CHOOSING A MOTOR FOR ALSOne of the main problems faced by the Amateur constructor engine aircraft — selection or production of the power plant the necessary power, weight and efficiency. This problem is solved normally, on existing capacities and experience of building such units.
There is no doubt that their designers can be as technically literate people, and are not sufficiently familiar with the basic provisions of the theory of internal combustion engines. In this article we will try to give an analysis of engines, presented at the last Moscow rally ultralight aircraft, and some tips on choosing the parameters of the internal combustion engine, the observance of which will reduce the relatively expensive and a long way to search, will help significantly reduce the likelihood of technical risk.
All internal combustion engine aircraft, presented at the meeting, can be divided into three categories:
1. Serial (boat, motor, internal combustion engine from the snowmobile equipment, automotive), adapted without major alterations.
2. Own design, with extensive use of series parts, motors.
3. Original design made from the paper.
These motors, including bankruptcy, are summarized in table 1. In column 1 vertically, provided their effective maximum capacity of Ne maxis spent on the rotation of the propeller, whereby torque on the shaft Mcu is converted to axial thrust. For judgments about the capacity of the power unit, building characteristics-rotor, screw selection and linking it with the engine you need to have the external characteristic curve of maximum capacities, which can develop the engine at different RPMs at full throttle. Accurate data can be obtained when the brake test stands that not every fan is available. There is an approximate way of constructing external characteristics on the basis of theoretical calculations, if there is at least one point of power and revolutions of the crankshaft (they are usually listed in the factory data).
Table 1Table 1
Table 1.
This method consists in the fact that at constant composition of the fuel mixture, the power required to overcome internal losses, changes approximately proportionally to the square of the speed.
N1 — display-power, HP;
NTr — power spent on overcoming friction forces of the pistons, pumping losses during blowdown, rotating assemblies, ignition distribution etc.;
Ne is the effective power;
N1‘, NTr‘, n’ rpm — current capacity values and speed.
N1‘=N1*(n’/n) (1)
NTr‘=NTr*(n’/n)2. (2)
Power NTr estimated mechanical efficiency (ηm), which is in the range of 0.8—0.9 for motors with the rotational speed of the crankshaft between 4,000 and 6,000 Rev/min and 0.6—0.8 for more high-speed.
For example, we have a building in this way the external characteristics of the engine RMZ-640.
Declared plant maximum effective power:
Ne max=27 HP at 5250 rpm.
Take mechanical efficiency ηm=0,87, then the indicated power of N1=Ne maxm=27/0,87=31 HP
The power of friction: NTr=N1-Ne max=31-27=4 HP
We define by the formulas (1, 2) N1‘, NTr‘, Ne‘, pre-specifying a number of values of the speed n rpm and bring the results in table. 2. These data build external characteristic Ne=f(n) (Fig. 1).
Table 2
Table 2.
Fig. 1. The external characteristics of the engine RMZ-640.
Fig. 1. The external characteristics of the engine RMZ-640.
There are maximum (or takeoff), nominal and maximum operating power. The maximum power of Ne max is getting when the engine is at full throttle on the ground. This mode for the engine is tight and limited to 3-10 minutes, the Power is less than maximum 10-15% is called the nominal (Ne nom). You can use it for an extended but limited time, no more than 1-1. 5 hours. Operating power (Ne ex) is less than the maximum by 25-30%, engine running, this power is not limited.
Momentum corresponding to the kinds of capacities, called the maximum, nominal and operational. By itself, the motor power still does not show on its merits, as it needs to correlate with its mass (see box 2).
Weight greatly affects the design of aircraft engine, determining the degree of tension of all its parts. Distinguish between dry weight and flight. In dry weight of engine in aircraft, it is customary to include a lot of components such as the carburetor, suction pipes, magneto, spark plugs and wires to them, the details of the launch system, the flanges of the exhaust nozzles (but not the pipes), deflektory, petrol and oil pumps. When calculating the dry mass not accounted for the air screw and its sleeve, hood, exhaust pipes, radiator, power generator, control and measuring devices and the wiring to them.
