I managed to get out to the field today and get in some flights with one of my Mig-35s with the 7x3 EMP prop and D2826/6 2200 Kv motor combo. Unfortunately my testing was cut short due to rain, so I have quite a bit more testing to do, but I did see some definite differences between the 7x3 EMP prop and the 6x4 APC prop under the same conditions as I evaluated both for speed today using the RC Speedo app on my phone.
Test conditions - light winds, no more than about 5 mph, temperature 7 C/45 F
I still launched the plane at about 50% throttle with the 7x3 EMP prop and I was surprised that I didn't really see any real increase in prop torque over the 6x4 APC prop. What I did notice is that the plane flew away and stayed stable without a whole lot of initial throttle input, I suspect because this prop is pushing a larger volume of air over the control surfaces and they have response almost right away as a result. Sometimes with the 6x4 APC setup I find that once stabilized, I need to advance the throttle to about 60% until the momentum builds up and the surfaces have full authority, then I can adjust power down again a bit from there.
Overall flight impressions up to about 75% throttle
This is my first experience flying with a 7" prop in a park jet, I found the response to the throttle inputs to be a bit "softer" than with a shorter prop. I felt like I had more control down to lower speeds, say about 40% throttle just gently cruising around due to again I'm assuming a larger volume of air going over the elevons/rudders. When I hammered the throttle, acceleration was a bit softer than a 6" prop, but as it wound out, it was perhaps a more "scale" type of acceleration if that makes sense.
What I did note is that my overall speed seemed to be higher as I was flying around and performing aerobatics in the 50-75% throttle range which is where I probably spend most of my time during my flights other than the odd bursts of full power for speed runs or vertical climb outs.
High speed runs and vertical performance
I clocked several speed runs with both the 7x3 EMP and 6x4 APC props to be able to compare top speed on the same plane in the same conditions. This is where the "rubber hits the road" quite often as numbers gathered on the bench don't always translate to what I might expect at the field.
I did about 10 full throttle runs with each prop set up and other than a mph here and there, they were essentially identical. I took the four best runs for each prop.
So perhaps there are a couple things going on here, the 7x3 EMP prop is definitely heavier at 12 gr than the 6x4 APC prop at 4 gr, so perhaps the motor can't quite get the RPM up as high swinging that much weight. Of course the difference in pitch would play a role as for each RPM the plane should move further ahead with the 6x4 prop than with the 7x3 prop, but perhaps total volume of air being moved by the 7x3 prop is still greater. What I think might be a big factor is "prop drag" because the blade is not only 1" longer, but considerably wider. I took another picture comparing the 6x4 APC prop sitting on top of the 7x3 EMP prop, also below is the comparative picture of the 6x4 APC, 7x3 EMP and 6x3 EMP. So I suspect that as that big blade gets spinning fast, the "prop drag" is definitely more noticeable as there is certainly a lot of blade in the way compared to the 6x4 APC.
Glide and approach to landing
I fly with my ESC brake "off" so that when I chop the power, the prop free spools, still putting some ariflow over the elevons and ailerons as I come into land. With this heavier, beefier prop, the glide distance is certainly longer than it is with the 6x4 APC prop. I have a pretty long runway to land on, but if you flew in a smaller area, you might need to start the approach and chop the power further back to land in relatively the same spot as you would with the 6x4 prop setup. The other alternative I suppose would be to select the ESC brake "on", but that could cause some adverse glide performance as suddenly you have this big prop slowing you down and chances of breaking the prop would be higher as it sticks down so much farther than the 6" prop.
Post flight motor/battery "health" and prop noise
I did the quick "five second finger test" with the motor after each flight to check for heat build up. Of course it was fairly cool today, probably in warmer conditions one would have to be more careful. The only time I noted the motor being warmer after a flight with the 7x3 EMP prop was after all the high speed runs when I was clocking the speeds. It was warm, but not hot so I couldn't keep my finger on it for at least 5 seconds. Again, as the weather warms up, throttle management might be of greater concern, on most of the flights, I didn't notice a significant difference in motor heat between the 7x3 EMP and 6x4 APC props.
I normally keep my flights to about 5 minutes not only to help out the long term health of my batteries, but after 5 minutes I tend to get cocky or bored, both of which lead to less than positive results :/ After all the flights today, regardless of prop, I was getting about 3.8 V per cell when I checked the battery, so pretty much the same. I'm assuming that as I said the mid range speed with the 7x3 EMP prop is a bit better that I wasn't needing as much throttle or having to adjust it as often to perform turns and basic aerobatics.
I mention prop noise as I know for some folks this can be an issue depending on where they fly. The 7x3 EMP prop was not quieter, but the frequency was lower, more of a "throaty" sound at full speed and less of a whine that the 6x4 prop produces. Sometimes it can be more about the frequency of the sound that annoys people rather than the decibel level I supect.
Overall initial impressions
So many areas of performance are quite similar between the 7x3 EMP and 6x4 APC setups from a standpoint of top speed, motor heat and battery consumption.
The overall "feel" with the 7x3 EMP prop is "softer", throttle adjustments aren't immediate, perhaps more subtle and scale from a standpoint of acceleration. Slow speed (40% throttle) to about 75% throttle is a bit better with the 7x3 EMP prop, the plane seems to fly faster and more efficiently with the bigger, lower pitch prop in that range, then at top speed everything evens out again.
So I will continue to test and compare, I think it boils down to "the right tool for the job" or what you want your plane to do. Obviously not all planes can accept a 7" prop, but I was impressed with what I see so far. Hopefully in coming tests I will be able to get some video as well :)