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Long Coot September 1, 2010

By petehdgs - Posted on 22 January 2011

Friday, September 03, 2010

Tail design:

Earlier this week I received my new copy of The Illustrated Guide to Aerodynamics by H. C. “Skip” Smith, I lost my old copy.  I will be using the formulas within to double check the Long Coot’s design parameters and redesign the tail.  I am concerned about the tail and how its existing design might not work as well with the longer and heavier Long Coot.  I am leaning towards making the tail structure out of geodetic wood and using a stabilator instead of a fixed horizontal stabilizer and tail plane.  I believe this stabilator system is more efficient and can provide a wider area of control thus increasing the CG range.  I believe this capability will be important on the Long Coot.  One of the design challenges will be how to apply the stabilator in such a way as to not interfere with movement of the rudder.  The stabilators I have seen have the anti-servo tab all the way across the back of the tail, with the rudder mounted above the tail.  I think the solution is to build a split stabilator.  This will make for a more complex linkage system to connect the anti-servo tabs across the split, but I think it is worth investigating.    

Low Speed Propulsion System:

Built onto the tail of the Coot is the water rudder. I would like to enhance this area with a low speed electric propulsion system equipped with a folding prop and a Kitchen Rudder.  Based on the 1.5 times the GW of the Coot, the maximum GW of the Long Coot will be 2925 lb with an estimated waterline length of 21 feet.  I used the formulas in “The Propeller Handbook” by Dave Gerr to calculate hull power requirements for GWs of 2100, 2500, & 2925 lb and speeds of 2 to 6 kts. 


SL Ratio

2100 lb HP

12V Amps

2500 lb HP

12V Amps

2925 lb HP

12V Amps

2 kts








3 kts








4 kts








4.58 kts








5 kts








6 kts








From this chart it seems clear that designing to a 1.0 SL ratio of 4.58 kts would be possible with an electric motor rated at 2.5 HP continuous that can be fed from a 12 volt, 220 amp alternator driven by a 5 HP gasoline auxiliary engine.  For low speed maneuvering at SL ratios below 0.5 (2.29 kts) the starting battery would be enough.  Having a rope & electric start auxiliary power unit along with a good sized battery would allow for many hours of quiet low speed trolling on the water, and the ability to cruise into a port under power at 4-5 kts indefinitely.    I have sent a request to Arlington Armature to see if the necessary hardware would be readily available.

Sunday, September 12, 2010

Boost Theory & Speed Control:

Engine Horsepower is a product of Torque X RPM X 1/5252.  In a properly adjusted gasoline engine torque is basically a function of Volumetric Efficiency, which at any given RPM varies directly with manifold pressure.  The wider the throttle setting, the higher the manifold pressure, the denser the air in the manifold and the more air molecules get into the cylinder during the intake stroke.  The maximum attainable volumetric efficiency also varies somewhat over the RPM range of the engine thus producing more torque at some RPMs and less torque at other RPMs and generating a torque curve over the RPM range that is slightly cambered.   This torque curve, and the HP curve derived from this information, is called the LUG curve and represents engine output at wide open throttle.  We can reduce the throttle to reduce power below the curve, but we cannot normally increase power above the curve. 

Even though the lug curve is cambered we can think of it as a straight line for this discussion, so maximum volumetric efficiency will be considered constant as RPM changes.  This means that HP and wide-open-throttle will vary directly as RPM changes, producing a straight line.  While this can be considered true of the engine lug curve, it can never be true of the propeller demand curve.  This curve, unless the propeller blades are stalled, follows the form of a cubic demand curve.  Demand HP is proportional to RPM cubed.  This means that the lug power curve of the engine and the demand power curve of the propeller do not coincide with each other and only meet where they cross each other.  Power output of the engine/propeller combination can never exceed the maximum engine power. 

Let’s say we have an O-360 engine with a fixed pitch prop at a constant airspeed producing 180 HP at 2700 RPM.  If we reduce the throttle and bring the engine back to 2400 RPM, how much power are we producing?  The new HP would follow the propeller demand curve and would become: ((2400/2700) cubed) X 180 HP = 126.4 HP, or 70.23% of the initial HP.  The new power output followed the propeller demand curve because the propeller pitch did not change, only the power feeding it did.  Since the new power was keeping it running at 2400 RPM, the new power had to be 126.4 HP. 

What if we maintain wide-open-throttle and instead changed the pitch of the propeller to bring the RPM down to 2400?  How much power would we be producing then?  The new HP would follow the engine lug curve and would become: 2400/2700 X 180 HP = 160 HP.  This time the new HP followed the engine lug curve because the engine throttle did not change, we only changed the pitch of the propeller which changed HP the propeller required.   