In flight the mass of the propeller installation is the mass of all aggregates necessary for flight, filled with oil and fuel tanks.
Flight weight as an objective criterion for weighing the quality of the motor is inconvenient that it takes into account consumable goods (fuel, oil), depending on the purpose and type aircraft. The total mass of these components is not easy to determine, so the mass of motor is characterized by a less complete but more accurately delineated by the concept of dry weight.
Column 3 shows the comparative evaluation of motors of various capacities on mass.
g=gDV/Ne max,
where GDV — engine dry weight, kg; Ne max — maximum power, HP
When calculating the specific gravity, as a rule, the dry mass of the motor refers to the maximum power. Specific gravity is one of the most important indicators of quality aircraft engine.
The specific gravity of a modern Western engine for ALS is 0.5—0.6 kg/HP, the best representatives of 0.25—0.4 kg/HP for Example, the specific gravity of the two-stroke engine for ULTRALIGHTS American company “Kolbe Corp”:
g kg/HP Ne max HP
0,32 6
0,25 18
25 0,23
Statistics on engines, presented at meeting, gives the following figures: 34% of Park the engine is from 0.61 to 0.91 kg/HP, the rest 66% — from 1 to 2 kg/HP, which is 4-5 times more than the special motors for ultralight aircraft.
The best indicator of the competitive engine M-18: g=0,34 kg/HP, the worst of 2.04 kg/HP of the engine “Dnepr” MT-10.
From the theory of similarity it is known that for geometrically similar engines, the mass is proportional to the cube of the diameter of the cylinder, and power is proportional to the square of the diameter, that is,
g=gDV/Ne max=A*(D3/D2)=AD.
In practice, this ratio is not respected, because of the strict geometric similarity between the same parts of different sizes it is impossible because sections of many items set the conditions of production; casting thickness, stiffness, mounting conditions, etc., so these section sizes can be considered constant. Then: GDV=AD2. Statistics show that the engines of medium and large size well follow this relationship, thus:
g=gDV/Ne max=A*(D2/D2)=A=const.
This dependence is violated in the region of small D in the direction of increasing mass, and is due not only to the above listed technological reasons, but also because the mass of service units — magneto, spark plug, carburetors, etc. — a little depends on the size of the motor. The relative weight of these parts is negligible at larger sizes the engine increases with decreasing engine size (see Fig. 2).
Fig. 2. The dependence of the mass of the engine from the working volume.
Fig. 2. The dependence of the mass of the engine from the working volume.
In the graph 4 shows the values litre capacity, this value is an important parameter of the perfection of the motor.
As is known, motor power:
Ne max=(Pe*Vs*nmax)/(225*i), where
Pe is the average effective pressure, kg /cm2,
Vs engine capacity, cm3,
n — rotational speed, rpm
I — numbers.
Here liter capacity will be expressed:
Nl=Ne max/Vl, HP/l.
With the increasing power density of the reduced size of the engine and its weight. At the power density of the highest performance two-stroke engine of IZH-“Sports”, N, l=91,5 HP/l, the lowest among the two-stroke engine “Skoda” — 39 HP/L. Approximately 80% of the motors are Nl from 46 to 63 HP/L.
At widespread in the West, two-stroke engines for ULTRALIGHTS “Recaps”, “Hirt”, “Qun”, “Kawasaki”, — Nл=80…105 HP/L. Thus, the engines presented at the gathering, there are reserves to boost.
From the theory of similarity it is known that the liter capacity is inversely proportional to the diameter of the cylinder, that is:
Nl=A/D, while
where fcooling — the ratio of cooling surface to volume of the cylinder,
Fcooling — cooling surface,
Us — volume of a cylinder
that is, with decreasing diameter of the cylinder increases the area of cooling surface per unit volume, which improves the cooling of the cylinder of small diameter, increases heat loss and reduces the thermal efficiency ηt, but at the same time it allows to increase the compression to compensate for the drop in ηt, i.e., growth of thermal efficiency to be expected.
In the graph 5 shows the numbers of engines.
