Take-off roll performance

[Piloto.Mendes]

Guys,

Does anybody know runway lenght required (aprox), with and without obstacles, for a RV-7A (180 HP, C/S prop) be able to take off at 3700 feet, 30? C (86F), maximum take-off weight, no wind, asphalt?

Thanks for reply.

Rafael Mendes

 

[Kevin Horton]

The formula for speed vs accelation and distance is:
V^2 = 2 a s (the square of the speed = 2 times the acceleration times the distance).

So, the distance required to accelerate to lift off speed, s, is proportional to (V^2)/a.

Assuming you become airborne at the same CAS in both cases, the TAS for the 3700 ft 30 deg C case is 1.1 times higher than it would be for sea level standard day.

Acceleration is proportional to engine power, if everything else is equal. My O-360 power chart suggests the power for the 3700 ft case would be about 84% of what it would be for the sea level case (assuming that the MP is one inch less than the ambient pressure for each case).

So, the ground roll for the 3700 ft case would be (1.1^2)/0.84 = 1.43 times the distance for the sea level standard day case. At gross weight, Van claims a ground roll of 575 ft (I assume this is for sea level, standard day). Thus the predicted ground roll at 3700 ft, 30 deg C would be about 825 ft.

For distance to clear an obstacle, I would need to know how high the obstacle is. I've got a very rough, completely unverified RV-8 flight performance model that I could run to find approximate climb gradients. The results should be equally invalid for the RV-7 as for the RV-8. But the accuracy is worth no more than what you paid for it.

Edit - But there is a problem I had not originally considered. Although I might be able to predict how climb gradient would vary with altitude, temperature, speed, etc, I do not know what the lift off speed would be for Van's claimed 575 ft ground roll. So I can't know how much distance it would take to accelerate to the climb speed to use for the climb to the obstacle height.

 

[fodrv7]

Southern coriolis.

So I can't know how much distance it would take to accelerate to the climb speed to use for the climb to the obstacle height.

Kevin,
I can't give you any figures, but I don't think that the acceleration phase from lift-off to Vcl would take more than 5 sec.
Lifting off in my RV-7 or my mates RV-8 at around 60KT would see 80KT on the clock by the time you had pitched up to the climb attitude; less than 5 sec. You should be able to establish a reasonable distance from that.

Rafael,
I see you are doing research into RVs.
It might be easier to ask the question from the opposite direction. ie. Can an RV-7A with an IO-360 and CS prop, take-off from runway at 3700 'AMSL that is #,000 ft long. How long is the runway?

I am sure that a bitumen runway in your capital city Brasilia (assume that is where the runway is, Brasilia being around 3,300 ft AMSL) is at least 4,000' long. If this is the case an RV-7A will have no trouble operating out of this runway.
The RV-7 will easily out perform a Beech Bonanza and according to the charts a Bonanza will clear a 50' obstacle at 3,700' Pressure Altitude, 30?C, nil wind, bitumen in 2,400'. AN RV-7 WILL DO BETTER THAN THAT.

Hope this helps a fellow Southern Hemisphere man.

Pete.

 

[Piloto.Mendes]

Kevin,
I can't give you any figures, but I don't think that the acceleration phase from lift-off to Vcl would take more than 5 sec.
Lifting off in my RV-7 or my mates RV-8 at around 60KT would see 80KT on the clock by the time you had pitched up to the climb attitude; less than 5 sec. You should be able to establish a reasonable distance from that.

Rafael,
I see you are doing research into RVs.
It might be easier to ask the question from the opposite direction. ie. Can an RV-7A with an IO-360 and CS prop, take-off from runway at 3700 'AMSL that is #,000 ft long. How long is the runway?

I am sure that a bitumen runway in your capital city Brasilia (assume that is where the runway is, Brasilia being around 3,300 ft AMSL) is at least 4,000' long. If this is the case an RV-7A will have no trouble operating out of this runway.
The RV-7 will easily out perform a Beech Bonanza and according to the charts a Bonanza will clear a 50' obstacle at 3,700' Pressure Altitude, 30?C, nil wind, bitumen in 2,400'. AN RV-7 WILL DO BETTER THAN THAT.

Hope this helps a fellow Southern Hemisphere man.

Pete.

Brasilia main airport has 2 runways wich lenght is 9700 ft. But hangar there is too expensive and the airport is too crowded.

I will put RV at a air club that has a runway of 1100 with some not too high obstacles on both thresholds.

Can I take-off on this strip with full weight, 90F at 3500 ft?

Thanks guys.

 

[dan]

I have a feeling you could take off without much of an issue. Landing on a relatively short field like that would require skill and diligence, especially at gross weight and relatively high density altitude.

 

[Kevin Horton]

Brasilia main airport has 2 runways wich lenght is 9700 ft. But hangar there is too expensive and the airport is too crowded.

I will put RV at a air club that has a runway of 1100 with some not too high obstacles on both thresholds.

Can I take-off on this strip with full weight, 90F at 3500 ft?