We are now ready to consider the application of Nitrous Oxide Boost.  The maximum power of a naturally aspirated engine is based on the amount of air that enters the cylinder on the intake stoke.  Air is 78% nitrogen, 21% oxygen, and 1% other gasses so the amount of fuel that can be burned in air is limited by the amount of oxygen the air contains.  Nitrous Oxide is 60% nitrogen and 40% oxygen so it contains nearly double the amount of oxygen that air contains.  If an engine could run on 100% nitrous oxide, instead of air, it would burn about twice as much fuel and produce about twice as much power as a naturally aspirated engine would.  This is because the temperature rise in the combustion chamber would be twice as high, expanding the gasses in the cylinder twice as much as would normally occur.  However, in an engine nitrous oxide must be mixed with air because the flame must be started before the oxygen contained in the nitrous oxide can be released and burned.  It is this property that makes boosting HP with nitrous oxide more difficult to sustain reliably the higher the desired boost is. 

While boosting with nitrous oxide can be finicky at high levels of boost, at low to moderate levels of boost it can be very reliable and predictable.  At these levels, boosts of short duration will provide added power without undue harm to the engine, especially if the engine is built to handle it.  These are the levels we are targeting for take-off operations of the Long Coot. 

Adding nitrous boost to the engine creates a new, higher output lug curve for the engine at higher torque and power.  In both of the previous examples we used only the tachometer and the power curves to determine the power being produced by the engine/propeller combination at different RPMs.  We started with a known power output at wide-open-throttle, then changed either the engine throttle or the propeller pitch and saw how that change affected the engine RPM and power.  Our naturally aspirated engine at wide-open-throttle produced 180 HP (-nw) at 2700 RPM and 160 HP (-nw) at 2400 RPM when we adjusted propeller pitch to change RPM.   If we maintained the same pitch and instead reduced the throttle the engine produced 126.4 HP (-nr) at 2400 RPM.  Now let’s look at adding boost. 

If we add a 50 HP boost kit to the engine (this is readily available) and it adds exactly 50 HP-b to the engine output of 180 HP-nw at 2700 RPM, what is the new HP and RPM combination?   180 HP(-nw) + 50 HP(-b) = 230 HP(-bw) at 2700 RPM, but if the propeller pitch remains unchanged the RPM will increase on the propeller demand curve to 2930 RPM.  ((230 / 180) cube root) X 2700 RPM = 2930 RPM.  This is clearly too fast, but it gets worse… Because the engine is turning faster, more air is being swept through the cylinders producing more power.  At 2930 RPM, the engine should produce 195 HP(-nw) (if it will run that fast), so the actual output at 2930 would be 195 HP(-bw) + 50 HP(-b) = 245 HP(-bw).  This higher engine HP will push the prop RPM higher to meet the higher engine output and a new iteration will have to be calculated.  After a couple more iterations, the engine will be turning about 3000 RPM or more.  The propeller and engine combination probably won’t take this and hold together, so let’s try again. 

If we adjust pitch to start with at 160 HP(-nw) at 2400 RPM and add 50 HP(-b) what do we get?
160 HP(-nw) + 50 HP(-b) = 210 HP(-bw) at 2400 RPM.  ((210/160) cube root) X 2400 RPM = 2627.7 RPM.  (2627.7/2700) X 180 HP(-nw) = 175.18 HP(-nw) at 2627.7 RPM.  175.18 HP(-nw) + 50 HP(-b) = 225.18 HP(-bw). ((225.18/160) cubed) X 2400 RPM = 2689.6 RPM.  (2689.6/2700) X 180 HP = 179.3 HP(-nw) at 2689.6 RPM. 
179.3 HP(-nw) + 50 HP(-b) = 229.30 HP(-bw) at 2689.6 RPM.  ((229.3/160) cube root) X 2400 = 2705.9 RPM. 
This is pretty close to 231 HP(-bw) at 2710 RPM.  This is about what we are looking for. 

Here is what we know so far:  The O-360 engine, rated 180 HP at 2700 will produce 180 HP(-nw) at 2700 RPM, 160 HP(-nw) at 2400 RPM, 231 HP(-bw) at 2700 RPM, and 126.4 HP(-nr) at 2400 RPM.  The term (-nw) means naturally aspirated at wide-open-throttle, (-bw) means boost operation at wide-open-throttle, and (-nr) means naturally aspirated at reduce throttle.  Obviously there are many more possible combinations, but this is enough for our purposes right now.  We are trying to put together the operating parameters of the engine/propeller combination for take-off operations of the Long Coot.  Other operations do not concern us at this time. 

