What is a 9x7 prop anyways? A 9x7 prop is a 9" diameter prop with a 7 pitch. The pitch number is the number of inches the prop would move forward with one revolution through a solid medium.
There are a few exceptions but nearly everyone agrees that the following are great prop options for a Beaver with the stock brushed motor: 1047- 2 blade, 9070- 3 blade or a 1080- 2 blade. I Personally am now flying with a brushless inrunner and have had great success with 8060 (8x6) 2 and 3 bladed props from Master Airscrew.
A couple of important points:
-When mounting your prop make sure the raised lettering on the prop indicating the size and pitch (i.e. GWS1080) is facing forward when you bolt it on. This is true of both GWS, APC and most other brands of props as well.
-If possible, it's a good idea to use a second nut as a jamb nut to keep your prop from coming loose.
-Be careful when tightening your prop as it is possible to actually pull the shaft too tight causing it to bind.
-GWS props tend to break easily if they come into contact with anything especially in the cold
GWS offers a wide range of props for a multitude of applications and although they will break easily with a prop strike they are very inexpensive to replace. APC and other manufacturers make more durable electric props at a higher price point however the do not generally come with the molded in hex to keep the prop from spinning on the shaft so they must be tight but not overly tight as noted above. APC props also require the use of a snap in APC bushing to bush them down to the appropriate size prop shaft diameter. An easy replacement for the APC prop adapter is to use a short piece of fuel tubing on the prop shaft with a nut and washer on both ends. Tighten the two nuts together and the fuel tubing expands to fill the hole in the center of the prop. Top off your prop with an appropriate GWS spinner and you are ready to fly
If you are using a 2-blade prop it is possible to use a prop saver which basically allows you to mount your prop to the shaft with a rubber band. The idea is to allow the prop to move sideways during a nose in landing thus saving your prop.
Rules of ye olde thumbe:
- Larger diameter = greater potential thrust and more efficiency, results in a higher current draw for the same gear ratio (shorter flight times), lower RPMs if a higher gear ratio is used keep the current the same.
- Slower turning props mean lower top end speed for a particular aircraft
- Higher pitch = higher current draw assuming the same diameter, less static thrust (poorer low speed performance, prop is stalled or bogged down at slow speeds), more "dynamic" thrust (better top end performance, i.e. higher top speed if the plane doesn't have too much drag) A high pitch prop on a slow flying plane is a poor combination, the prop never really gets to strut it's stuff.
- Lower pitch = lower current draw assuming same diameter (therefore longer flight times), better low end performance (better for 3D work or slow flyers), limits the top end speed of the aircraft.
In the end; a higher current draw results in a shorter flight time, a lower current draw results in a longer flight time.