How can I calculate the speed gain from using multiple STC modifications together?

Looking at a parts manufacturer website, they list various STC'd modifications for Mooneys with estimated (cruise?) speed gains for each.

However, I've heard that those gains are not additive. For instance, if you applied three mods with 5mph gain each, the total gain would be less than 15mph.

Is there a way to calculate what the gain should actually be if you applied several at once, assuming the gains listed are accurate when applied in isolation?

Most appear to be aerodynamic improvements (i.e. reducing drag) but a few are engine improvements (i.e. increasing thrust). I'd prefer an answer that addresses both sides in an integrated way, if possible.

• Can you explain further what you mean by speed gain? Is this cruise speed? Or Vmo? Hopefully not takeoff and landing speeds...
– JZYL
Jul 28 '19 at 15:58
• It seems pretty obvious that by reducing drag and increasing thrust that you would improve cruise speed. Jul 28 '19 at 17:48
• @Jimmy Not specified, but I assume either cruise or maximum. I'd think the relationship would be the same. Jul 28 '19 at 18:38
• @michaelhall Increasing thrust can also increase takeoff speeds, especially if it's a multi engine airplane.
– JZYL
Jul 28 '19 at 18:48
• Can you list the original source where you found this? Knowing what the mods are can help. Since cruise drag and speed are square related, they should not be additive.
– JZYL
Jul 28 '19 at 18:53

Not practically speaking no. With sufficient engineering data you could perform calculations to come up with a model for what the additive effects would be of the various modifications.

The problem with the aerodynamics to horsepower relationship is that almost all of the basic calculations have exponents. So a mod that gives you 5 knots over a given 'stock' airspeed will not give you close to the same number at a different higher base airspeed. So with three 5kt estimated improvements you end up with maybe 5 knots for first mod, 3kt for 2nd, and perhaps 1tk for the 3rd. Add to that the complexity of really subtle interactions in aerodynamics could mean that one mod may make the other 2 yield no differences in cruise.

Take for example a TBM850 which cruises 300mph on maybe 800hp vs a Cessna 210 at under 200 mph on 300hp vs a piper cub going 100mph on 75hp.

Not apples to apples, but to give a sense of the scale these effects have.

I'm just making this up as I go along, so your mileage may vary. Also I'm only dealing with the drag-reducing mods. It seems this method ought to give a conservative estimate of the combined speed gains.

For each mod, seperately, calculate a "drag coefficient reduction factor" equal to stock airspeed squared divided by promised airspeed squared. The number will be less than one.

When you' ve done this for all the mods you want to do, multiply all these values together. Then invert that number, take the square root, and multiply by the stock airspeed to get your expected airspeed.

You may be able to treat power-increasing mods the same way, just treating them as an apparent drag reduction.

• Shouldn't that division be inverted? If the factor is always less than one, the result will always be slower than stock. Jul 28 '19 at 20:25
• Yes, thanks, fixed. Jul 28 '19 at 20:42