range it was reduced to a manageable +12º/-12º for the YS at 1900-2000 NR. For partial evaluating of the 53 engine similar pitch values and rotor rpm are used, along with piloting impressions. In the final appraisal a judgment is made based upon the best setup for the Redline engine.
Using a crankshaft-holding tool is a good means to secure or torque down a screw on type fan and is also the best method for correct wrenching of the crankshaft nut.
As usual the clutch was dialed into less than .002″ total indicated run out using a dial test indicator before the engine is mounted into the airframe. I installed a Thunder Tiger 50 muffler and governor from the YS. The engine was fired up and hovered for a tank of fuel at reduced rpm with collective spikes thrown in once in a while for a better rich break-in loading. It was clear at this point that the engine has very decent low rpm torque, which totally surprised me under this operating condition. The governor controls RPM during engine break-in evaluation using a 1600NR setting. The second tank of fuel idle up is intermittently used at a governed 1900NR and it is clear that the helicopter climbs out like a “home sick angle”. The mixture is at this point is still rich for a slight power drop off. Throttling is excellent with absolutely no lean spots. The machine is loaded and unloaded both short and long term with consistent smoke, engine noise and stability. The power is smooth under all conditions. I looked for flying situations other engines would have issues with hunting induced by mixture instability and could find none with the governor both on and off. The collective pitch on the subsequent flights is topped out to +/-13.5 degrees with no sign of bogging during both +7/-7 degrees of cyclic and full collective applications. At this point we decided to change the rotor blades from 600mm Funkey to wider chord light inertia 600mm Radix “Stick Bangers”. The pitch values are left alone and the machine now can be intentionally forced to bog down slightly with both sticks at extremes, abet with a more sprightly performance. The helicopter is still however very manageable and overloading the pitch to the point that the rotor speed sags must be very intentional. The engine has not been maxed out in its rpm power band due to being new and not fully broken in yet. Because cyclic rate is a function of RPM further enhancement is expected. At this point I’m confident in reliability and have to say this is the most powerful and smoothest 50-sized engine I’ve encountered to date. Not often does something exceed my preconceptions to cause such personal excitement!
Main needle valve has a fine thread and low taper resulting in more precise fuel metering and reduced needle adjustment sensitivity.
High-powered engines can entail finicky operation and tuning, this one does not. The high-speed needle is very insensitive making for perfect adjustments plus there is no peaky setting. The main needle taper is very shallow and the thread pitch being fine certainly contributes to the excellent carburetor design. Our needle is set at about 5 turns out from fully closed. When you consider this to a needle running at say 1.5 turns out it is very easy to see the added metering precision. The low speed adjuster has a wide slot easily accessible for accurate adjustment through visual orientation. This adjustment works well to acquire a good idle and does not cause midrange transition difficulty. The designers wisely place the default value at flush with the carb housing to make life very easy.
Power is not everything if you cannot take full advantage of it. Because the stock Raptor was developed for the best of the day engine, for some it now lacks the pitch range for full power extraction in a symmetrical, moderate to high rpm setup. Because the Raptor control geometry is near perfect below the swashplate it is easily and safely modified above this point at the rotor head. Many have purchased complete aftermarket rotor heads for this reason. Most duplicate the stock head mechanical (lever) geometry with one major difference; the mixers are of a higher throw design. Herein lay the key; simply altering the mixers will contribute to the same increased potential performance level. Thunder Tiger has realized the advantage by offering high throw mixers in the 90-3D. Running this kind of increased stress level unfortunately necessitates the use of a TT metal head block to prevent eventual plastic fatigue. Because of this, purchasing the mixers or a metal head block with the mixers will reduce required upgrading cost.
We chose to modify existing TT parts because we have easy access to them and the fact that we had the pleasure to try a similar modification made by an American individual at IRCHA by the name of Eric Lis from Riverview, Florida this 2007 season. We have been using modified mixers on our 90s for the same reason over the last three years. It was nice to reap the benefits of someone else’s labor through a couple of evaluative test flights. While Eric’s modification worked extremely well I found it a bit too aggressive for my tastes. He uses an available pitch range of 32 degrees, which makes for a very sensitive expert’s setup along with the possibility of control lock-up. I have to admit that in-flight this never occurred even though bench tests indicate a potential. I found the cyclic to have a large expo like feel around center before it went into the ‘play’ mode suddenly.
Three mixers compared, top is the stock mixer with a 22 degree collective range. Middle is a modified 27 degree mixer manufactured from a shortened R90 washout arm. The lower lever is the stock R90 washout arm prior to modification with a potential for a 32º collective range. Average, hot and O.M.G. throws are shown.