So far in our discussion we have used only the tachometer to determine power output of our engine.  We can use all kinds of other instrumentation, but I would prefer not to.  A good accurate tachometer is all that is necessary to properly operate and lean an engine, so long as the propeller pitch doesn’t change while you are adjusting it.  I have flown several years with a fixed pitch prop and tachometer as the only engine power indicator and know this is true.  While it is relatively simple to operate in this manner, constant speed, adjustable pitch propellers do bring advantages, especially to seaplane operations. 

Propellers work by accelerating air rearward to push the airplane forward.  The amount of push of any given propeller is partly based on the speed that the propeller is moving through the air.  A fixed pitch propeller can only produce optimized thrust over a narrow airspeed range.  Outside of that range, propeller efficiency decreases producing less thrust.  Seaplanes demand high thrust at low airspeeds, but if a fixed pitch propeller were optimized for this it would not be able to cruise very fast, as higher pitch is necessary to support higher airspeeds.  Constant speed propellers continuously change the propeller pitch as the airspeed increased to maintain maximum thrust at the engine RPM the pilot sets.  Once set, this all occurs automatically without pilot input so he can pay attention to more important matters like flying the airplane.  This is the perfect arrangement for takeoff operations of a seaplane. 

My biggest problem with constant speed props is the added complexity of instrumentation and operating procedures that are required after leaving the traffic pattern.  In addition to the tachometer, you must have exhaust gas temperature and manifold pressure gauges, and you need to know how to use them to properly adjust the engine in flight.  Wouldn’t it be nice if we could have the adjustable pitch prop and the prop governor without more instrumentation and complexity?  That would be the best of both worlds.  I think we can. 


Monday, September 13, 2010

Governor Theory and System Operation:

A governor is a device that maintains RPM by controlling either the power output or the power demand.  Lawn mowers and garden tractors have engine governors that control power output by adjusting the carburetor throttle plate setting.  Airplanes have propeller governors control power demand by adjusting propeller pitch.  There are two basic types of governors, variable speed and limiting speed.  Variable speed governors always have control of the adjusting device and constantly make adjustments to maintain RPM where desired.  The operator can only adjust the governor setting and does not have direct control of the adjusting device at any time.  Constant speed governors on aircraft are actually variable speed governors because the operator can adjust the governor for different RPM settings and does not have direct control over propeller pitch.  Limiting speed governors allow the operator to have direct control over the controlling device and only operate to keep the RPM within desired maximum limits. 

Ivo-Prop has an adjustable pitch propeller that the pilot can adjust while in flight.  They also have an electronic governor that can be connected to simulate a constant speed propeller arrangement.  I propose to use the Ivo-Prop in-flight adjustable pitch propeller wired to two of their governors interconnected to the nitrous oxide boost system to act as a limiting speed governor system that automatically controls the propeller pitch and RPM during takeoff.  See the drawing “Long Coot O-360 with Nitrous Boost and Limiting Speed Propeller control schematic REV-0:” on the next page.  This drawing is not complete, but shows the basic concept of the system and how it will operate. 

The pitch motor will be wired to two relays, the Raise Relay (RR) and the Lower Relay (LR).  The raise relay (RR) will RAISE engine RPM by operating the pitch motor to decrease propeller pitch, and the lower relay (LR) will LOWER engine RPM by operating the pitch motor to increase pitch.  These relays may be energized by the pilot with a Raise / Lower Manual Control Switch, by governor 1 set at 2400 RPM, or by Governor 2 set at 2700 RPM.  

The Raise / Lower Manual Control Switch will always be active and available for the pilot to use.  This switch gives the pilot direct access to control pitch of the propeller, even if the governors are turned off. 

The pilot can select which governor controls the prop with the Boost System Control Switch.  The 4 positions are Off, Gov1-2400, Boost-Armed, and Gov2-2700. There will also be a Wide-Open-Throttle Switch that operates at full throttle and a Trigger Switch that operates when the pilot pulls the trigger.  In the Off position the governors and boost are disabled and the Manual Raise / Lower switch is used to adjust pitch.  In the Gov1-2400 position governor 1 is turned on and nitrous boost remains disabled.  The governor can increase pitch to limit maximum RPM to 2400 but can only decrease pitch to raise RPM if the WOT Switch is closed or the trigger switch is operated by the pilot.  The GOV2-2700 position has the same restrictions, but the RPM is limited to 2700 instead of 2400 RPM. 