An RV-9A would be a better match to this airport. If you must build a short wing RV, then build it light, and consider doing take-offs and landings at a reduced weight, and loading passenger and fuel at another airport. Vortex generators may also be a worthwhile mod, as they should give a small reduction in stall speed, and better controllability at low speed. Some sort of active stall warning system would also be a good idea, as you will be operating at low speed close to the ground, and the short wing RVs (with the possible exception of the RV-8) have very poor natural stall warning.

It would also be a good idea to do the flight test program at another airport, and to only move to this one once you have demonstrated that you can consistently achieve take offs and landings in the required distance.

 

[Piloto.Mendes]

Thanks for all help, guys. This space is amazing.

I was figuring out that it would be really difficult to acomplish, without any error margin.

It?s gonna be difficult to leave it light, because I want a full IFR aircraft to make cross country flight. I live in countryside and I love beaches, between 1000 km and 1500 km I can get to most beautifull ones. So, I have to find another solution.

I think that landing wont't be that trouble, I've some experience and mostly I will land with low fuel.

What do you think about putting a IO-390 and a 3 blade MT propeller? Would it make MUCH diference at take off roll performance? I am going to lose some knots at cruise speed (compared with Hartzel), but...

Thanks again

 

[Kevin Horton]

I think that landing wont't be that trouble, I've some experience and mostly I will land with low fuel.

Actually, take off distance is much more consistent from take-off to take-off than landing distance, as there are fewer ways for the pilot to screw it up. I'm more worried about the landing at your airport than I am about the take-off.

What do you think about putting a IO-390 and a 3 blade MT propeller? Would it make MUCH diference at take off roll performance? I am going to lose some knots at cruise speed (compared with Hartzel), but...

If everything else was equal, the IO-390 should have a take-off roll of about 180/210 = 86% of the value for an 180 hp engine. So, the predicted ground roll at 3700 ft, 30 deg C would be about 705 ft. Note: this assumes a smooth, hard runway surface.

I don't know how the performance of the MT props compare to the Hartzells during take-off. The prop blade twist is optimized for performance at much higher speeds, so large portions of the blade may be stalled during portions of the take-off roll. Most people only really look at cruise performance, and maybe climb performance. I have never seen any data from anyone who tried to compare take-off performance between these two props. It is hard to say which one would be better.

 

[brianw]

Hi Kevin,

Fellow Ottawan curious about the impact of vortex generators on stall speeds, particularly on an RV-3...any way to provide a ballpark idea of how much a reduction there might be on the 51mph stall speed of a 750lb -3?

Thanks!
Brian

 

[Kevin Horton]

Hi Kevin,

Fellow Ottawan curious about the impact of vortex generators on stall speeds, particularly on an RV-3...any way to provide a ballpark idea of how much a reduction there might be on the 51mph stall speed of a 750lb -3?

Terry Jantzi did some flight testing of VGs on his RV-6A. He fitted a flight test airspeed boom, so he could get proper calibrated airspeed data at the stall. He found that the stall speed with full flap went from 47 kt to 44 kt CAS. Some people report reductions in IAS at the stall that are more than that, but changes in IAS stall speeds are pretty much meaningless, as you don't know how much the error in the airspeed system might be affected by the higher angle of attack you get at the stall once you put the VGs on.

 

[gmcjetpilot]

Yes

Short answer YES, especially with no obstacles.

 

[N908RV]

I was figuring out that it would be really difficult to acomplish, without any error margin.

I think that landing wont't be that trouble, I've some experience and mostly I will land with low fuel.

Rafael,

I'd have to concur with Kevin's recommendation that you might want to consider an RV-9 with lower stall speeds than the -7. This would give you even more margin on landing and it still has a great climb performance from what I have seen.

To give you a real world number - I have an -8 with a 180hp and Hartzell BA CS prop. Max gross weight take offs are not a problem at sea level - take offs are usually about 500' on asphalt. Even on high DA days, I generally don't use more than about 600' at full gross. Landings at gross are another matter. I find that at full gross I generally eat up about 1000' to 1100' before I can come to a full stop. Part of this is because I fly at slightly higher speeds at full gross due to higher stall speeds. But If you are light on fuel and solo, then this would be easier of course, but then you have to factor in your local DA conditions on take off and landing and you mention that you have some "small obstructions".

As George said, the short answer is yes, you can theoretically do the 1100', but you have to be close to perfect everytime.

I look at Van's numbers of 500' for a full gross (sea level I'm sure) landing in the -8 and think, OK, that is not a landing, it is an approach at probably less than 1.3 VS and it is a smackdown three point and on the brakes as soon as possible affair. Not exactly easy on the airplane and most -8 drivers don't do this. Admittedly though - 8's like the wheelies which require more speed and I can land my friend's -6 a tad slower and 3 point it, so your -7 will fair a little better in that regard. But only just a little.

We also tend to get sloppy in the states as many of us have long, wide runways with good approaches. We're spoiled!