Try to decide which engine is more suitable for ALS — a four-stroke or two-stroke. Let’s start with the level of fuel consumption. From two-stroke engine 400-450 g/HP-HR, four-stroke internal combustion engine of 200-250 g/HP-HR, that is, the specific consumption from the two-stroke engine average 2 times higher than the four-stroke. But the latter may be less beneficial for SLA because of the greater mass and greater air resistance, as part of the effective power will be spent on moving the heavier engine in the air and to overcome the harmful resistance. Therefore, the flight efficiency is most fully characterized by the fuel consumption per tonne-km.
This figure, in addition to efficiency, also takes into account the magnitude of the air resistance of the propeller installation, the efficiency of the propeller and a number of other indicators, in a word, the totality of the factors determining the degree of perfection of the aircraft.
Calculate the total mass of the engine and watch fuel for four – and two-stroke engines. Take used on ALS close in power and volume motors “Dnepr” MT-10 “Vortex”. Fuel for 1 hour to MT-10, with gc=200 g/HP / h is 7.2 kg, and for the “Vortex” with gc=400 g/HP-h — 12 kg. total weight of engine and fuel 67,2 kg — for engine “Dnepr” MT-10 and 36 kg for the engine “Whirlwind”. Thus, the rotor installation on the basis of a four-stroke engine is much heavier than on the basis of the two-stroke. The weight IUD for ALS is important, as is 25-35% of the empty weight of an ULTRALIGHT.
Used for production of ALS of new materials, technologies, profile will cause the appearance design with the low weight of the airframe. In this case, the relative weight of VMG will grow even more. Four-stroke engines will have a distinct advantage over two-stroke during long flights, when it’s specific fuel consumption.
We have already talked about the impact of the volume of the cylinder (see table 1) on the specific weight and litre capacity. Now consider the effect of the size of the cylinder on the indicated efficiency. Recall that the indicated efficiency ηI is the ratio of thermal energy converted to work, all summed up in the engine.
Since the volume varies as the cube of diameter D3, and the surface is the square of the diameter of the cylinder D2, then the heat loss in engines of similar structures is inversely proportional to their size. It follows that, ceteris paribus indicated efficiency increases with the increase of the diameter of the cylinder (at the same speed of the piston).
Thus, the thermal efficiency of internal combustion engines of small size is relatively low and specific fuel consumption they will be higher.
Table 1 shows the dimensions of the cylinder, the piston and its relative course S/D. These parameters are closely linked, so we will consider them together.
Almost all the engines referred to have the relative move is less than one, and short-stroke engines have several advantages over clinohedrite: here and the possibility of placement of channels of large cross section, which increases the filling of the cylinder; and a decrease in the average speed of the piston, thereby increasing mechanical efficiency. Finally, short-stroke engine more compact dlinnonogih.
The next factor is the speed of the piston
VSR=(S*n)/30, where
S —piston stroke, m; n — frequency of crankshaft rotation, Rev/min. The average speed of the piston engines shown in the table, from 8.4 m/s to 17 m/s. This indicator seriously affects the dynamic load of the parts of the engine, filling the cylinder and the amount of energy expended on the friction of the pistons and bearings. Mean piston speed special motors for ALS 12-15 m/s.
The frequency of rotation of the crankshaft (see table 1) under consideration of power plants — from 4500 rpm to 8000 rpm. it is Known that the power of the engine depends on its specific speed. However, accompanied by a sharp force (proportional to the square of the speed) is increased, the inertia forces of the rotating and translational moving mass of the engine parts and, as a consequence, the increase in friction loss, which requires increased mechanical strength of the parts of the engine and the change of the bearing working conditions. On the other hand, the increase in speed is limited by the cooling of the cylinder head, piston, spark plug, since an increase in rpm increases the heat removal from the cylinder. In addition, the speed of rotation is limited to average speed of the piston, which increases pressure loss in the purge increase dramatically (in proportion to the square of the speed of the piston), which reduces filling and reduces engine power. However, the speed increase up to a certain limit improves ηI.
