| Model Aircraft Kits | Plan Packs | Materials & Accessories | Contact | How to Purchase |
| EVENTS | Links | Club talks | Useful hints & tips | |
| HOME |  |
HOME | ||
How do I tell if my engine is over propped?
While for the most part, there is little need to question whether or not the engine is over propped, occasionally you may experience funny quirks that get you baffled.
You may be experiencing one or more of the following;
1) Excessive fuel use, 2) Engine varnishing (blacking) quickly, 3) Aircraft seems to speed up (leap forward) when throttling back; 4) Slow to take off and gain airspeed.
The diagram on the right shows how the airflow is when the prop is working correctly.
Under normal circumstances this cavitation is present only when the aircraft is stationary at full throttle, and it reduces as the aircraft increases speed allowing the prop to gain full purchase on the air, this in turn now allows the engine to be fully and efficiently active in towing or pushing the aircraft forward.
When an engine or electric motor is purchased, it usually has a prop recommendation with it. This is a very good guide point to go by for most applications, however, there is always the exception to the rule. If for instance, the engine/motor is to be used in a bi-plane, it is very likely that the recommended prop pitch is too steep (FOR THOSE WHO DON'T KNOW, the pitch of the prop is the second of the two numbers stamped on the prop, the first is the diameter of the circle the prop will make) eg, 10 x 6 means 10 inches diameter by a 6inch pitch (these measurements may be given in cm but the principle remains the same). The pitch of the prop is how far forward the prop and what it is pulling should move forward in one complete 360o rotation of it.
You can easily work out what airspeed can be expected at a given set of rpm for the pitch of the prop;
If we look at the 10 x 6 prop at say, 10,000 rpm:-
10,000 x 6" = 60,000" Now convert this number to feet by dividing by 12 to give 5,000 feet (per minute). Divide that by 5280 (feet in a mile) = 0.947 (miles/min). We now times that by 60 (minutes) = 56.82 MPH at 10,000 rpm. Just simple, logical, longhand arithmetic.
When the final figure from the above is worked out, it will then need about 15% taking off to account for what is known as "prop slip", this is the amount that the prop effectively uses in order to do its job, so in this case the final airspeed of the aircraft will be nearer to 48.3 MPH
The average 60" span bi-plane will be screaming through the air at that speed and the engine will be working quite hard and more than likely giving the characteristic leap forward at a lower throttle rate.
When the aircraft gives the leap forward, it is because the pull of the prop cannot match the inherent drag factor of the aircraft above a certain rpm. This means that even in flight, there is still a large amount of cavitation occurring at the prop at the higher rpm due to the drag of the aircraft at that speed being significantly greater than the pull of the prop, therefore, when the revs are reduced, the prop regains full purchase on the air and becomes fully efficient to its maximum at that point.
If we now fit a 4" pitch prop, using the same method of calculation above, we now achieve near enough 38mph at the same rpm, the engine now has an easier job, cavitation is now either reduced or fully eliminated, our aircraft flies at a more scale speed, fuel consumption is reduced, flight time increased, engine blacking reduced and we have no leap forward at lower throttle settings. Our aircraft now is much easier to take off, to fly and more importantly to land as there is far greater control at lower speeds.
IT MUST BE NOTED; that if the pitch is reduced, the diameter should be increased slightly to offer the engine enough load in order not to over-rev too easily. Some engine info does give guidance on this, but as rule of thumb, the above 10 x 6 prop would be exchanged for an 11 x 4, this means that the engine is now driving a larger circle but with much reduced parasitic drag in the form of cavitation.