Engine choices

[ALMARTON]

Dear folks, this forum is of most outstanding source of information for everything about RVs. Flying , building, so on...

I am faced with a difficult choice, I plan to operate a RV7A on a very high altitude field (4500ft having common days of @6500ft ) and with a grass runway 1650ft long (510m) .

I am buying an assembled RV7A that until now has by contract a Lycoming YIO-360-M1B 180hp c/s prop Hartzell C2YR-1BFP/F7497? 72

But I can still change that. My main goal is:

A) to maintain SAME fuel consumption because in Brazil some regions the facilities that carry fuel are very distante from one another

B) I don't want to increase much weight to the plane nose, so I have to consider that on planing a new engine

C) I want the most horse power with the lowest consumption and weight possible.

That will made my take off in such a high field more safe on a hot day. I did some experience with 180hp from a friend and the landings are dificult in such high place, but the take-off are leaving to little margin for safety. Taking about 75% of the field on our trials.

So I was considering to put instead the Lycoming either:

- Superior 360 with high compression pistons and cold air induction sump (185hp)
or
-AeroSport Power IO-375 (high compression with 205 HP)

I HEARD that due to balanced crankshafts (with less friction) and other improvements both this engines yields more power and less fuel consumption than the stock IO360 Lycoming, I will like to read your opinion in comments does figures are real?

I will have more power with less fuel consumption and thus be able to maintain my range and take off run more safe ?

Many thanks in advance for all the opinions that are based on your Experience in flying one of those.

[Kevin Horton]

Engine power is important, but don't forget that light weight is even more important. If you increase the engine power by 10%, the take-off distance should decrease by 10%. But if you decrease the weight by 10%, the take-off distance should decrease by 20%.

Once you have your new aircraft, study every possible way to safely reduce its weight. Also consider a diet for the pilot, unless he is already a super lightweight guy.

Fuel consumption: fuel injection is a big advantage if you cruise at low power, as it makes it practical to run lean of peak EGT. If you are prepared to fly 20 kt slower than maximum cruise speed, the distance traveled per litre of fuel will increase very significantly.

[BobTurner]

Originally Posted by Kevin Horton

Engine power is important, but don't forget that light weight is even more important. If you increase the engine power by 10%, the take-off distance should decrease by 10%. But if you decrease the weight by 10%, the take-off distance should decrease by 20%.
y.

I'd be interested in seeing a derivation (or reference) for those numbers.

[Kevin Horton]

It's classical physics:

V^2 = 2 a S, or

S = V^2/(2 a)

S = the distance required
a = the average acceleration
V = the ground speed at lift off

I.e. The distance required to accelerate to a given speed is proportional to the square of the speed divided by the average acceleration.

Acceleration, a = F / m

a = acceleration
F = force (i.e. the thrust)
m = mass

i.e., the acceleration is proportional to the thrust divided by the mass (I'll use the term weight from here on instead of the more technically correct mass). The thrust comes from the propeller, and the thrust should be roughly proportional to the horsepower, if we keep the propeller unchanged. Thus if we keep the weight the same, the acceleration is proportional to the power.

Combining the two above equations, we get:

S = m V^2 / (2 F)

If we keep the weight the same, the take-off speed should be the same, so V^2 does not change. Looking at last equation, we see that the distance S is inversely proportional to the thrust (or power). So, a 10% increase in power should yield approximately a 10% decrease in take-off distance, assuming everything else is the same.

==============
Now let's look at the effect of weight.

It is reasonable to assume that the angle of attack at lift-off will be approximately the same for different weights, thus the lift coefficient should be approximately the same.

The classical equation for lift is:

Lift = 1/2 rho V^2 A Cl

rho is the air density
A is the wing area
Cl is the lift coefficient.

At lift-off, the lift produced equals the weight. Thus V^2 at lift-off is proportional to the weight.

From above we saw that:

S = m V^2 / (2 F)

If we decrease the weight 10%, the mass is 10% lower and V^2 is 10% lower. The thrust, F, is unchanged. There are two terms that reduce by 10%, so S will reduce by approximately 20%.

[BobTurner]

Originally Posted by Kevin Horton

and the thrust should be roughly proportional to the horsepower, if we keep the propeller unchanged. Thus if we keep the weight the same, the acceleration is proportional to the power
.

All of your equations depend on the Force (thrust) being constant. Unfortunately, the above quote is a very poor approximation. A better (although still not great for cruise props) approximation is to assume that the prop efficiency is constant, in which case F=P/v. You can then solve Newton's law to get that the takeoff distance S is proportional to mass, velocity cubed, and inverely proportional to power.

So you got the right functional dependencies for the weight and power, but for the wrong reasons!

[Kevin Horton]

Originally Posted by BobTurner

All of your equations depend on the Force (thrust) being constant. Unfortunately, the above quote is a very poor approximation. A better (although still not great for cruise props) approximation is to assume that the prop efficiency is constant, in which case F=P/v. You can then solve Newton's law to get that the takeoff distance S is proportional to mass, velocity cubed, and inverely proportional to power.

So you got the right functional dependencies for the weight and power, but for the wrong reasons!

Prop efficiency is not even constant - it'll be fairly low at the start of the take-off run, and increase as the air speed increases (the prop efficiency data I have from Hartzell and MT show this clearly). A proper assessment of take-off distance vs weight and power would likely require a numerical solution, using data from prop efficiency maps. The only way to describe it in simple terms is to make gross assumptions.

But, I'm glad we agree that weight is a much more important than horsepower if you want to decrease take-off distance.

It was interesting to look at the STOL take-off and landing competitors from Valdez at Oshkosh. The extremely lightweight Cub-like one with an O-200 had a shorter take-off distance than the O-360 powered ones.

[RV10inOz]

Simple answer, go to Barret Precission Engines, ask for a parallel valve high compression engine (most power and lower fuel burn) and fit it with the Hartzel BA prop.

You will get light weight (of the stock engine) and better HP than a stock 180, and have better fuel burn, but it will need Avgas not mogas.

[N941WR]

If you are going to operate out of a high density airport, go with the -9 rather than the -7.

The long wing and unique airfoil on the -9 take the worry out of high and hot operations.

As I understand it, Aerosport Power simply assembles kit engines or rebuilds cores. Thus you could end up with a Superior, ECI, Lycoming, or a mix of all three.

[GeneL]

I see you have listed the B/A prop good thing, but in a 72 inch. Go with 74 inch. Much more bite in thin air.

[AndyRV7]

I have an Aerosport Power engine in my purchased 7. I Have about 50 hours on it in the past 6 weeks. I love the power of the engine but haven't owned it long enough to tell you how it will perform over a longer time frame. But the extra power is very nice.

I cruise at about 23 squared at 175mph TAS or better and burn about 8.6 GPH for what it's worth.

Andy

EDIT: I also have a Hartzel CSP.