Table 1 shows also the mean effective pressure and compression ratio. From the formula of power shows that there are two main directions of increase power is to increase the speed and pressure Pe. The effect of speed on power we’ve seen so far. See how you can improve Re.
This is easily achieved by increasing the E — compression ratio (for two-stroke engines applies effective compression).
EEF=(VEF+VKS)/VKS, where
EEF is the effective volume described by the piston from the top edge of the exhaust window before TDC, VKS — combustion chamber volume (see table. 3).
Table 3
Table 3.
Graph of the effect of compression ratio (solid line) and boost (dashed lines), pressure at end of combustion. Pz and specific fuel consumption Ce (%).
Graph of the effect of compression ratio (solid line) and boost (dashed lines), pressure at end of combustion. Pz and the specific fuel consumption Ce (%).
This method is good because it is simple and, except to increase power, reduces fuel consumption. However, it has disadvantages.
The increase in E is accompanied by increasing the temperature and pressure at the end of the compression stroke, causing a sharp rise in combustion pressure Pe, and consequently is a need for more durable parts, tightens the requirements for fuel and oil. However, the effect of increasing energy from the increasing Re is the physical border more than 15-20%, so the power does not increase. When compression ratios 10-12 power increase is already negligible. Up to what limit it is possible to increase the compression ratio from the point of view of the practical benefits? The rise of Pz and ηt can be traced with an increase from 4 to 8. Omitting the design side, here is the result.
The compression ratio E, equal to 4, 5, 6, 7, 8, correspond to the combustion pressure Pz 25.3 kg/cm2, 34 kg/cm2, 44,0 kg/cm2, 54,2 kg/cm2 and 65.5 kg/cm2. This shows that increasing from 7 to 8 we will win in the efficiency ηt is only 4.6%, whereas the combustion pressure is increased from 54,2 to 65.5 kg/cm , i.e. 20%. Therefore, in practice, we need to compromise between optimal compression ratio and ηt (see chart).
For practical use we can recommend the most favorable value of the degree of compression when working on fuel, not detonating at all times.
Another method of increasing Re is to increase the pressure of the mixture at the inlet.
From two-stroke engines increase Re is achieved by using the resonant pipe on the suction and exhaust (effect of Cadence, which he discovered in 1903 and first implemented on the engine of the company “YuMO” in 1923, when it was received a capacity increase of 60%). Tuned exhaust system, for example, increases the power up to 30-40% without much increasing the mass of the motor, besides improving its efficiency.
The increase in Re have four-stroke engines is associated with considerably greater difficulties. Even a simple change of valve timing will put the designer in front of a serious technological and design challenges of manufacturing camshaft, bore the seats and install new valves, etc.
Our statistics gives the following Re: for four-stroke internal combustion engine from 9.5 to 10 kg/cm2, two-stroke have from 3.6 to 6.6 kg/cm2, 40% two-stroke engines Re ranges from 5.1 to 6.5 kg/cm2, which is a good indicator. However, the engine RMZ-640 (one of the most common at the rally) Re is only 3.6 kg/cm2, indicating reserves of increase of its power. Bringing Re to 5 kg/cm2, that is, to the average values for the two-stroke engine, we increase Ne max 30-35%, getting 38-40 HP
The author of the work has been done to improve this engine. Alteration was the manufacture of four additional vent channels with phases 2-3° less than the main Windows in the piston and the increase of EEF. This improvement allowed to withdraw 84 kg of thrust on the screw Ø = 1,08 m with a step of H=0.5 m, vs 70 kg to alteration.
In table 1 it is possible to trace the importance of the reduction on the screw. It is known that the efficiency of the propeller depends on the magnitude of dynamic steps:
λ=V/nc*D, where
V — airspeed, m/s; nc is the number of propeller revolutions per second; D is the screw diameter, m.
The efficiency of the propeller has a maximum at the value λ=1-1,5; with a larger and a smaller value of λ the efficiency of the propeller decreases. This shows that the flight speed and the number of revolutions of the screw must be in a certain ratio.
