Sport Pilot Talk

Jfly67 wrote:Appreciate all the responses and it’s not about going 115kts to 120kts it’s more like 100Kts to 120KTs and better climb rates as most our missions require 9500 ‘.

I’ve never been very good at math. I’m trying to figure the hp increase necessary to get from 100kts to 120kts.

I’m taking the percentage increase, 20% and representing it as 1.2. I then square that and get 1.44, indicating it would take a 44% increase in power to go 20% faster.

Am I doing that right?

FastEddieB wrote:

Jfly67 wrote:Appreciate all the responses and it’s not about going 115kts to 120kts it’s more like 100Kts to 120KTs and better climb rates as most our missions require 9500 ‘.

I’ve never been very good at math. I’m trying to figure the hp increase necessary to get from 100kts to 120kts.

I’m taking the percentage increase, 20% and representing it as 1.2. I then square that and get 1.44, indicating it would take a 44% increase in power to go 20% faster.

Am I doing that right?

If you’re not I’m sure Tim will let you know.

FastEddieB wrote: Am I doing that right?

I can't say without digging into some of my engineering books, Eddie. I know that parasitic drag goes up with the square of velocity, but I'm not sure if there's also a square relationship between power and speed.

drseti wrote:

FastEddieB wrote: Am I doing that right?

I can't say without digging into some of my engineering books, Eddie. I know that parasitic drag goes up with the square of velocity, but I'm not sure if there's also a square relationship between power and speed.

I do recall there’s not a direct relationship between the drag increasing as the square of the increase in velocity, and the power required to overcome that drag. Clearly the thrust has to exactly balance the increasing drag, so must follow the square increase. Not sure about the power.

Time to dig out Aerodynamics For Naval Aviators*.

In any case, my point was mainly that the hp increase to increase speed by 20% is a lot more than 20%.


*I have a hard copy, but a pdf version is readily available for free online.

All else being equal, (which never happens) I believe power required goes up as the cube of speed.

However, more power lets you fly higher... thinner air = less drag for a given speed = less power for a given TAS... sort of... the Rotax 912 seems to run a but rich at altitude, despite it's auto mixture control, so some of the advantage in flying high may be lost.

Pete

FastEddieB wrote:
Time to dig out Aerodynamics For Naval Aviators

My aerodynamics book is from the 1960s, by Roger Bate (Air Force Academy professor). I wonder how that differs from what they teach to naval aviators?

There used to be an old saying that to double your speed you need 4 times the power. For a Cub if you increase the power from 65 HP to 180 HP, and increase just shy of 300%, you will get around a 50% increase in speed.

3Dreaming wrote:There used to be an old saying that to double your speed you need 4 times the power.

That certainly agrees with Eddie's estimate. And probably works up to the point of diminishing returns. Where exactly you hit the wall is uncertain. But at some point, increasing drag will clearly prevent any further speed increase, regardless of power.

proemer wrote:All else being equal, (which never happens) I believe power required goes up as the cube of speed.

That sounds familiar.

If correct, for a 20% increase in speed, you need about 73% more power (1.2 cubed = 1.728)

I suppose the difference is inefficiencies in converting power to thrust.

Some relevant stuff below...





I don’t see any specific reference to power required increasing as the cube of the increase in velocity, but perhaps it’s hidden in the equations.

I found this link: https://www.aviationschoolsonline.com/f ... rticle.php

Which shows a "power curve" probably similar to a typical LSA, as attached here It's been a long time since I was studying this sort of thing, so I could be all wrong. Ironically I find my self flying at 50-55 % power more often than not... I'm usually sight seeing and it's quieter at lower power!

Pete

Will Edge Performance warranty your engine mods? Rotax engines are durable and if maintained they will serve you well but start messing around trying to get more horsepower, 100hp to 160hp, there will be trade offs, usually not in your favor. Reliability and durability will probably be the first to go and at the wrong time this could be bad. The extra force on the rods and crankshaft was not designed for 60% increase. TP modded his 912ULS to get 20 hp more and broke a rod in flight. Modified engines always need (or not really but we want to) "tuning" resulting in a cascading effect of errors snowballing into a major problem. Why, we are operating them outside of their design limitations. More problems are caused by tuning instead of getting back to factory specifications and not understanding to the full extent of what they are changing.

I have an SLSA and it will stay that way as long as I am trying to help our flight school attract and train new sport pilots. Dealing with the manufacture can be difficult and a blessing. I had to wait until late 2018 for the service bulletin for the ADS-B modification. If I had to wait on a A&P to do the install well I would still be waiting, they are swamped. My previous E-LSA was fun and I made a lot of changes to improve the avionics and reliability of the 912 ULS including carb heat, soft start, carb overhauls, 2000 hour TBO, so the freedom is an advantage and people are not afraid of experimental aircraft and with the new rules we hope to see something that inspires more confidence for kit builders products.

Good Luck,
Randall Bearden
C162 SLSA N3037T
Sport Pilot
Light Sport Repairman Mechanic

Yep, I am sure there is added risk when dealing with EP mods but it should be noted that one of things they install is their “performance camshaft“ and also new cylinders etc ... in other words, this is not just a performance boost - everything else be damned - it is a package but then again, they are not Rotax and don’t have necessary resources to test everything and they may very well be another weak spot that will reveal itself even if their new and improved camshaft and other parts work as designed ...