The Boost-Armed position is used to automatically switch between Governor 1 & Governor 2 and operate the application of nitrous oxide & additional fuel to the engine.  When this position is selected at part throttle Governor 1 is armed and is free to increase pitch to limit RPM at 2400.  At this time it is assumed the prop is spinning slower than 2400, but that may not be the case, so the governor will increase pitch if necessary to bring the engine down to 2400 RPM.  When the throttle is pushed to WOT the engine will go to full HP as defined on the Naturally Aspirated Lug curve and governor 1 will operate to increase or decrease pitch as required to maintain 2400 RPM. 

During WOT operation with the BCS set to the Boost Armed position, and after RPM is stable at 2400, and the pilot is ready to apply boost, the trigger is pulled and held closed to 1) disable governor 1, and 2) apply metered nitrous oxide and additional fuel to increase engine HP and RPM to the Boost Lug curve (about 231 HP at 2710 RPM), and 3) governor 2 is enabled and will operate to increase or decrease pitch as required to maintain 2700 RPM.  This condition will last as long as the trigger is pulled and held in this position at the WOT setting, or until 10 seconds has expired.  At that time, if the trigger is still being held the boost application will be ceased and locked out and Governor 2 will be latched in the ON position.  The engine will reduce HP to the Naturally Aspirated Lug curve and governor 2 will operate to increase or decrease pitch as required to maintain 2700 RPM. 

Long Coot O-360 with Nitrous Boost and Limiting Speed Propeller control schematic REV-0:



If the trigger is released before the 10 seconds have expired 1) the application of nitrous oxide and added fuel is stopped, 2) Governor 2 is disabled, 3) the engine reduces power to the Naturally Aspirated Lug curve and slows down again to about 2400 RPM, and 4) governor 1 is enabled and will operate to increase or decrease pitch as required to maintain 2400 RPM. The Boost system is still armed and unlatched and boost may be applied again if necessary by simply pulling and holding the Boost trigger.  For safety reasons I have assumed that the Ivo-Prop governor cannot control the pitch fast enough to keep up with the sudden change in engine HP when boost is first applied.  Defaulting to the lower RPM Gov1 setting will prevent an overspeed condition should the boost trigger be pressed again after it had been released.  At this time the pilot may choose to select the Gov2-2700 position thus disabling boost and enabling governor 2 to adjust the pitch as necessary to maintain 2700 RPM, but the pilot must select this position to do so.  (The drawing will have to be revised) 

So with this system in place how do we fly it? 

The Boost Control Switch with allows the pilot to set up each governor on the NA Lug curve first.  Simply select the governor desired, apply WOT or the boost trigger and adjust the governor to maintain the correct RPM.  Once this adjustment is complete on both governors the system is ready to use and should not require readjustment. 

To takeoff and cruise without boost, select Gov2-2700 on the Boost control switch.  Apply full throttle and takeoff normally.  The engine will stay at 2700 RPM for the takeoff run and climb out so long as WOT is maintained.  Once at your cruise altitude, reduce the throttle and bring the engine RPM back to 2400 or 2500 RPM as desired.  Trim the nose for proper airspeed, trim throttle to remain level, trim the mixture to peak RPM, then lean gently to drop 10-15 RPM.  Double check cruise airspeed and RPM and manually adjust pitch if necessary.  You are done! 

To takeoff and cruise with boost, select Boost-Armed on the Boost control switch.  Apply full throttle and begin to takeoff normally.  The engine will stay at 2400 RPM for the takeoff run so long as WOT is maintained.  Pull the boost trigger and hold it, the engine power increases and RPM will jump to 2700 and the governor will keep it there as long as the trigger is pulled.  Adjust the nose as necessary to maintain desired climb airspeed or angle of attack, and keep the trigger pulled until you are clear of obstacles.  Once the trigger is released and the engine power is reduced it will resume and maintain 2400 RPM.  Adjust the nose as necessary to maintain desired climb airspeed.  If desired, the pilot could select Gov2-2700 and the engine speed will increase to 2700 RPM on the engine NA Lug curve.  Once at your cruise altitude, reduce the throttle to bring the engine RPM back to 2400 or 2500 RPM as desired.  Trim the nose for proper airspeed, trim throttle to remain level, trim the mixture to peak RPM, then lean gently to drop 10-15 RPM.  Double check cruise airspeed and RPM and manually adjust pitch if necessary. 

Thursday, September 16, 2010

Found out yesterday NO IVO PROP on Lycoming Engines!!  They suggested Franklin. 
Rersearched Tiny PLC and ordered a starter kit for the E10-npn yesterday. 
Called Woodward Governor & found no limiting speed governors for aircraft.  IvoProp seems best bet.
NOTE 10/30: MT propellers also has electrically operated Props and they will work on Lycoming Engines. 

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