In modern high speed engines the efficiency of the propeller decreases dramatically to 0.3 to 0.5, by reducing the dynamic step, especially when installing the motor on low-speed aircraft. Therefore, it appears advantageous to bring the screw from the crankshaft and through a reduction gear.
Almost half of the engines on SLA is the reduction of the screw from 0.38 to 0.7, which leads to higher static thrust at 80-100%.
Thus, application of the reduction gearing for high speed motors installed on low-speed SLA, is highly desirable.
Table 1 shows the effect of the D screw on the static thrust.
The thrust of the screw R=L a*R*nc2*D4where a is the coefficient of thrust; p — mass density of air; nc is the number of revolutions of the screw; D is the screw diameter, m.
It is seen that the gain in thrust by increasing the propeller diameter is obtained significant. For example, increased D by 5% and increases thrust by 21% and 10% gives a rise of 46%.
Will briefly describe possible ways of constructive solutions of DVS for ALS. There are two ways. The first is the development of new engines using the latest advanced technologies, with optimization of workflow parameters; the second is to develop them on the basis of already existing and tested by long practice, by necessary modifications.
The first way will give the best results, but require large material costs, research and theoretical work. And the timing of the creation of such internal combustion engines will be high, since the technical production of aircraft piston engines largely lost with the transition to the gas turbine.
The second path is associated with less technical risk and could be implemented in significantly less time. The starting point for creating engines can serve as produced by our industry and widely used by lovers of “Vortex”, RMZ-640, “Neptune”, “Hello”. These machines are compact, have a small “head” that is dynamically balanced, has an even torque and low rotation speed of the crankshaft.
Referring to the particular construction of the engines, it can be noted that the main number of the internal combustion engine of a meeting (78%) had a rotational speed of the crankshaft 5000-6500 rpm, which can be considered optimal. Applying the reduction to the screw of 0.4—0.6, it is possible to obtain a compact gear reducer (V-belt or a simple gear). With increasing rapidity grows reduction on the screw that will require transition at multiple pulleys due to the reduction of the angle of coverage of a driving pulley for V-belt transmission that will draw in an increase of the length and diameter of the screw shaft of the console (and as a consequence — the weight of the installation) or will necessitate the transition to the planetary gear (engine V. Frolov, n=8000 rpm). The specific gravity of a properly designed and fabricated gear for internal combustion engines of small volumes is 0.14—0.15 kg/HP, and at high engine speeds, it can “eat” the entire gain on mass.
The author seems to be another solution of the two-stroke engine for ULTRALIGHTS. Remembering that the weight of the engine is inversely proportional to the diameter of the cylinder, you can increase the volume of motor up to 1.5—2.0 l, limiting the rotational speed of the crankshaft in the range of 2400-2600 rpm. Moderate average piston speed (7-8 m/s) have a beneficial effect on mechanical efficiency. This engine is easier to organize the dynamics, and this will lead to an increase of the filling ratio of the cylinder. The system of direct fuel injection low pressure will put this engine in a row with a four-stroke machines in specific fuel consumption. Application devilsbane cylinders and nikosilver coating or ceramics will further reduce the proportion. This engine can be lighter than a speed of the internal combustion engine of the same capacity with gear.
In conclusion, we note another problem posed to designers of future SLA meetings associated with the silencing of the exhaust noise. 87% of the fleet of engines of the rally operated without mufflers. Sound pressure exhaust two-stroke engine without a muffler at a distance of 2 m from the edge of the exhaust window reaches 130 to 140 dB, which corresponds to the pain threshold of feeling. To be under the influence of this power is very tiring and harmful. For two-stroke engine tuned muffler even desirable, as it improves the economy and power.
Based on the review it is possible to formulate a General approach to the creation of DVS for ALS:
— small overall dimensions,
— low specific weight g≤0.5 kg/HP,
— dynamic balance,
— good throttle response (1-2 sec),
— high efficiency, not more than 200 g. l/h
— high reliability and durability (1000-1500 h),
— easy mounting and Dismounting,
— easy maintenance
— low noise level (not exceeding 100 d),
— low unit cost in mass production.

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