Choosing the right engine for your model might seem straightforward, but is not always as simple as peaking the cheapest offer on the web.
Jet models are meant to be flown and usually feature a fairly high wing loading ratio. The powerplant choice will significantly affect this ratio and might render a plane tricky to fly if not done properly. Consideration like price is also very important and must be integrated into the equation.
The usual workflow I use for this choice is the following:
1. Check the weight of the aircraft
2. Check the wing loading vs intended type of flying
3. Define a class of thrust from these considerations taking into account kit manufacturer recommendations
4. Pick up the highest thrust-to-weight-to-price ratio in this class
5. Consider the following complementary factors in decreasing priority:
· Does this engine fit in the Airframe?
· What is the build quality/ reliability of the engine?
· Is the idle thrust low enough for my plane?
· What about warranty conditions?
· What is the cost of the bearing change and at what interval?
· What fuel/oil combination does it burn?
Also bear in mind that oversizing the engine for the airframe will cost you a lot more than picking up the right size. You see increased airframe an landing wear due to faster touchdown speeds and high impact energy as well as higher airframe stress in flight.
I have included a table listing the power to weight to price ratio for some engines available on the market. This is using web published manufacturer data, including price offered in the USA.
|Designation||Thrust N||Weight gr||Price USD||Thrust/price/weight ratio|
|K-60G (-20 )||58||830||1800||3.88|
|Jetcat P-90Rxi (-80)||105||1504||2455||2.84|
|Jetcat P-100RX (-20)||100||1060||2595||3.64|
|Jetcat P-140RX (-20)||142||1570||3295||2.74|
|Jetcat P-180RX (-20)||175||1575||3995||2.78|
|Jetcat P-300RX (-20)||300||2610||5800||1.98|
|Merlin M100XBL (-20)||100||955||2495||4.20|
|Merlin VT80 (-80)||80||860||2000||4.65|
|Merlin M140XBL (-20)||140||1285||2995||3.64|
|JC Bee II||60||954||2595||2.42|
|JC Rabbit 100||100||1120||2795||3.19|
Below is the overall ranking for all the engines listed.
|Ranking by Thrust/price/weight|
Ranking for 20 to 60 class
|JC Bee II|
Ranking for 80 to 100 class
|JC Rabbit 100|
Ranking for 120 to 160 class
Ranking for 170 to 180 class
|Ranking 170-180 class|
Ranking for 200 to 300 class
|Ranking 200-300 class|
Using the procedure described above and the tables will result in a wise and appropriate choice.