What I did like when using his 32º (+/-16/16) mechanical pitch range is that the machine tracked better in fast flight. The flybar authority was one of the parameters increased along with the added collective and cyclic throw. Having a mechanically sensitive high throw collective control system makes for a very quick and sensitive stick especially near the half travel and close to the hover point. A 5-point pitch curve did accommodate matters in this regard leaving me with an uncomfortable feel even when using a slight expo like “S” shape. Since the engine will not pull 16 degrees with cyclic applied, for most mortals it needs to be reduced with ATV, I decided to forgo the ATV reduction in favor of lower throw mixers ranged between the stock units and Eric’s. Basically we take Raptor 90 washout arms and carefully shape them into 50 sized mixers after drilling the hole at the short end more inboard. Simple washers or spacers installed under the arms correctly locate the mixers to clear the head structure. As a side benefit autorotation hang time after the flare is increased due to better energy extraction from remaining rotor inertia. This is also good for budding pilots or those having difficulty managing energy reserves. The question I often ask myself in jest, when does a “rednecked” RC helicopter component become an accepted modification!
The installed 27 degree mixers use spacers or a washer stack-up to clear the flybar teeter fork on the head block. 3mm self locking elastostop nuts can be used in a bind for correct spacing.
After full break in and numerous play sessions we decided to raise the engine and rotor speed. I was quite happy running at approximately 2000 head speed, however two modelers associated with ACE Hobbies (the TT North American distributor) are running 2300 and have been doing so for quite some time. I’m a pretty conservative person with engine rpm; yet knowing these individuals convinced me to do so. So while using the TT canister muffler off I went.
Findings demonstrate that the engine power curve remains stable up until 2300 rotor rpm increasing cyclic control power but without a sudden bog down during large power demands. Because the engine is ported for a higher rpm segment and retains a relatively broad (sweet spot) a useful maximum power band running at 1800- 2300 on the rotor via 8.5:1 gear ratio offers very usable top end engine performance intended for a variety of contemporary flying styles. This is very good news, however by using a more rpm aggressive high flow exhaust like say the Curtis MP-5, power levels might be increased further. At this point in time I wonder why I consider even making a change in exhaust system since present performance is excellent for my needs.
Note the area needing attention when mounting a MP5 to a Raptor/Redline. Simply adjust the boom support mounting to clear or add a thick metal gasket between the engine and MP5. It’s well worth the small effort involved.
Installation of the longer MP5 required me to counter sink a boom support screw into the fitting for muffler clearance. Not a big deal and I’m sure there are other means to mount the boom support or shorten its standoff height. The MP5 runs nice and smooth at first glance, but surprisingly it’s very quiet. It tends to oil up the machine a little when rich, as it is slightly longer with a short deflector, thus a different exhaust deflector is installed as a corrective measure. I also found a very large change in high-speed needle settings, the setting is reduced from about 5 turns out to 2 and ¾ which I suspect is at least partially due to a higher fuel tank pressure. Performance is up another rung on the ladder as the engine breaths more freely. I’d say this is a very good match for power hogs. The pipe resonant frequency is where it should be for exceedingly sprightly flying qualities but still it maintains muffler like smooth running qualities at all useful rotor rpm settings. The engine cuts quite nicely into throttle-hold with no hang up. The end result is 90 sized helicopter performance levels but with no bogging and the economy of a 50 machine. In fact due to the smaller helicopter mass a performance setup can actually be more agile or cat-like. We used both the CY 600mm SBs and the heavier inertia CY 600mm blades.
This engine is going to be very popular in various branded helicopters, TT has just moved the 50-class performance bar up. Modelers will not be disappointed in this engine selection and I expect the first production run to sell very briskly. Thanks to Darren Altbaum at Hobbycraft Canada for supplying the Redline 53 engine and for his generous support to our fine hobby.
We used:
Redline 53 helicopter engine
Raptor 50V-II helicopter with a TT metal head block
Thunder Tiger exhaust
Youngblood MP5 exhaust
Radix 600mm main rotor blades
Coolpower 30% helicopter fuel
Futaba 72mhz PCM avionics with ATG and Solid G
Miniature Aircraft tachometer
3 views below
Addendum:
Thunder Tiger have a newer carburetor that comes standard with the current production Redline 53 engine. A few modelers were eventually having a sticky throttle action. TT as I am told has changed the coating on the throttle barrel to extend service life. I have not personally gone to the newer carburetor as problems have yet to be encountered even after many cases of fuel.
Presently there is a free exchange program should you have the old type and want to swap out. The new carb has a groove formed in the center of the main needle housing wrenching or nut flats. Unknown to most, there is a 3rd factory set midrange needle hidden from view under the throttle arm bolt. When using 30% glow fuel and based upon ambient conditions a small adjustment might be required.
The adjuster is very sensitive so richening by 1/8-1/4 turn is the maximum needed. The low speed adjuster will need to be set to compensate as it is affected, however this 3rd needle adjustment has no effect on the main needle adjustment. Remember the 3rd needle rotates in and out with the throttle barrel while the low speed needle does not. This is why it has a large effect on the midrange area.
After the weather warmed up and while running at top rpm in a full power leaned condition I opened my 3rd needle 1/8 turn richer or in the CCW direction. The first indicator of an adjustment requirement is when coming out of idle up in a low power situation, the engine will sound lean and rise in rpm with a possible short duration hunting situation, before is self stabilizes. If the idle needle will not correct this situation with richening and still maintain a reliable idle the hidden adjuster is where you should go.
Stephen Bell
http://www.scotiabladerunners.ca/redline53.htm
2 Comments Already
Pingback & Trackback
Related Post
Please Leave Your Comments Below