The APC 7x3 is quite a heavy load with a lot of blade area (I used to run this prop on a Cox Conquest .15 in F1C)-the Master airscrew 7x3 is a nice prop-lighter, and with much narrower blades. Try this on the TT07 and you'll find it much happier. Two other props to try are the Master 6.5x3.5 and the very thin bladed 6.5x3. Remember that revs don't fly aeroplanes-THRUST does-and thrust is determined from a fairly complex dynamic interaction between diameter, pitch, blade area and rpm. A particular prop will suit your plane better than others-but only test flying will give you the answer as to which-you might find the aforementioned 6x3 or 6x4 prop works better than the larger sizes I mention-alternatively you might find the opposite. Somewhere in that 6-7" diameter and 3-4" pitch range will be a prop that suits the engine and flies the model well-but you might need half a dozen different ones before you find the best-and even in the 6x3 and 6x4 sizes you have a choice of at least 3 or 4 manufacturers-in the US-all with different blade shape, thickness, weight, airfoil and pitch distribution. Unfortunately, manufacturers have never come up with a common standard for prop pitch-some use 'average pitch, some use max pitch, some use pitch at 70% blade radius-this is why props vary so much even in the same nominal size-and why it is impossible to be specific in recommending a particular size and brand of prop as 'best'